HomeMy WebLinkAboutresolution.apz.016-73Idovcr,u~:r, _ ~ 1_,
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G7Il-~R1.AS, it is the. responsil,i.J.ity of the Aspen
P7.anning ~,Olnn1J.SS.'LCtt to plan for the. orderly growth and
development of the City to promote the hea7.t1-~, safety and
welfare of its citizens, and
LJIII~.I:F,AS, [dx-ight-I~1cTaughlin I~;ngineers has pr.c-
pared 2~id submitted for planning commission consi._deration
an "Urban Runoff Management Plan" for. the City of Aspen, and
l•:IlIsPrAS, upon legal notice a public hearing to
consider. comments on above said plan c;as held l~ovember 6,
].973, and
WHEP.>iAS, the pJ_anni.ng conmiission has reviewed
said plan and finds that its adoption ;could appreciably
reso7_ve stream pollution problems caused by urban runoff,
minimize pr.opcrty damage from flooding, and otherwise
improve Aspen's envirormcnt t: ):rough the proper management
of stoz~n runoff,
NC[d TI}]P.EFOR); 13E TT 1:LSOLVEll, that the Aspen
P7_annin;; and 7.on:i.nl; Conmrissi.on hreby adopts the "urban
I:unoff 2~1o.nagen,ent Plan", t:c~;t and maps dated Au;ust, 1973,
prepared by idr:ight-13cL~aughlin Enginec~-s, En~i_neeriixg
Consultants,
Urban RunoLt PlanagemenL' i'1an
Page 1t~~o
Bl3 ~IT P'UKTA>/R l:LSCLVLD, thaY_ dur.i_n~ implemen-
tation tle ).ocation of_ detenti.on ponds, 3rainage swales
and storm sewers may he varied from that shown on the
"Urban l:mloff Management Plan" so long as the objectives
of the plan are accomplished.
r~
_ \ : ~ __
Chaixmh:i~
Aspen T~lannr.~ Commission
llated this ; ~~ ~5j- day of ~/O(~=,, I973.
Urban Runoff Management Plan
Wright-McLaughlin
August 13, 1973
WRIGHT•McLAUGHLIN ENGINEERS oorlL{n [NDIN{{Rlwo eurletl
RONALD C. MCLAUGXLIN• IN TM[[I[OIALTr I1[LD{01
KENNETH R. WRIG XT• EN6IN EERING CONeU LTANTe
1ALFORD E. ERICKEON ~
3130 ALCOTT ITRE[T WATtl [WRY AMD OI[Tlel[UTION
WAT[11 AND t{W AO[ TII[ATY[NT
lOl1GLA$ T. SOVERN• OENV[R~ COLORADO [0211
![W AO{ COLL[RION AND R{U[[
JOXN T. MILAN E• 190!1 Aee.i301
IN OUITRIAL WA[Ttl
KENNETH AB N. MwNwD[R•
!TORY 011AIMA0[ -
ASPEN OIFICE 13 August 1973 ILOOD CONTROL AMO
p.0. e0%[!10
ASPEN. COLO. elell OTNtl WATtl•O{I{NT[D I{OJ{CT{
Mr. Stacy Standley, Mayor
City Council City of Aspen
James Breasted
Jack Walls
Jenifer Pedersen
Peter De Gregorio
Michael Behrendt
Ramona Markalunas
Mr. Russell Campbell, Manager
P. 0. Box V
Aspen, Colorado 81611
Gentlemen:
~~2y~8~ ~ 4S~ ~~~J
Our master plan report "Urban Runoff Management Plan" for the City of Aspen is submitted
attachede This report was undertaken in accordance with our agreement of October 4,
1972.
The plan's purposes are to resolve the stream pollutional problems caused by urban
storm runoff, to minimize property damage from flooding, and to otherwise improve
Aspen's environment through the proper management of storm runoff,
Hydrological analyses were made from 2, 5, and 100-year frequencies of occurrence of
runoff. The plan incorporates major drainage criteria of 100-year frequency of occur-
rence throughout the study areae For the initial drainage svstem, i•e., the curbs and
gutters and the storm sewers, the design criteria are based upon the 2.3-vear fre-
quency of occurrence of storm and snowmelt runoff for most areas and a five-year basis
for the business area. Pollution control design criteria are related to the 2.3-
year frequency of storm runoff, (or more where the initial drainage system has addi-
tional capacity for carrying runoff)e
Basin and sub-basin descriptions are presented in the report along with an explana-
tion and evaluation of snowmelt and storm runoff quantities, a presentation of pollu-
tion loads, treatment opportunities, and a tabulation of design criteria.
Tested runoff qualities from downtown Aspen have shown suspended solids concentrations
six times higher and organic solids (BOD) about half that of raw sanitary sewage.
The Master Plan is described in Section VIII and illustrated on Drawings 1 through 5.
The Pu n's basic approach involves:
• Jnterceotion of the upstream tributary mountain runoff, prior to its becoming
contaminated, and segregated routing to the Roaring Fork River.
C
;Letter of Transmittal, 13 August 1973 Pcye 2
• Collection of •t he relatively contaminated flows from urban Aspen. Providing
detention storage for these flows and then controlled-rate treatment using the
existing Aspen Metro plant.
' Collection of runoff from the lower density developmentareas, These flows to be
stored in sedimentation/detention basins and then used for irrigation, released
to the stream, or further treated if required.
Prevention of ollution and reduction of eak flow rates throw h institutional
measures Section IX .
A recommended initial program has been selected to resolve a majority of the immediate
needs. The major features of this program include:
1. Channels, detention basins, and floodways as required to handle mountain runoff.
2. Construction of a storm sewer and floodway in Mill Street in conjunction with the
proposed mall project. Separate aesthetic low flows of good quality water to be
provided.
3. The main sewers, channels, as well as initial drainage improvements, and other
appurtenances needed to collect contaminated storm runoff from the urban Aspen areas.
4. .Detention basins and controlled discharge facilities to permit storm runoff (SRO)
treatment at the Metro plant.
5. Miscellaneous initial drainage, transmission and treatment facilities in the West
Aspen area having high value to cost ratios.
The estimated project budget for the initial recommended program is $440,300. In
addition, a continuing cost for maintenance and treatment by the Aspen/Aspen Metro
Sanitation Districts would be incurred.
We will 6e available to discuss the Plan and initial program with you,
Respectfully submitted
by
----~ZijLE~IL~ ~
William T'agga
Kenneth R, fight
~` t ROBERT G,
WRIGHT-McLAUGHLIN ENGINEERS
Kenn sh ~~~~-
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Ronald C. McLaughli
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A. In_ •.
CITY OF ASPEN
ASPEN, COLORADO
URBAN RUNOFF
MANAGEMENT PLAN
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WfaIONT-M~LAUOMLIN ENOfNEERS
ENOINEERINO CONSULTANTS
DENVER, COLORADO
AUOUST, 7573
TABLE OF CONTENTS
LETTER OF TRANSMITTAL
Section No. Title
Page
_ 1 INTRODUCTION ,
•
I-I
Study Area
Scope of Work I-I
I-2
II PROJECT SETTING
.
R II-I
oaring Fork Watershed II
I
The Aspen Valley ~ -
Population Growth and Development II-2
11_3
Vehicular Population II-4
I11 DESCRIPTION OF DRAINAGE BASINS .
• -
III-I
Aspen Mountain Dralnage Characteristics III-4
Dralnage Basin Description III-4
' Major Stream Basins III-9
IV HYDROLOGY
. . .
Aspen Desl
R
i
f IV-I
n
a
n
all
~ IV-2
_Design Runoff IV-5
Snowmelt Hydrology IV-8
V SNOWMELT AND STORM RUNOFF POLLUTION V-1
Urban Runoff Pollution Characteristics V-I
Aspen Runoff Pollutants
~ ~ ~ V-3
Field Tests
Institutional Reduction of
Runoff Pollutl V-3
onV-8
.
Receiving Stream Quality V_9
VI DESIGN CRITERIA
VI-I
Hydrologic/Hydraulic Criteria VI-I
Pollution Abatement Cr;terla VI-4
Design Criteria for Sewage Treatment
Water Quality Standards VI-6
Y VII TREATMENT OF URBAN RUNOFF VII-I
Aspen Sanitation D(strict Plant (ASD) VII-I
Aspen Metro Plant
Land Application VII-2
Seperate Plants VII-3
VII-6
'.
TABLE OF CONTENTS
Section No, Title Page
VIII MASTER PLAN DESCRIPTION VIII-I
Dense Urban Runoff VIII-3
West Aspen Basins VIII-3
Outlying Basins VIII-4
Capacity of Aspen Metropolitan Plant VIII-4
Agreement for Treatment VIII-5
Costs and Phasing VIII-5
Urban Runoff Management Plan Cost Estimate VIII-9
IX INSTITUTIONAL RECOMMENDATIONS 1X-I
Land Use X-I
Upstream Ponding X-I
Pollution Control Regulations X-2
APPENDIX A REFERENCES A-I
APPENDIX B EXAMPLE FLOOD PLAIN REGULATION B-I
LIST OF TABLES
Table No, Tltle Page
II-I NUMBER OF AUTOMOBII.I.S ENTERING ASPEN
PER DAY i l-5
II-2 NUMBER OF MOTOR VEHICLES IN ASPEN AREA II-5
III-I GENERAL BASIN CATEGORIES III-I
III-2 DRAINAGE BASIN CATEGORY AREAS III-2
III-3 ASPEN STUDY MAJOR DRAINAGE BASINS III-3
III-4 ASPEN DRAINAGE BASIN CHARACTERISTICS III-8
III-5 ROARING FORK RIVER - MAJOR TRIBUTARY BASINS
AFFECTING THE STUDY AREA III-10
IV-I ASPEN DESIGN RAINFALL IV-4
IV-Z ASPEN DRAINAGE BASIN IMPERVIOUS CHARACTER IV-5
IV-3 RAINFALL RUNOFF PEAK FLOWS IV-6
IV-4 COLORADO URBAN HYDROGRAPH PROCEDURE STORM
RUNOFF HYDROGRAPH AREA 3B 2 YEAR STORM IV-9
IV-5 COLORA00 URBAN HYDROGRAPH PROCEDURE STORM
RUNOFF HYDROGRAPH AREA 38 5 YEAR STORM IV-10
IV-6 COLORADO URBAN HYDROGRAPH PROCEDURE STORM
' RUNOFF HYDROGRAPH AREA 38 100 YEAR STORM IV-il
IV-7 ESTIMATED SNOWMELT RUNOFF (Inches of Water
per Month) IV-13
IV-8 MAXIMUM DAILY PEAK SNOWMELT FLOWS AND VOLUMES
' (One Percent Probability) IV_13
IV-9 MONTHLY RUNOFF VOLUMES AND EQUIVALENT DAILY
FLOWS FOR AN AVERAGE YEAR IV-14
IV-10 PEAK FLOWS FOR INTERMEDIATE REGIONAL AND
STANDARD PROJECT FLOODS (9) IV-16
' IV-11 ROARING FORK MONTHLY FLOW DISTRIBUTION (10).
(Including Drougth Period of 1954-'S6) IV-17
LIST OF TABLES
Table No. Title page
IV-12 ~ AVERAGE DAILY STREAM FLOWS NEAR ASPEN (cfs) IV-18
V-I TYPICAL POLLUTANTS TO A RECEIVING STREAM FROM
A CITY OF 10,000 PEOPLE FROM A 0.5-INCH STORM
.N ONE HOUR (12) V-2
V-11 TYPICAL HEAVY METALS FROM A CITY OF 10,000
PEOPLE FROM A 0,5-INCH STORM IN ONE HOUR(13) V-2
V-3 RUNOFF CHARACTERISTICS OF SEPARATE STORIA
RUNOFF (14) (Values in Milligrams per Liter) V-3
V-4 FIELD TEST SUMMARY (Values in milligrams per
L f ter) V-5
V-5 PROJECTED MONTHLY POLLUTION LOADS (FOUNDS) V-7
V-6 PROBABLE ASPEN POLLUTANT LOAD TO ROARING FORK
RIVER IN ONE HOUR FROM RUNOFF RESULTING FROM
A MEAN ANNUAL STORM HAVING A FREQUENCY OF
OCCURRENCE OF ONCE EACH 2.3 YEARS V-8
VII-I AVAILABLE CAPACITY FOR REGULATED INFLOWS TO
ASPEN METRO SEWAGE TREATMENT PLANT VII-5
LIST OF FIGURES
Figure No. Title page
I-I ASPEN STUDY AREA I-4
IV-I TIME-INTENSITY FREQUENCY CURVES ASPEN COLO.
T.105., R.85W. IV-3
V-I INCREASE OF BOD AND COD CONCENTRATIONS IN
SOLIDS SAMPLES WITH INCREASED ELAPSED TIME
SINCE LAST RAINFALL (12) V-4
V-2 TEMPERATURE (Recelving Stream. Quality) V-10
V-3 SODIUM (Recelving Stream Quallty) V-11
V-4 DISSOLVED SOLIDS (Recelving Stream Quallty). V-12
V-5 CHLORIDE (Recelving Stream Quallty). V-13
VII-la,lb TOTAL ASPEN SEWAGE FLOWS VII-4
VIII-I MALL FOR MILL OR GALENA STREETS
'r' VIII-2
LIST OF DRAWINGS
Sheet No. Title
I COVER - ASPEN AREA
2 CITY OF ASPEN - EAST
I 3 CITY OF ASPEN - CENTER
4 CITY OF ASPEN - WEST
I 5
I
I RED BUTTE AREA
SECTION I
INTRODUCTION
The purpose of this report is to present a comprehensive storm
runoff master plan for the City of Aspen and its immediate environs.
The master plan differs from most urban storm runoff investigations in
that its major emphasis is placed on water quality control. Collection,
detention storage, and transmission have been studied to the extent
necessary in order to result in a coordinated and complete plan.
The Urban Storm Drainage Criteria Manuall provides well thought out
I and generally accepted methodologies and planning and design criteria
for the management of urban storm runoff. This report, published in 1969
by the Denver Regional Council of Governments, demonstrates that urban
storm drainage can be managed in an environmentally sound manner. The logic
and philosophy adopted for the Aspen area is essentially consistent with
that as presented in the Urban Storm Drainage Criteria Manual.
The City of Aspen lies in the Roaring Fork Valley of the Colorado
River basin on the western slope of Colorado, in Township 10 South,
Range 85 West of the 6th Principal Meridian, in Pitkin County. The City
and its residents are concerned with maintaining and enhancing the quality
of the environment-the environment at Aspen being strongly oriented towards
the character of the river which flows through the City, and its adjacent
tributaries. The community is well aware of its responsibility to the
river downstream from the City, not only because the Aspen residents and
visitors utilize the stream for recreational purposes, but also because
the quality of water in the Roaring Fork River is now excellent and
degradation of this quality cannot be advantageous. It is for these reasons
that significant emphasis on the urban storm drainage management plan
includes water pollution control' aspects related to storm drainage.
The Aspen area is subject to urban snowmelt runoff and urban storm
runoff. On the average, Aspen experiences approximately 29 rainstorm
days each year. Nevertheless, the Aspen area has grown over the last
93 years to its present level of development without ever having laid
out any type of planned urban drainage system. Growth of the last 25 years
has resulted in more intense development, more impervious area, and more
storm runoff, and it is this which has emphasized the need to implement
a master plan for urban storm runoff.
STUDY AREA
The study area has been defined using natural boundaries of drainage
basin areas proposed for development, and includes areas which could be
annexed in the future, or those which would materially impact Aspen in
regard to the urban storm runoff. The study area selected is bounded on
' 1Refers to Reference I listed at the end of the, text.
on the west by Maroon Creek, on the north by the Roariny fork River,
and on the east to approximately the south line of Section i7 at
the Ute Cemetery. For the southern boundary of the study area the
toe of the slope of Aspen fountain was selected for system planning
purposes; but for hydrologic. purposes the full Aspen Mountain drainage
basins tributary to the City of Aspen were studied.
The study area is shown in Figure I-I in relationship to the Roariny
Fork valley and the surrounding mountains.
SCOPE OF WORK
Th(s assignment was alined at developing and investigating alter-
nate storm drainage plans for the City of Aspen and immediate environs
by taking into consideration the eccr~omics of the city's comprehensive
plans and programs, th,e oi:e:; space objectives of the community, the
storm drainage reyuirem~nts to the urban area, the need for water quality
management to protect the Roariny Fork River, and with full consideration
to environmentally sound design concepts,
The intended result is the preparation of a recommended coe~pre-
hensive storm drainage plan for each of the basins involved. The purpose
of the comprehensi•/e plan is to provide sufficient information to permit
inal planning and design of drainage and related facilities as a part
of a long-term master plan-so as to avoid unnecessary future conflicts
and wasted investment.
The report was undertaken in three phases categorized as evaluation,
monitoring, and preli,iilnary design, Specifically, these are described as
follows:
A.
Evaluation.
1. Review and anlysis cF the existing storm drainage system.
2. Review of de•/eloprnent planning c,f FuC~ire development, parks,
and other related featw'es.
3. Hydrologic studies to determine design discharges, bath peak
rates of flow and volume.
4. Review of existing bridges, drainage structures and stream
channels.
5- Review of existing City ordinances and regulations related to
drainage.
B. Monitoring.
1. Selection of testing and observation points for storm runoff
quantity and quality.
2. Analyzing of snowmelt and storm runoff water samples,
' I-3
' 3. Laboratory tests and published data were collected and analyzed
[o reflect degrees of pollution involved, and,design criteria
were developed for effluent water quality to reflect national
~, oals as articulated by the Water Pollution Control A r nmo d
ments of 1 72.
C. Preliminary Design.
1, Providing and describing of present practices, i.e., institutional
' approaches to urban storm drainage management.
2. Preparation of basic urban storm drainage design criteria
for the City coupled with recommendations for regulations
' covering urban drainage practices.
3. Conceptual layout and design of treatment processes and
facilities to attain the desired water quality goals.
4. Preliminary layout of the initial drainage system (characterized
by storm sewers) and major drainage waterways which are typically
' represented by open channel swales and streams.
5. Provide preliminary cost estimates for improvements proposed
' for near-future construction.
LJ
1-4
ASPEN; STUDY AREA
WAIOMT- M~LAUOMLIN ENOINEEAH
24HO ALCOTT 8T. DENVER, COLO. B021'1
SECTION
PROJECT SETTING
The defined study area for the City of Aspen urban storm runoff
' and snowmelt management plan represents the core of the Aspen metro-
politan area. This study area Iles southwest of the Roaring Fork
River, commencing at the confluence of Maroon Creek and extending
' upstream a dlstance'of approximately three miles. The width of the study
area paralleling the Roaring fork area ranges from a
roximatel
o
-h
lf
pp
y
ne
a
to one mile. As noted In Figure I-I, the study area includes the
City of Aspen and the adjacent more densely populated environs.
' Thi
ti
f
s sec
on o
the report presents a general description of the
area to provide the reader with the project setting so that he may
' better understand the more technical aspects of the hydrological and
pollutlonal characteristics of th
b
i
h
i
f
e
as
n. T
e C
ty o
Aspen, lying at
an elevation of about 7,950 feet, impacts not only the Roaring Fork
' River but also Hunter Creek, Castle Creek, and Maroon Creek, all of
which join the Roarin
Fork withi
th
l
d
d
g
n
e. se
ecte
stu
y area boundary.
Waterways directly receiving storm and snowmelt runoff include the
' Roaring Fork, Castle Creek and Maroon Creek. Hunter Creek enters the
Roaring Fork on the right bank.
The present a;nd future urbanized area of the City of Aspen and its
' environs lies on the historic flood pialn and terraces of the Roaring
Fork and the tributaries mentioned above. The flood plain and Its
' terraces are approximately 3000 feet wide at the upstream end of the study
area and broaden to over a mile wide in the vicln(t
of Maroon Creek
y
.
1 ROARING FORK WATERSHED
The area tributary to the Roaring Fork River at Aspen is comprised
' of 285 square miles ranging in elevation from 7,630 feet at the confluence
of the Roarin
Fork and Maroon C
k
14
6
f
g
ree
to
,2
5
eet on Castle Peak.
The average djscharge of the Roaring Fork River at a point two
' miles upstream from Hunter Creek is about 110,000 acre-feet per year, w~ich
represents an average flow of 152 cfs. The maximum discharge of record
was 3,170 cis, which occurred on June 18,1917. Prior to the construction
of the TH~in Lakes Tunnel, the minimum daily flow occurred In July, 1934
at 15 cfs. However, since the construction of Twin Lakes the minimum
flows have dropped to 0.4 cfs in September of 1956 representing for
most practical purposes, a dry stream.' These low stream flows quickly
recover at the confluence of Castle Creek and then with Maroon Creak..
The setting of this basin Is spectacular, with many 13,000 to 14,000-
foot peaks, abundant vegetation and wildlife, streams and natural lakes.
II-2
However, the natural beauty has been adversely affected by man in some
places such as the mining areas, town areas, and in particular the natur~l
flood plain near Aspen, as documented by the Roaring Fork Greenway Plan.
The geology of the valley is basically lacial alluvium to great depth
which often extends up the adjacent mountain slopes. The valley floor is
naturally well-covered with vegetation ranging from ground cover of grasses
and sage to shrubs and trees such as scrub oak, cottonwood and blue spruce.
I The natural precipitation runoff process tends to hold back water where
i[ falls. That is, some of the falling precipitation is held on vegetation
surfaces, some inriltrated into the porous natural ground, and what runoff
does occur is often detained in small ponds and lakes. The final result is
I a pattern of steady stream flows which fluctuate rather slowly with rainfall
precipitation events.
I Nevertheless, the Roaring Fork is considered hydrologically to be a
flashy stream because of the large accumulation of snow pack in the basin
and its rapid runoff with the coming of warm weather. The normal non-
' snowmelt period stream flow in the Roaring Fork upstream from Hunter Creek
generally ranges from 15 to 40 cfs from August through April. In May
the average stream flow often increases by a factor of 10, with the beginning
of the ss owmelt runoff season, reaching a peak in June when the aye rage run-
' off is generally increased by a factor of 20 to 30 times. The flow in the
river drops rapidly in July as the snow pack is depleted.
' THE ASPEN VALLEY
I The valley floor of the Roaring Fork and its tributaries in the Aspen
area is bordered by Bell Mountain, Aspen Mountain and Highland Peak
to [he south, Smuggler Mountain to the east, Red Mountain to the north, and
Buttermilk ski area on the west. The Roaring Fork flows from
southeast to northwest through the City of Aspen, ranging from a rather
steep bedded stream upstream of Aspen on a narrow flood plain to a fast
flowing stream incised into a broad flood plain downstream of Aspen. Just
upstream of the southeastern edge of the study area a glacial terminal
morraine exists which has resulted in an old lake bed which is now filled
with organic silt containing sand and cobbles.
I The Aspen urban area was initiated in 1880 and was originally knovm
as Ute Springs or Ute City. The mining industry was strong in early Aspen
and early populations before the turn of the century were significantly
greater than the permanent population of Aspen today, In 1880 the popula-
tion was 35 people, but by 1888 this had grown [0 8,000 and then by 1892
the population peaked at II,000. After the silver bust in 1893 the city
was quickly depopulated and remained relatively static with a fairly minimal
economy based on mining. The Smuggler and the Free Silver shafts were
flooded in 1910 by ground water inflow, though the situation was corrected
soon after. The Aspen Daily Times stopped publishing in 1920, and during
the following two decades the Aspen area-was at a low point in terms of
II-3
I!
`I
=1
~~~
population and economic activity. Members of the Tenth Mountain Division
and Mr. Walter Paepcke recognized the potential of Aspen as a recreational
area shortly after World War II, and since that time the Aspen population,
economy and development has risen steadily 4
The effects of the City of Aspen and mining on the natural drainage
system can be traced from the early beginning to the present. The pollu-
tional aspects of an 8,000 to 11,000 population city in [he period from
1888 to 1892 are significant, considering the relatively small town site.
The effects of mine dumps and smelter wastes created by men such as
D. M. Hyman (Durant Mine), Henry Turtelotte (Turtelotte Park), and
Jerome B. Wheeler (Aspen Mine) are probably still felt as present day
runoff carries some leachate pollutants to the Roaring Fork. The actual
drainage patterns were altered by various developments. For instance,
the railroad grade for the Colorado Midland Standard Gauge Railroad
fortunate) blocks some drainage from Aspen Mountain. The various grades
of the Denver and Rio Grande had a ects on the ocal drainage patterns
in Aspen itself. The various mine dumps on the mountains have undoubtedly
varied the historical stream thalwegs so that recognition is often diffi-
cult, if not impossible. The miners also constructed various drainage
works to prevent mines and other works from receiving runoff flows.
Inflow of ground water to the mines requiring regular pumping to keep
them workable indicates the availability of ground water in significant
quantities which is recharged by rainfall and snowmelt that has infiltrated
through the ground surface to great depths.
The hydrological impact of the City of Aspen results basically in a
number of water-oriented changes which are:
The Roaring Fork River floodplain is altered. for example, bridges
create backwaters which widen the floodplain, and adjacent develop-
ment tends to straighten and narrow the river, making a faster flowing
more eroding body of water.
2. The runoff volumes and peak rates of flow are increased because vege-
tation has been removed, surfaces made impermeable, and the general
flow path to the main stream shortened.
3• The urban snowmelt and storm runoff carry pollutants picked up from
overland flow on parking lots, streets, gutters and other surfaces
and flow routes. Such pollutants include various heavy metals, grit,
trash, and debris, organic material and various salts.
POPULATION GROWTH AND DEVELOPMENT
The present permanent population of the City of Aspen and immediate
environs, which represents the study area for this report, is approximately
5500 people, representing about 50 percent of the entire permanent popula-
tion of Pitkin County. This present population is greatly increased for
' Il-4
about lour and one-hall months of the year during the ski season with
' peak populations, including visitors, el about 25,000 people.
During the winter period approximately 200,000 people visit Aspen
' for skiing and related activities for an average stay of 5.3 days, which
represents an average visitor population from Thanksgiving to mid-April of
approximately 7,500 people, During the summer visitor period including
' June, July and August, the total population of the City of Aspen and
its immediate environs is estimated to average about 10,000 people.
During the last five to ten years the total population of the area has
risen dramatically. Using trend forecasting to estimate future popula-
' tion numbers would quickly demonstrate the fallacy of trend forecasting
because the carrying capacity of the valley would be greatly exceeded
' in a relatively few years. The recent population and visitor growth
has resulted in environmental awareness and the creation of citizen
activitists who are working towards a stabilization of both permanent
population and visitors. The population can be limited by controlling
' building permits, limiting new beds for visitors, controlling neo-i
industry and commercial establishments -- all related perhaps to the
measureable carrying capacity of the ski lifts and ski trails.
' As a result of the recent population growth situation. a changing
.
political scene, and anti-growth sentiment, future population pYO-
' jections have not been incorporated into this re ort. Rather, the
approach to uture development as it relates to the generation of
snowmelt and urban storm runoff has been to relate to a relatively
minor increase in development density in the presently developed portions
' of the study area and'to assume relatively low density development
in those portions downstream towards Castle and Maroon Creeks.
t VEHICULAR POPULATION
' A significant source of air and water pollution in present day
American cities is the automobile. The automobile affects urban runoff
water pollution because of constituents exhausted from the internal
combustion engine and from oil and grease on surfaces exposed to runoff,
' such as parking lots and streets.
Lead is currently a major pollutant resulting from automobile
exhausts being deposited on urban surfaces ranging from leaves and
grass to paved surfaces. The surface soils adjacent to heavily travelled
roadways for instance, might contain 100 to 200 milligrams per liter,
while natural soil would be only a fraction of this. It should be
realized however, that natural soils in the Aspen area ran
e widel
in
g
y
lead because of the heavy mineralization which was the basis of the
mining industry. During snowmelt and storm runoff periods, vehicular
pollutants tend to be carried with the runoff for discharge to the streams.
L'_1
II-5
Automobile activity and population in the Aspen area is high. The
number of automobiles in 1972 entering the study area on state highway 82
from either upstream or downstream ranges from approximately 1,340 to
' 1,950 per day. An estimate by months is presented belay in Table II-1
TABLE II-1 "
' NUMBER OF AUTOMOBILES ENTERING ASPEN
PEA DAY
' Month No. of Autos
January 1,300
February 1,500
March 1,600
April 1,500
May 1,600
June 1,800
July 1,900
August 2,000
September 1,700
October 1,600
November 1,500
December 1,500
It is estimated that in 1972 the number of registered vehicles in
Aspen and its immediate environs amounted to 4,500, of which about
75 percent were automobiles, the rest being trucks. This represents
approximately .8 registered vehicles per permanent resident. The number
of automotive vehicles in the Aspen area, including both permanent
t .residents and visitors, has been estimated for 1972 on a monthly basis
as shown below in Table II-2.
1 TABLE II-2
NUMBER OF MOTOR VEHICLES IN ASPEN AREA
1 Month No
of Vehicles
.
January 5,800
February 5,900
March 6,000
April 5,900
May 6,000
June 6,200
July 6,300
August 6,400
September 6,200
October 6,000
November 5,900
December, 5>900
II-6
During 1971 spot. traffic counts were made at specific key locations,
and it was determined that on an average daily basis approximatel
23,000 vehicle trips occurred through the
city y
boundaries, either in or
out. Interior auto trips within the city not crossing the city boundaries
are in addition to these 23,000 trips per day.
1
SECTION III
DESCRIPTION OF DRAINAGE BASINS
The drainage basins located within, or tributary to, the study
area of the City of Aspen and its immediate environs are described in
this section of the report. The basins are numbered 1 through 10 and
are shown at the end of this report on Drawing No. i.
The drainage basins which have been defined fall into three general
classifications. The first classification represents the Aspen Mountain
basins which are tributary to the presently developed City of Aspen.
The second category of basins represents [hose situated in the already
urbanized City of Aspen area, and the third classification consists of
outlying areas, primarily of open space such as Hallam Lake, the Aspen
golf course and low density development lying between Castle Creek and
Maroon Creek.
At the end of this section an additional discussion of the major
stream characteristics, including the Roaring Fork River, Hunter Creek,
Castle Creek, and Maroon Creek, Fs presented to provide a continuity
and relationship to the urban basins for potential additional analyses,
considered outside the scope of this report. The three categories of
drainage basins are segregated as follows:
TABLE III-I
00 GENERAL BASIN CATEGORIES
Category Description Sub Basins
I Mountain basins tributary to the City of IA, 3A, 5A
Aspen
II City of Aspen urbanized basins 16,~2A, B p,
SB, 6A, 6C
III Outlying low density basins SC, 6B, 6D, 7A, 8A,
9A, 9B, IOA
The character of development of drainage basins relative to snowmel[
or urban storm drainage runoff is important from the hydrological point
of view because basins which have more developed conditions with greater
amounts of impervious area create significantly more runoff than would
' low density developed basins or those still in their natural state.
In other words, the more people and more development there is, the more
runoff there is with associated pollution.
III-2
' Far purposes of this study, aerial photographs, mapping, and fleid
inspections were used to determine present characteristics of the basins
affecting runoff. This Information presented a bench mark from which to
begin hydrological studies. It should be pointed out, however, that all
hydrological investigations and computations were based u on future ulti-
mate eve opment o the basins, taking into consideration the present
Aspen Planning Department estimates of what robabl would ha en to
each o t e areas assuming a present continuation of the community effort
to control the level of development in the Aspen area. Therefore, descrip-
tions of the drainage basins in this section of the report, and upon which
t the hydrological computations have been based, represent best estimates
of the level of development of each of the basins for a condition which
would be expected to exist in approximately 1990.
1 The total area of the three categories of drainage basins which have
been studied (s summarized as follows in Table III-2.
TABLE III-2
DRAINAGE BASIN CATEGORY AREAS
Category Area in Square Mlles Area in Acres
I 1.457 933
' II 0.546 349
Ill 1.920 1,230
Totals 3.923 2,512
Each of the drainage basins situated in the study area, or tributary
to the study area, is listed in Table III-3, in tabular form, together
with the respective areas. It is evident that the total area of 2,512
acres represents a significant watershed and therefore a significant
potential flood hazard and water pollution contributor.
The drainage basins as presented in this report are generally defined
on the basis of "major drainage". Major drainage basins describe the
tributary drainage systems which funct on urino a large storm when minor
drainage facilities cease to function, curbs are overtopped, and the
runoff water tends to follow natural depressions or historic routes.
The major drainage system Is that storm drainage system which carries
the runoff from a storm having a frequency of occurrence such as once in
25,50 or 100 years. "The major system will function whether or not i[ has
been planned and dgslgned, and whether or not improvements are situated wisely
In respect to it."
In urban storm drainage planning and engineering it is also necessary
to consider_the "Initial drainage system", which is that system which
functions during small storms. The init(al drainage system is "that drainage
system w c s requent y used for collecting, transporting, and disposing
III-3
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III-4
of snowmelt,lmiscellaneous minor flows and storm runoff up to the capacity of
the system."
The initial drainage system is often characterized by storm sewers
and curbs and gutters of city streets. This report deals primarily with
the major drainage system,and to a lesser degree it deals with the
initial drainage system relative to the magnitude of the rate and volume
of runoff. In regard to the pollutional a'soect, the initial draina~
system takes on more importance in this study and report because it is
the initial drainage system which will primarily carry those runoff waters
which will be captured, collected, and treated prior to discharge to the
river.
ASPEN MOUNTAIN DRAINAGE CHARACTERISTICS
Historically, miners maintained drainage ditches in a similar manner
to the present Aspen Ski Corporation method to prevent roads from eroding
and to prevent water flooding of the lower mountain mines. The Aspen Ski
Corporation has maintained a drainage system for many years which sheds
the upper mountain runoff into Keno Gulch in the Castle Creek basin and
distributes lower mountain drainage into heavily wooded areas to mitigate
erosion on the grassed ski slopes. Thus, Aspen seldom has to pass signi-
ficant flows from the mountain as the precipitation which falls on the
lower mountain infiltrates into the alluvium/glacial material at the
base of the mountain. The Town of Aspen would be annually inundated by
runoff without this drainage system.
Because Aspen is
Mountain, no planned
system has been const
little service. A mai
generally unaware of drainage problems from Aspen
major drainage system exists. An initial drainage
or drainage system is needed in Aspen.
DRAINAGE BASIN DESCRIPTION
Hydrological studies to determine the amount and character of storm
runoff, utilizing standard 1engineering techniques such as the Colorado
Urban Hydrograph Procedure, require the definition of parameters such
as the area, the distance from the point of concentration to the upper
limit of the drainage basin, the distance from the point of concentra-
tion to the centroid of the drainage basin, the average slope of the
thalweg of the drainage basin, and engineering determinations of the time
of concentration. These have been determined for each of the basins and
sub-basins presented in Table III-3. Some of these basin characteristics
are also presented at the end of this Section in Table III-4.
A narrative description of each of the basins which presents some
of the more nebulous aspects of the basins is given below by basin
grouping.*
* Reference is also made to Town Basins 16 as East, 2A, 3B, 4A, and 5B as
Center, and 6A, 6C as West for purposes of determining polluted loads.
III-5
Basin lA and Basin 16:
Spar and Copper Gulches form the largest portion of this basin.
Basin IA is the mountain portion of this basin, while 18 is the flatter
urban portion.
The drainage diversion system maintained by the Aspen Ski Corpora-
tion on Upper Spar Gulch, as shown on Drawing No. 1, is an important
consideration. Charles Maddalone of the Ski Corporation related that the
ditch capacity has been exceeded twice in about 15 years which correlated
to two major flooding incidents in Aspen. Besides flooding down the
natural thalweg of Spar Gulch, it is possible for water to flood into
Basin 3 or Pioneer Gulch from a short length of open ditch between diver-
sion culverts under the upper ski slopes. Farther down, roadside
diversion ditches in Basin 3A, Pioneer Gulch, can divert water to Basin
5A. Thus it is possible for Spar Gulch runoff to enter into West Aspen.
Another drainage consideration farther down Spar Gulch near the
bottom terminal of Chair No. 5 is the general condition of the channel.
When the lift was installed, a large flat area was formed for the terminal
facilities out of a ridge which appears to have separated Vallejo Gulch and
Spar Gulch. The creek was diverted around the lift in a small ditch.
However, due to the adjoining flat area and lessened channel slope, this
channel is steadily silting. It is conceivable that flood flows could
eventually travel towards Vallejo Gulch of Basin 3A and into Central
Aspen. Thus an obvious improvement to the major drainage system will be
the improvement of Spar Gulch here.
Just above Aspen, Spar Gulch flow joins the Durant Mine outflow,
which eventually empties into the Original Street storm sewer. Develop-
ment has constricted the stream in this reach and this creates he ne d
for major drainage improvement. Stream changes by local residents and
developments in Spar Gulch above and near the Durant Mine have adversely
impacted Spar Gulch.
The lower sub-basin, 1Q is an urban area comprised of light com-
mercial, condominium and residential developments. Most of the streets
are paved and curbed. The Original Street storm sewer serves the area,
but inlet capacities are inadequate.
Basin 2A•
Spring Street serves as the major drainage system for this small
urban basin. A portion of the lower part of Bell Mountain is tributary
to this basin. There is a double cross-pan across Main Street which
carries flows often. Presently this area is developed in commercial uses
and is curbed and guttered. Conceivably,major flows in Basin 1 could
cross into this basin at the lower end.
III-6
Basin 3A and 38:
Pioneer and Vallejo Gulch are major tributary streams [o Central
Aspen. Their significance has no[ been clearly designated before, but
runoff from this basin would create considerable problems.
' Presently, the lower portion of this basin is being altered for
developments. It is quite probable that the development and relocation
of the Roche Cup Finish Area will cause drainage to be diverted to the
west instead of what appears to be the historic thalweg near Galena and
' Mill Streets where it should remainT_
The lower urban end of this basin, 3B, is not served by storm sewers,
but merely drains in a cross-pan system. The flow across Main Street and
Galena is the most troublesome, causing road deterioration.
The land use is represented by condominiums on the upper end and
commercial in the business center of Aspen.
Basin 4A:
This is a small urban basin with little mountain area historically
' tributary. The area is largely commercial with condominium development
similar to Basin 38. The area is served by a small storm sewer, but
this is largely inadequate and has insufficient inlets. ~'!
I Flood flows can become tributary, with "cross flows" traveling in
and through this basin to the west to Basin 56.
1 Basin 5A, 56 and 5C:
This basin includes a large portion of Aspen which can become
tributary to Spar Gulch runoff. The mountain portion of Basin 5A would in-
filtrate all but highly infrequent rainfall events as it is an absorptive
area of angular cobble and boulder talus. However, other materials are
present and the area is subject to slide and runoff flow hazard during
high intensity/long duration rainfall events. Runoff from the western
third of Basin 5A is detained to some extent by the abandoned Midland
Railroad embankment.
Runoff from this area, SA, enters the urban basin, 5B, which is
largely residential and is picked up in several sump areas by the Center
(Garmisch) storm sewer. There is no planned major drainage system.
Flooding would occur until the storm sewer eventually drained the area.'-
. The inlets are also inadequate, and the existing 36-inch storm sewer
would not flow to capacity because of this inadequacy.
III-7
Basin 6A, 68, 6C, and 6D:
This basin contains residential urban areas, 6A and 6C, the Aspen.
Institute Area, 6D, and the Hallam Lake Game Preserve,'68. Presently
the upper urban areas, 6A and 6C, are served by a small storm sewer
which drains the area into Hallam Lake. The capacity of this storm sewer
is small, causing flow to go from 6C to 6D, the Aspen Institute Area. The
drainage is basically carried through these areas in a system of .roadside
and irrigation ditches.
Major runoff from Basin 5 would flow into 6A and therefore into
Hallam Lake.
Basin 7A:
The area immediately adjacent and tributary to Castle Creek within
the study area forms Basin 7A. Bath sides of this basin are formed in
steep, natural bluffs that generally prohibit major development. However,
some light residential and condominium development exists.
Basin SA•
Light residential development exists in this area along with the Red
Butte Cemetery. A small portion of the east side of the golf course
drains through a 15-inch culvert near the intersection of the power plant
road and cemetery (Red Butte -..County Road No. 19) which then drains along
the east side of Cemetery Lane to the north. The upper portion of the
basin is relatively flat, but the middle of the basin near Mountain View
Drive is steeper, being a natural bluff which slopes to the river. The
drainage ditch alongside Cemetery Lane is considerably larger below, but
ends at Red Butte Drive. As there is no clear outlet to the Roaring Fork ,
River, the runoff either infiltrates into the ground or runs overland
I across Red Butte Drive to the Roaring Fork River. This basin is largely
covered by natural grasses, sagebrush and vaFious trees near the river.
This area has a somewhat higher infiltration rate than in the City of Aspen.
Basin 9A and 9B:
The upper portion of this basin, 9A, is the lower portion of a
ridge that runs down from Highland Peak. The steeper upper portion
of this basin is covered by evergreen, spruce and aspen trees. As the
slope becomes gentler, the vegetation changes to grasses and sagebrush.
Here again the natural infiltration rates are high. Presently, the
only development in this area is the Meadowwood Subdivision and the
water filtration plant.
It has been anticipated that this area will largely be developed for
light residential uses. If denser development occurs, urban storm runoff
would become a problem in this, area.
III-8
~_ The drainage from this area is picked up by a roadside ditch alon
Highway 82, .which drains through a 30-inch culvert through the inter- g
' section of County Road No. 13, 15 and Highway 82. It then drains through
the golf course.
' The golf course comprises a large portion of the lower basin 96.
The Wes[ Aspen Subdivision abuts [he north side of the golf course and
drains to Maroon Creek. The Marolt Irrigation Ditch handles urban
storm runoff from this entire basin. The 30-inch culvert under Highway 82
' •carries this ditch water, presumably to irrigate the golf course, and must
also carry drainage from the upper sub-basin 9A. At the lower end,
drainage ditches from the West Aspen Subdivision link with this ditch
' before discharge into Maroon Creek.
' Basin 10A:
This basin is quite similar to 9A with the upper portion being moun-
tainous, covered by evergreens and aspens. The lower portion of the basin
is somewhat flatter and is covered by grasses and sagebrush. Some develop-
ment exists in the lower portion of this basin, such.as the Aspen High
School and the Aspen Tennis Club Subdivision. The assumption that this
area will be dev=loped for light residential uses has been made.
TABLE III-4
ASPEN DRAINAGE BASIN CHARACTERISTICS
Basin L
No. (Miles) Lca S Perviousness Area
(Miles) (Percent) (Percent) (Acres)
IA 2.20 1.19 28.5 90
16 0.416 0.265 590
2A 0.568 0.22 2.0 60 70.3
3A 1.02 7 2.0 40 32.1
0.530 46.2 90 220
36 . 0.606 0.303 2.0 40
4A 0.644 0.227 I.9 40.0
SA 0.568 0.170 41.6 40 36.7
58 0.454 0.265 16.0 90 123
5C 0.189 0.10 6C 81.2
6A 0.379 0.227 3.3 80 6.9
t.6 60 56.0
6B 0.341 0.170 14.0 80
6C 0.379 0.190 2.3 60 73.5
6D 0.379 0.170 2.0 80 56.6
8A .148 0.379 6.25 80 180
0.474 3.33 80 150.
98 0:940 0.379 17.8 80 368
IoA 1.33 0.568 4.6 8a 200
195
I
III-9
MAJOR STREAM BASINS
The 285 square miles of the upper portion of the Roaring Fork
water-shed tributary to the confluence of Maroon Creek and the Roaring
Fork River can be best described from a hydrological standpoint by examining
each of its major tributary stream basins. The respective basins.,
' tributary areas, elevations and forest cover are given in Table 111-5,
Each basin has its own character with respect to geology, ecology
and hydrology, However, an important phenc-^-^^ *~_+ nrrirrs in the
mountain watersheds is the inFiltration ant / ~/~ / the
vegetation cover and physical soil system. ~j~Cl(~P~V/ It
runoff is naturally treated and maintained ~ ,m.
I For instance, runoff on the high slopes lri ;_
to i ned i n h i gh a l t i tude I akes and niarshc:s v ~ :an-
while the high altitude tundra slopes in(i -;~~1,'~S t'U ith
it emerging later as a groundwater contribr ,n ~ ~lO „\ -~ in
slopes are tree and vegetation covered, wh MJ heds
moisture to the ground below. The lower mr ~~ Y r y
very porous and readily infiltrate large ar s
water flows down in ground water or travel ~-~ Q U ~- its
eventual joining to the river or creek bel~ ain-
tained in a high quality state as to most ~, G~~~~(-'~~
In this process, however, elements ar
resulting in an increase in the total diss gh
the basic result is a clean stream and "pu condi-
tions. When man-made wastes or developments c.r~a„y~ ~ ~ ,,,_____,.the
results can be adverse for the receiving stream, For instance, high per-
centages of impervious area prevent infiltration and transmission via the
ground in the urban area and redirect the water via curbs and gutters
to a receiving stream. The stream flow is proportionately larger than
naturally created flow, and it also carries more solid matter and
pollutants. .
This latter process occurs in Aspen. However, it can be mitigated
by prevention of more impervious areas and by treating the urban run-
off,
The various basins are described as follows, with particular reference
to the hydrological nature of each, The particular hydrological system
in each .basin will reflect how the basin responds to rainfall or snow-
melt runoff in forms of flows and qualities.
Roaring Fork:
' An i~ortant feature of the Roarin Fork basin in and above Aspen
is the glacial deposits/alluvium. The deposits directly underlying
Aspen are largely sand and gravels con ainin8 cohhlnc any/nr i,.,,~t.io~~
Above Aspen to the gravel pits lie sands and silts containing cobbles.
and/or boulders. farther above this area to the Aspen Park is another
III-10
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III-11
' alluvial deposit formed basically of organic silt containing sand and
cobbles. These various alluvial materials and flat areas act somewhat
like a sponge, retaining runoff in the ground or in small ponds, and
' attenuating high peak flows. Later, when the runoff drops, ground water
inflow helps maintain the strewn flew.
' Many smaller creeks enter the Roaring Fork such as MacFarlane,
Diffi
l
P
cu
t,
tarmigan, and Coleman Creeks. Just above the Devil's
Punchbo
l
d
w
an
Weller Lake, Lincoln Gulch joins with the Roaring Fork.
Farther above this
oint
ff i
' p
runo
s diverted, stored, and sent to the
eastern slope by the Twin Lakes Reservoir and Canal C
ompany.
This diversion system, and associated storage, is significant from a
`h ydrologica St
i
an po
nt because it has the effect of detaining and divert-
ing high mountain snowmelt runoff
thus lowerin
st
fl
,
g
ream
ow in the Aspen
area. evera o the upper drainage snowsheds are diverted to this
' system.
For instance, the area above the confluence of Lost Man Creek and the
Roaring Fork, this confluence being just below the ghost town of
Independence, can be diverted by Tunnel No. 2 over [o Grizzly Reservoir,
which is loc
t
d
1 a
e
on upper Lincoln Creek. Much of the basin runoff above
the confluence of Lin
l
C
co
n
reek and New York Creek also is diverted to
Grizzly Reservoir. Grizzly Reservoir
water is then sent via Tunnel No. 1
to the North Fork of Lake Creek on the eastern slope of Colorado.
Hunter Creek:
Hunter Creek has a geological character similar to the Roaring Fork.
A[ the l
ower end of the basin, the material is a sand and silt alluvium
containing cobbles and/or bould
' ers. This glacial alluvium, and two marsh
areas higher up the thalweg, behave in a similar absorptive manner t
o atten-
uate runoff flows.
Castle Creek:
Castle Creek basin is quite different, with less glacial alluvium
material and more material of a sandstone and slltstone ;,edrock nature
Th
.
e hydrological nature is more responsive with relatively higher peak
fl
ows generally due to a somewhat steeper channel slope and a less
absorptive surface. The stream divides into the upper reach of Castle
Creek and Conundrum Creek some six miles below Ashcroft. Conundrum
Creek i
"
s steep and has a
flashy" hydrological character. The upper
portion of Castle C
k i
i
t ree
s m
lder, particularly near Ashcroft where a
marshy area provides some retention. A few lakes hi
her u
i
hi
g
p
n t
s
portion of the basin also provide retention such as Cathedral and
' American Lakes.
i
1
1
1
1
1
1
1
1
1
1
1
1
ill-12
Maroon Creek:
Maroon Creek has a character similar to Castle Creek from the con-
fluence with the Roaring Fork up about three and one-half miles to
Willow Creek. Because of a steeper streambed, less absorptive alluvial
material and more silts tone/sandstone, peak flows tend to be relatively
higher than the Roaring Fork and Hunter Creek flows. The upper portion
of the basin is drained by Willow, West Maroon and East Maroon Creek.
Willow Lake, Maroon Lake, Crater Lake and numerous small ponds provide
a large natural retention capacity.
SECTION IV
HYDROLOGY
The size and character of major drainage works, as well as other
urban storm drainage facilities, are determined by the magnitude of
the design runoff, that is, how much water should they carry before
being overtopped. Furthermore, the design of, the works must be
based upon expected sediment problems, including both degradation and
aggradation. This field is the domain of the hydrologist. Estimating
of pollution loads is a related, though a somewhat new effort in
hydrology, usually performed in conjenction with sanitary engineering.
Any master plan must be based on adequate and thorough hydrological
studies if the plan is to have significant value.
Hydrological studies must take into consideration volume of runo
as well as peak rates of flow. It is when the volume characteristics
are known that one realizes the value. of detention storage, both pond
type and channel storage, to reduce the peak rates of flow.
The magnitude of a flood can be fudged by its maximum rate of flow
even though the maximum rate may last for only ten minutes. The frequency
of a flood is judged by its return period, The return period is defined
as the average interval of time within which a given event will be equalled
or exceeded once. Thus a flood having a return period of 100 years has a
one percent probability of being equalled or exceeded in any given year.
Flood flow estimates have been made to determine frequency
in the Aspen area. Floods with return periods of 2, 5, and 100
were studied. The mean annual flood, with return period of 2.3
is approximately o greater than the 2 year runo
The 100-year return period has been chosen for major drainage
design purposes at Aspen because it is reasonable to provide against
major damage and loss of life in an urban area for a hazard which has
a one percent chance of happening in any given year. Experienced planners
and engineers throughout the country have generally chosen this
frequency as a basis of design for urban areas. Furthermore, the Corps
of Engineers studies for flood plain purposes define the Intermediate
Regional Flood which generally is a 100-year flood.
The 25-year flood is generally inadequate for a community to
use for planning. The 50-year flood is also generally considered to
be inadequate for major drainage design. The difference in project
cost between the 50-year and 100-year design is normally not sufficient
to justify the lowering of flood protection standards. For pollution
_control and treatment, a 2-3 year frequency storm runoff is a reasonabl
design parameter. Larger design runoffs usually tend to reduce cost
effectiveness.
relationships
years
years,
ASPEN DESIGN RAINFALL
GI
~'
U.S.
Rainf
IV-1
lates
ine c-. ~-, ana iuu-ye
Weather Bureau Technic
all Intensity-Duration
is the Time-Intensity-
the 2-, 5-. and 100-v
Infiltration
rai
requency ~urv
equency Curve
r storm based
Iv-2
lls were dete mined from two sources:
and No. 40~'- which present
s and Isohyetal mapping. Figure
for the Aspen area. Table IV-1 tabu-
--- --
upon Figure IV-l.
Part of the rainfall that occurs infiltrates into the ground and does not
become part of the immediate storm runoff. As Aspen further develops, the
overall infiltration rate will change because larger areas will become impervious
due to building and paving, and other areas will change from natural vegetation to
lawn grass. In some cases, the infiltration will increase, and in other cases it
will decrease.
. _;
The infiltration rates used in this study are based on the probable future
land cover. To obtain these rates, field infiltration observations were made.- ~""~
The values selected were for steady state infiltration.
Aspen Mountain was given an appreciably higher infiltration rate than the City
because of vegetation and forest litter cover and a permeable geology. The in-
filtration value assigned to the mountain was 1.8 inches per hour, while the
pervious area of the City was assigned a value of 0.5 'inches per hour, a rate
typical of many urban areas and judged suitable for Aspen.
Impervious Land Cover
Land that is totally impervious will produce a runoff approaching 90 percent.
Pervious land will absorb and retain a significant amount of rainfall. In this
study a probable future development of Aspen was assumed in order to arrive at
realistic future hydrologic conditions.
The imperviousness of a basin is determined by field inspection, study of
aerial photos, and determination of probable future land use. A review of
_Asoen Mountain revealed that because of the steepness and trails out through the
woods, coupled with roadways for access, the effective imperviousness would be
approximately ten percent.
Table IV-2 presents the estimated future effective imperviousness of each
of the categories of drainage basin.
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4
`~°
~'
I
iv-3
8
s
r5
s 4
} IOJ Y
3
2
S war
2 Ysar
I
0
0 10 20 30 40 50 60
~,~ -;_,:~.i~;9,,_~~,.~ TIME IN MINUTES - -
~ze~,„~( ~3~c,.a ~ec _~;p TIME-INTENSITY-FREQUENCY CURVES
1 ASPEN, COLORADO, T.IOS., R.83W.
Wg1ONT- M~LALIOMLIN ^NOINttgs
~4Y0 ALCOTT ST. DENVEq, COLO. X0219
I z
iqur -
IV-4
l~ TABLE IV-1
ASPEN DESIGN RAINFALL
1 Y.
Frequency
Period
(min) 2-Year 5-year 100-year
(in) (in) (in)
0 .00 .00 .00
to .03 .05 .l0
20 .06 .14 .18
30 .32 .48 .98
40 .10 .10 ,29
50 .05 .06 .12
60 .03 .05 p7 '
70 .oz .04 ,05
80 .02 .04 p5
90 .02 .04 .05
l00 .02 .04 .05
110 .02 .0 3 .05
120 .02 .03 .04
130 .Ol .03 .03
140 .Ol .C3 .03
150 .ol .03 .03
160 .01 .03 ,p3
170 .Ol .02 .03
180 .01 .02 ,03
Total Rainfall 0.77 1,26 2 21
T
I1
`1
I
Iv-s
TABLE IV-2
ASPEN DRAINAGE BASIN IMPERVIOUS CHARACTER
Basins Percent of Imperviousness
lA, 3A, SA ( Mc.~.:~-t,res~ 10
5C, 6B, 6D, 7A, 8A, 9A, 96, l0A ~ ~cw ~le,,;.~r~ 20
18, 5B, 6A, 6C ~ l..~hly da.rl.:~k.l «•..+z.~h.-1~ 40
2A, 3B, 4A L~-,•~.•ti2,~~1~
6D
DESIGN RUNOFF
r: ~cw
Eighteen hydrological desig_n points were selected•for this project.
However, the runoff through any basin is also a function of the runoff entering
from upstream basins. Also,desired goals or planning constraints may call for
diversion of flows in the upper basin to other basins resulting in a variety
of flow possibilities.
The initial and major drainage in their present flow patterns from various
basins are shown on Drawing No, 1 and are presented in Table IV-3. The Urban
Storm Drainage Criteria Manual recommends the initial drainage design storm to
be the two-year for residential areas and the five-year for commercial areas.
Note, however, that a two-year flow is given for all urban basins for treat-
ment data relationships.
Determination of Design Flows
1
The Colorado Urban Hydrograph Procedure was used in determining the mag-
nitudes of the 2, 5, and 100-year flood at: the e~hteen design points. This
procedure is based on synthetic unit hydrograph theory.
The unit hydrograph principle was originally developed by Sherman in 1932.
The synthetic unit hydrograph, whichis used for analysis when [here is no
rainfall-runoff data for the basin under study, as is the case in Aspen, was
developed by Snyder in 193$•
Selected examples of the unit hydrograph printouts are presented in
Tables IV-4 through IV-6 at the end of this Section of [he report.
A_unit hydrograph is defined as the hydrograph of one inch of direct
runoff from the tributary area resulting from a unit storm. A unit storm _is 'A
a rainfall of such duration that the period of surface runoff is not appre-
ciably less for any rain of shorter duration. The unit hvdrograph thus
represents the integrated effects of factors such as tributary area- shape,
street pattern, channel__capacities, and stream and land sloes.
~• r ..
_ - z ~ _ ~.~., u,~
TABLE IV-3 - - - _,. - --,,,1
RAINFALL RUNOFF PEAK FLOWS
2-Year Frequency 5-Year Frequency 100-Year Fre quency
Pe ak Flow Volume Peak Flow Volume Peak Flow Volume
Basins Cu.Ft./Sec. Ac.-Ft. Cu. Ft./Sec. Ac-Ft. Cu.Ft./Sec. Ac.-Ft.
lA 22 3.1 302 27.4
~IB 12~ 1.4 91 7.6
lA, 18 30 4.5 351 35.0
2A C~;.,~.e.<,;.( 14 0.9 38 ~ 1 .8 93 4.0
3A 12 1.1 176 9.8
36 w..,,»F.~::.I 16 1 . 1 43 2.2 105 4.9
3A, 36 25 2.2 221 14.3
4A ~ _..,,„t,z,;.1 15 1 .0 42 ~ 2.0 102 4.4
2A,3A,3B,4A 45 4.0 123'_ 6.0 295 22.8
5A 11 0.7 164 5.7
56 14 1.6 102 8,7
5A, 56 24 2.3 254 14.4
2A,3A,36,4A, 61 6.3 558 37.8
SA,SB,SC
16.2A,3A,36, 73 7.8~'°-~ 648 45.4
4A,5A,56,5c
6A 11 1.1 78 6.1
6B 6 0.7 79 6.4
6C 7 0.6 48
3.5
60 5 0.5
59
4.8
6A, 6C 17 1.7 126 9.6
7A 15 l.7 188 15.4
8A 13 1.4 157 12.8
9A 31 3.5 385 3t.4
96 17 1.9 209 17.1
IoA 16 1.8 204 16.7
Without 3A '~* 6.0 A.F. without 3A, SA
~. _ _ ;~
Iv-~
To apply the unit hydrograph, the effective precipitation depth for
the "unit storm" periods are multiplied by the ordinates of the unit
hydrograph and added to obtain a design storm runoff.
The basic premise of the unit hydrograph is that individual hydro-
graphs resulting from the successive increments of rainfall excess that
occur throughout a storm period will be proportional in discharge throughout
their length, and that when properly arranged with respect to time, the
ordinates of the individual unitgraphs can be added to give ordinates
representing the total storm discharge. The hydrograph of total storm
discharge is obtained by summing the ordinates of the individual hydrographs.
The derivation and application of the unit hydrograph are based on
[he following assumptions. --
1. The rainfall intensity is constant during the storm that produces
the unit hydrograph.
2. The rainfall is uniformly distributed throw hout the whole area of the
drainage basin. The small basins in Aspen make this a valid assumption.
3• The base or time duration of the design runoff due to an effective
rainfall of unit duration is constant.
4. The ordinates of the design runoff with a common .base time are directly
proportional to the total amount of direct runoff represented by each
hydrograph.
5• The effects of all physical characteristics of a given drainage basin,
including shape, slope, detention, infiltration, drainage pattern,
channel storage, etc., are reflected in the shape of the unit hydro-
graph for that basin.
Equations
There are two basic equations used in defining the limits of the syn-
thetic hydrograph. The first equation defines the lag time of the basin in
terms of time to peak, tp, which, for the Colorado Urban hydrograph Pro-
cedure, is defined as the time from the center of the unit storm duration
to the peak of the unit hydrograph. This study used a ten-minute unit
storm duration.
tp = Ct (L Lca)•3
where t = time to peak of hydrograph from midpoint of unit rainfall
p in hours
L = length along stream from study point to upstream limits of
the basin in miles
Lca = distance from study point along stream to the centroid of
the basin in miles
Ct = a coefficient related to time to peak
IV-8
The second equation defines the unit peak of the unit hydrograph
C 640
cl -~_
p tp
whet ~qn = peak rate of runoff in cfs per square mile
Cp = a coefficient related to the peak of the unit hydro-
graph.
The C and C values are empirical and should be based on similar
basins tha~ have bteen instrumented to obtain the actual rainfall-runoff
relationships. The C and C values used in this study are based on
values recommended in Table -1 o the Urban Storm Draina a Criteria Manual.
When selecting the Cp and Ct values for the various basins, the area, shape,
and slope were considered along with probable future conditions such as
type of development and percent of impervious area. The urban Ct values
ranged from .25 to .55, and the urban Cp value ranged from .35 to .60.
The process of computing excess precipitation, and then the unit hydro-
graph, was followed by multiplying the two to obtain the design storm
hydrograph. This is lengthy and time consuming if done by hand. For this
reason, a computer was used. The data fed into the computer for each
basin consisted of: the area, the basin length parameters, Cp, Ct, percent
pervious area, and the precipitation infiltration along with the design
retentions for the pervious and impervious areas.
A11 of the unit hydrograph and storm hydrographs that were obtained
from the computer were plotted and studied for reasonableness. Checks
for peak flow per unit area and the widths at 50 and 75 percent of the
peak flow were made on each unit hydrograph that was used.
To obtain most of the design flow hydrographs, two or more of the
"computer hydrographs" were added graphically, This was necessary,
especially on basins tributary to Aspen Mountain, because the upper por-
tions of the larger basins have different hydrologic characteristics than
the lower portions of the basins, thus making it necessary to compute the
different hydrologic regions separately. When adding hydrographs from
different basins together, such as where Basin 3A enters Aspen in Basin 36,
the absolute time of the hydrograph had to be determined and the addition
lagged appropriately since the peak flow from each basin would not reach
the confluence at the same time.
SNOWMELT HYDROLOGY
Normally snowmelt runoff is not considered in urban storm drainage
studies; however, it must be considered here because the heaviest concen-
tration of pollutants occurs from this runoff. Three methods have been
use to etermine the monthly averdge runoff flows and volumes, and an
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IV-9
rasl_E I v-~
COLORADO URBAN HYDROGRAPH PROCEDURE STORM RUNOFF HYDROGRA~H
AREA 38 2 YEAR STORM
TIME ~RECIP INFIL RETENTION EXCESS HNpT
Y STORM
N A ES
C Z NYO
0
-
0.00
0.00
0
01
~ o.oo
o.oo
o.oo -LCE6L
o.oe
20 0.06 '
0
02 --- - 0.02 0.00...._..- 120.68- --~
p
-
30 0.32 ,
0,04 0.04
O.1S 0.00
0.13 47124
27
30 0 O
p
•
i
-- --40-- -- 0.10... . 0.04...
0.00
0.06 .
18
50 ~,~:K~2,,.,
1S 6
~ ~ h ~`~
50
60 0.05
0,03 0.02
0
01 0.00 0.03 __.-_
.
13. 8
6 __
~~-~"'"" 3a
9~7t
---- 7Il - ---_0.02 .
0.01_ 0.00
0.00 0.02
0
01 4
6.8 7,3~
80
90 0 02
0,02
0,01
0
01
0,00 ,
_.
0,01 0.00--
0.00 - SrL~---
4,49
--14D
-
II'~2--
.
---O,DL- ~
0 00
-- -0.OD---
0,01
IlI
O
0.00
~
3.20
!30
!20
0.02
0 02
0.01
0
01
0.00 .
-
- ---
0.01 -0.
0.00 2.71
2.Sf
- -LiO --
--X41-_- ,
--0, 0.0.. 0.00
- . - 0.00- 0.01
01
0 0.00 ~~5~
140
0.01
0.00 .- _
0.00 _._ _
.
0
01 - D.O~ ~_e"
150
-1
~
0.01
01
~
0.00
0.00 .
0.01 0.00
0.00
1,6p
1.4a
-
--
170 a
__.
-
0,01 _- 0.00_
0
00 0.00 0,01- 0.00--- --1`38
180 0.01 .
'0.00 0 00
0.00 0 01
001 1.30
_..190- - 0.01-- 0.00 0 DO 0~
.ol 0.00
o
oD 1.26
'
zoo o:oo o.oo
0.00
0.00 .
---
0.00 ~~?o=--
0_.61•
TOTAL
• PRINT O.TB
OUT STOPS 0.24 0.20 0,34 2
WHEN STORM HYpRpGRAPH DWOPS 34. 2-
2 __ _
7d~9~
AREA 0.06 Sp.MI. CT
0.25 BELOW 1 CFS.
SLOPE 0.020 00 CP Oa~S
L~BASIN 0.61 M
lLES P RVIOUS
E 0.40
L•CA
__ynLllME 0,30
Si-RM RUN MILES
OFF_= IMPERVIOUS
1
088
R 0.60
!`•
`~
'` ` ' K
~.'° ___
TP =
OPK ^ 0.150
120.675 HOURS
CFS .
AC
SpP = !91 iEET
5.481 CFS/Sp,pl~ - ~~-
Pp~T =
0.200
TN SP ^ 0.387 CFS/ACRE
ILPC ^
10,000
; TRFT_^ 0 2Q0 tN.
a_
~~~~~p. ~~~,~)
L ~~^ c -
-' ,-" ~ r -~ ~. L ~- -
/,' _ ~
' TABLE IV-5
C
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IV-10
COLORADO URBAN HYDROGRAPH PROCEDURE STORK RUNOFF NYDtOGRA~N
AREA 3B 5 YEAR STORK
PRECIP UNIT STORM
TIME PRECIP INFIL RETENTION EXCESS NYD NOD
(MiN) (iN) CIN) (iN) (IN) (CFS) («81-_
0 0.00 0.00 0.00 0.00 0.00 000
t0 0.05 0•~ ___D.D~ --D.IlO --120-68 0..0.0
20 0.14 0.03 0.1! 0.00 47.24 0.00
30 0.48 0.06 0.06 0.36 27.30 43.1! yr,k
30- - 0.10 _ _004 __-- 0.00 ---_._ 0.06_ .18.50 ~ 3.
50 0.06 0.03 0.00 0.03 13.68 16.61
60 0.05 0.02 0.00 0.03 6.84 13.10
_~~_ __.0.04___. _ 0.02 0.00 _0,-02 0.00._ l0~lit
80 0,04 0.02 0.00 0.02 0.00 8.26
90 0.04 0.02 0.00 0.02 0.00 5.58
__ _ioo___ _._D.oa.. 0.02 0.00 0.02 0.00.--. --3.21
110 0.03 0.01 0.00
~ 0.02 0.00 4,44
120 0.03 0.01 0.00 0.02 0.00 4.15
--- 130.._-_ -_ 0.03 _ 0,01 0,00 0.02- p~00__ _ _ n~_ol_
140 0.03 0.01 0.00 0.02 0.00 3.91
150 0.03 0.01 0.00 0.02 0.00 3.8~
--- -1-60- - --0,03_ _- _-_ O.D1 _ 0.04 0.02 O+OD - __3.79
1T0 0.02 0.01 0.00 0,01 0.00 3.14
180 0.02 0.01 0,00 0.01 0.00 2.89
---- -1-90- -._ -_ Da02- __ __.D•O1 0.00 0.01_-
0.OD--
- 2.74
200 0.00 0.00 0.00 0 00 0.00 1.34
210 0.00 0.00 0.00. 0.00 0.00 0.75*
-- -- - -----
. - ---
TOTAL 1.28 0.38 0.20 0.70 234,22 162.8
• PRINT OUT STUPS NHEN STORK HYDR OGRAPH DR OPS gEL01( 1 CFS•
- AREA. 0.06 _ SO.NI. CL _- 0,.25-
SLOPE 0.020 0p CP 0.45
L•BAbIN 0.61 MILES PERVIOUS 0.40
_1•CA 0.30 MILES INPERVIO pS_Oa60
VOLUME STORM RUN OFF = 2.248 ACRE•FEET
TP = 0.150 HOURS SOP = 1915.481 CFS/SO,NI•
~Pli- _ __ ..-_120z6j5 -~F_~_ _.SP_ _ _= 1.07 LFS/ACRE
PRET = 0.200 IN• IRET ^ 0.200 INS
ILPC ^ 10.000 Y
iv-11
TABLE IV-6
COLORADO URBAN HYDROGRAPy PROCEDURE STORM RUNOFF HYDROGRA'N
AREA ~B 100 YEAR STORM
TINE PRECIP PRECIP UNIT STORN
INFIL RETENTION EXCESS HYD HYD
rMrN) [iN) (IN) riN) rIN) __ (CFS) rec51_
0 0.00 0.00 0.00 0.00 0.00 0.00
>!o n.1~ _0a4,} ~.D7__ ~.00_. __--1.20,48 e~0n
20 0.18 0.04 0,10. 0.04 47.24 521
30 0.98 0.09 0.03 0.86 27.30 705.44: ~;k~k
a0 029 9._Q~ _ _...0.00...... 0.24 ---_1fl.50 Y0.71
50 0.12 0.04 0.00 0.09 13.68 43.31
60 0.07 0.03 0.00 0.04 6.84 31.3•
re __ .__0..0~_ ___D.D2._ _. - 0.00 0..03. .. O.DD-- 23.7!
80 0.05 0,02 0.00 0.03 0.00 16.21
90 0.05 0,02 0.00 0,03 0.00 8,72
~00_ ___0~-0.5- ---4.02 ---- O.oo -0.03.. _ .O.DO_ __ ~re4
110 0.05 0,02 0.00 0,03 Oo00 6,40
120 0.04 0.02 0.00 0.02 0.00 5.61
--134--- -9~~-- -0•.01 -- -0.00 - .__0.02- --- D.DO- a•7f
140 0.03 0.01 0.00 0.02 0.00 q•36
!50 0.03 0.01 0.00 0.02 0.00 4,11
-----16D---- ~0~ 0.,.0_L- -- --_O.OD - -- - 0.02_ _ ._ O~AD_ _ joop
170 0.03 0.01 0.00 Oa02 0.00 3.93
180 0.03 0.01 0.00 0.02 0.00 3.79
---140--- -0~Q3-- ._0.01_ 0.00 -- 0.02._ .. - Oa00-- --.3.79 -
200 0.00 0.00 o.od o.oo o.o0 1,a~
210 0.00 0.00 0,00 0 00 0.00 1,0~
_-~29 _-0.`00 __ ---0,00.. 0,00 0
00 0
DO D
6
_ _ ._ .
._ .__
•
__ __
;
1!
TOTAL 2.24 0.51 0.20 1.53 234,22 358.21'
____+..PRINT DUI_STOPS MHEN S TORM HYOROGRAPy Dk DPS gELOM ~CFS,
AREA 0.06 SC•MI. CT 0.25
SLOPE 0.020 00 CP 0.45
-1~HASIH _ -_01.6.1. MILES- PERVIOUS. n.ae
L•CA 0.30 MILES IMPERVIODS 0.60
VOLUME STORM RUN OFF = 4.935 ACRE'FEET
TP _- ~1150 __IiIIURS__ _SaP_ 1915.481_ CES/So~M2•
OPK = 12.675 CFS Sp = 2.616 CFS/ACRE
PRET s 0.200 IN• IRET = 0.200 IN•
I IV-12
' attempt has been made to establish a limit on the maximum flow and daily
volume. Of the three methods used to establish an approximate evalua-
tion of7t~ie above items, the most applicable proved to be the Degree Day
I Method.
Degree Day Method:
' This method is a simple procedure using daily mean temperature records.
A Degree Day is defined as a departure of 1° in mean Gaily temperature above
32°F. The number of degree days is multiplied by a degree day factor to
result in the number of inches of melted water runoff. This factor is
dependent upon humidity, wind, solar radiation and nprmally ranges between
0.05 and 0.15 inches per degree day.. A value of 0.1 was used in Aspen
because o compensating factors of moderate and low humidity against high
solar radiation. The historical temperature records taken in Aspen from
1944 to 1954 were used to evaluate the monthly degree days as follows:
I December January February March
Degree Days 11.2 8.8 8.2 44.8
Snowpack ;
.Examination of recorded snowpack accumulations on a day to day basis
provides basic data on potential monthly runoff volumes and peak daily flow.
A general water content of IO percent was assumed for this study. Records
of snowpack for Aspen are taken at the bottom snow gage of Aspen Mountain.
Runoff Sampling Analysis:
During [he winter of 1972-1973, winter water quality sampling of
snowmelt runoff was performed. Observations were made of approximate
flows from various tributary areas as the samples were taken. These values
were compared to snowpack decrease on corresponding days.
Results:
The actual estimate of runoff for the snowmelt period is best given
by the egree ay met o owever, one must a ante t e snowme t capacity
against the actual precipitation and accumulated snowpack. Also consider-
ation must be given to [he fact [hat snow sublimates (evaporation from a
solid to gaseous state) and loses nioisturc, requiring a readjustment of
[he precipitation record. About 25 percent would be lost tq sublimation -
during [he winter months, or about one-half inch7per month.' The final
result is then adjusted or Marc an pri ecause some snow is still
likely to be on the ground in the City in the early part of April.
Table IV-7 illustrates the estimated runoff occuring from snowmelt.
~k equivalent snowmelt water runoff
Iv-13
TABLE IV-7
ESTIMATED SNOWMELT RUNOFF (Inches of Water per Month)
Nov. Dec. Jan. Feb. March APril
Precipitation as 1.3 1.52 1.77 2.01 1.80 1 69 '
Snow
Snowmelt 1.3 1.12 0.88 0.82 2.65 2.01
The relationship of Snowmelt to snowpack and temperature was reviewed,
and based upon the appropriate records it has been determined that there
is a one percent probability of Snowmelt runoff occurring from the City
basins equal to or exceeding the values given in Table IV-8.
Table IV-8 illustrates that daily Snowmelt runoff peaks will not
5ontrol the capacity or sizing of pipelines and treatment facilities for y
runoff. The storm runoff peaks override the importance of the Snowmelt
in this regard.
Snowmelt is important on a monthly basis because of the storage
needed in the.avent of the use of land treatment. For conventional sewage 1;
plan[ treatment of polluted Snowmelt, the peak sanitary flows occur in the
winter, and thus the average Snowmelt volumes and rates of flow on a daily
basis are critical to insure hydrograph compatibility and short term
storage needed.
TABLE IV-8
MAXIMUM DAILY PEAK SNOWMELT FLOWS AND VOLUMES
(One Percent Probability)
Flow Volume
Basin Category (cfs) (Acre-Feet)
East 3 I
(IB)
Central 9 3
(2A, 38. 4A. SB)
West 6 2
(6A, 6C)
Monthly Total Runoff Hydrology
The previous hydrological data for storm and Snowmelt runoff were
synthesized to determine the monthly runoff rates of flow and volumes for
the City basins. This establishes the volumes requiring treatment. Note
that the volume units in Ta le IV-9 are changed to million gallons (MG)
and million gallons per day (MGD) from terms of acre-feet and cubic feet
per second.
,.
IV-14
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IV-15
The monthly volumes of runoff given in Table IV-9 were based on
average precipitation for Aspen coupled with the amount of imperviousness
in terms of acres in the urbanized basins, less five percent. The losses
due [b evaporation and sublimation, and minor events creating no runoff,
are compensated by street flushing water and lawn irrigation spillover
to gutters.
Mainstream Hydrology
The final receiving water in the Aspen area is the Roaring Fork
River. To evaluate impact on the stream, the flow character of this river
is of importance to the hydrologist and sanitary engineers.
The basic information for the streamflow hydrology was taken
from Water Supply Paper 1683 which is Part 9 in a series of reports pub-
lished by the U.S. Geological Survey, Composite frequency-magnitude curves
were used as a basis for developing the relation between the mean annual
flood (or about the 2.3-year frequency flood) and the 100-year frequency
flood. A verification of the extrapolation of the USGS graphs to the
100-year frequency was done by examining the flow records of the streams
of concern.
The mean annual flood was derived from a graph of tributary area
versus mean annual flood for the mean altitude of the tributary area.
The particular graph(s) used depended on the type of hydrologic area; and
as mentioned in Section III, Maroon and Castle Creek are simil~r and
desginated in hydrologic area A6 by the USGS and use Figure 8. Roaring
Fork and Hunter Creek Basins are designated in hydrologic area A5, and
use Figure 7.
A special consideration that had to be evaluated was the cumulative
100-year frequency flood; that is, the 100-year frequency flood from the
total basin will not be the simple addition of the 100-year floods in each
individual stream basin. Basically the flows were proportioned according
to the total area and mean elevation in-each hydrologic area. These
flows were then c$mpared to the U.S. Corps of Engineers Flood Plain
Information Study for Aspen, Colorado and found to basically agree.
Table IV-10, presents the 100 year flood (Intermediate Regional) and
the Standard Project Flood which is the largest presumable flow as
determined by the Corps.
Also the average monthly streamflow must be determined to relate
the pollutant load to the receiving body. The various data were taken
from Compilations o Recor s o ur ace ater of The United States
for 1950-1960 and "Part I, Surface Water Records" of Water Resource Data
for Colorado, for the years 1961 to 1971, which present average stream-
flows on Hunter Creek and the Roaring Fork with some background on
low and high flows. Table IV-II presents one of the tabulations
for the Roaring Fork. Table IV-12 represents a tabulation of Average
Monthly Flows for each of thel~a;q tributaries with an indication
of the lowest recorded flow. '
IV-16 i
TABLE IV-10
itc7~.
PEAK FLOWS FOR INTERMEDIATE REGIONAL AND
STANDARD PROJECT FLOODS (9)
Intermediate
Location Reg_(onal Flood
c.f.s.
Roaring Fork River
upstream llmlt of study 3,300
Roartng Fork Rlver
below Hunter Creek 4,600
Roartng Fork River
below Castle Creek 5,700
Hunter Creek 1,700
Castle Creek 1,300
Standard
Protect Flood
c.f.s.
4,400
6,200
7,800
2,500
1,900
IV-17
TABL"r' IV-11
ROARING FORK MONTHLY FLO':1 DISTRIBUTIOIJ (10)
(Including Drougth I'erlod of 1954-'S~)
776. Roaring y4rIC at lepen, Colo.
~iut~D~. --IJt 29.11160•, long 108.16 X66•, 1n ue.T, t1d 7. 9.BA Y., m rldlb helot ai
-IIpi9, tMeyu.rGn of ..Sle upltnY f2'4. gWUr Cne{I.
Dntnlxl wa. --109 Ig U.
Reeards Iw 1Lbb.--0etaDlr 1910 CG gepta.Gr 19E1 OctaMr 1921 to JaDt1^Wr 1960. NmNlf,
ear larder for Icu pBrlodl DuE1LMd In Y4p 1717. Rfrord. rtnn NYy II., 1976, .n
e ulvalent to prior ncorda tt dlwroton to hln LIm larval to added W floe Dut au•
ton.
~Bp -Wbrataga rocorde r. Mtu^ of 61go I] 7,88..58 tt BDaw ^e an wa level, dales of
-
7929. Jen, 1, 1911, to PID. EJ, 1915 It aft gage •t brldgl 1 800 tt Wltnu It differ-
ent data.. Ya b. 2., 1915, to 9eDt. 36, 1921
staff 6e6e and dDr . Ye, 1972 to Oct, g,
,
1976, uNr-etgl ncolder, •t bridge N1f I .Ile OoYlutnu •t dlfhnnt data..
van dteenar a.--10 years (1910-21 19J1.60), 157 efa {130,600 Ian-tt pef )earl Snelld-
van on1 Dy tY1n Iabe tu]w t.
{xtttnee.--1910-Y1 1971-60; MuL~a dlecN rge, 2,170 eh Jura 16,
-
~
d 1917 (pga b1 t
a
7.Z
Tt, site In
flatus then Sn uea, fros rlOOdsVke) Ira ntin g cutw eiNMed I
era
1 200 eh; •Snlnuv dally Dr14r to construction Gf Te{n ]a be tunnel, 15 eh July 15, 16,
147.; ^1n1au^ daily •Ince dlw re~on tnrougn hSn Lakes turw 1, 0 ./ eft 9eDt. 2a, !7, t^,
1956.
Raeerke.-dnnsaountlln dive relGn 15 •I b1 upltrou.througn hfn 4
- ba larval W lrtanul
Al r<r Desln slow Rey E. 1575 (ue ales YAsn Sn Ch11 npOrt). Bllrltlon diten dl wfta
aster above Itltlon for {rrlgetion of about 1,000 wro Belo+,
^rtell am Iur31 ran IbaNge, In ueC• Mt Mr eees,l
e.u
V Oet. Ia, p1. Jtn. Ise. rr. Ve. ^er )um Jull aW• MOt. i1r wV
Ilsl ZB.a i1.1 21.0 r0.1 20.L 21 .U 36.L 2l] ]t> lE{ Sf .l l!.] ]l.t
31R 28.0 25.1 21.] 3].0 22.1 le .L 01.1 Z9 C]1 119 W.2 a].1 II!
Ua] ]l.4 251 Z6.Z Z1.0 21.1 22.1 3L .3 Ile afi] YtJ R.B TT .I 4.0
tB% 11.0 28.1 Z].D 21 .a 20.1 IaA L1.4 311 1U] S4.p ]Iq Tl.e IaJ
31% 13.1 Y1.1 9a.B 21.1 le.e 11.E V.I IL' 226 E6i ]1.0 31A tl,0
19L1 11.0 22A XJ.a ]1.] 11.6 18.1 ]].] SZ Z1Z Q. la.l l.It ^l.B
1x51 1.13 ILa ]8.6 1B 1 18.L ]eA 3),L I2L 8]L BYB 1]1 aa.0 le]
1958 ]1.] %.l ]9.0 ]U.1 21.L ZL .L 33.9 UI EEI C6. 32J 1Le 111
1958 28.1 25.1 32.0 20.8 21.3 22.] l0.] laL ltl] t9.a ZO.B Z0.2 e0.0
1x60 11.0 35.2 ]3.] 21.8 20.1 32.C ll3 1B] lLE Wd 11.9 33.6 15.1
bnt11> em per31 ILeN qe, In aen-Mt
Ylv Oet. ^ee. de. JV. bY. Rlr. epr. 41 Jvr JWI IW• YOt. lbe pV
]161 3,1)0 1, U0 ],180 1,ZLC I,1lG 1,3Y. t,1lC I!,]:: 23,C 9. L6 2, i8 M,1W
l aa2 ],tM 1,510 1, E50 I,IZO 1,ilU ],11J ].BY. lI,LV 3e, lEL Y,I1 3.1 2, 6] N,I10
195] l,J90 1, lyl I, 630 1, Itl0 1,194 1,It4 2, ILC a,4Y 28,1W a, 11 3,62: 1,1 aq 390
19% 2.000 I, 110 1, U0 IrJ10 1,124 1, i1G S. ZI4 IO,GU C, 11 1, 1,1 1.t !t,]SD
19% 3,830 1,6]0 1,650 1,]b D]3 l,Oh 3,9W 11, 13,EW 1,1 t.2i w aJ,a fO
31% 01 1,]50 L360 1,130 1,010 1,110 2,160 16,61 16,s60 E,6 1,0]4 Ill U,1]d
I9E1 1E] 1,060 1,110 ],130 916 1, 01L l,9]L 1, ]SO q, X.O 6],190 I,l]L 2. 31],000
19% t, t90 t,OIO 3.100 I,B50 I,IUJ L24J 2,024 21,2Yi 39, 1.'!i I,4W I, UO 3r01 al,1W
1151 1, iBO I,aW 1,]Y 1,280 I,IBO 1,114 3,]Y Il,l Yi Z6 rB ]L 1,2i4 1,25J 1, 2LG 61,e10
11]4 t.690 2,090 S.OIO ],53U 1,194 i, :b4 L1dL 11,3W 21 150 6 !CO 1 220 1Q @ 100
Turl1 BLeNge, In euBie fat pr ewaM
Yebr
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TeV q1 ^Ornu rrtn~ ^mW
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1180 - w.0 H.T10
If 6l Its! lb June M, 1161 a,! 81.1 69,160 tIJ 6f,i10
1167 ilia 1,180 Jum l4 IAi )t ]]t •I, tlO 11] U,IIO
lean 1TU l.vo Jum u, 195] n aT.4 a,s94 ud ae,la
n% lw Sw anT u, nsa I,o 11.1 ax, sLO 16.1 3), IOD
If 60 .119] 656 Jur 9. 1116 •.t 80,0 U,1N at.a l0,ftd
Lw ]113 RI Jum 3, 1968 e0.1 ll,eb %.a al, tOD
1961 lbl5 1,910 Jum [9, 1961 IU 1]1.000 111 l]6, 140
195e 1D1] 1sW June 1, 1968 1[ Ill BI, WO Ile Br110
1161 loll 1.020 Jum 11, 1189 1.1 10,0 01,910 UJ r,te0
p8p 1711 Is1 Jury [l, leM 9.1 16.1 Bt,IW
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iv-is
SECTION V
SNOWMELT AND STORM RUNOFF POLLUTION
Runoff from urban areas carries with it pollutants picked up from
roof tops, gutters, pavement and other surfaces. Both Snowmelt and
storm runoff represent a flushing action in much the same manner in
which a City public works crew flushes the streets and gutters with
a tank truck.
Runoff flows by gravity to the receiving streams, and it carries
with it the pollutants picked up in the urban areas. These pollutants
are similar in many respects to sanitary sewage, though constituents
vary quite widely, both in respect to time and concentration.
This investigation and report deals with the determination of
pollutant loads in several ways, including:
a. Collecting of field samples for laboratory analyses.
b. Observation of street and gutter accumulations of debris,
oils and common litter.
c. Notes regarding animal population.
d. Review of published data on storm runoff pollutant loads.
It was determined that while the storm runoff pollutant load
from the Aspen study area is not great in terms of the normal flow
of the Roaring Fork River, the storm runoff contributes pollutants
which tend to degrade the water quality of the stream, particularly
during stream low flow periods. Snowmelt runoff is somewhat greater in
the pollutional impact because of longer periods of accumulation between
runoff. Snowmelt runoff in Aspen generally occurs earlier than the
region-wide Snowmelt, and thus it enters the Roaring Fork prior to
the spring rise in river discharge, thus having a greater adverse impact.
Adverse impact is worse with low Roaring Fork dllutlonal flows.
This section of the report presents estimates of runoff pollution
by month in an average year, by annual total, and by individual Snowmelt
and storm runoff events.
URBAN RUNOFF POLLUTION CHARACTERISTICS
Storm runoff and Snowmelt pollution varies widely from community
to community, and with time. Furthermore, little attention has been
paid to runoff pollution because o~ the more serious problems associated
with sanitary sewage and the fact that the latter has not ye[ been brought
under adequate pollution control on a National basis.
V-2
However, to put runoff pollution into perspective, several pollutant
characteristic estimates are presented below.
TABLE V-1
TYPICAL POLLUTANTS TO A RECEIVING STREAM FROM A CITY OF 10,000 PEOPLE
FROM A 0.5-INCH STORM IN ONE HOUR (12)
Pollutant Pollutants in Runoff in One Hour
Settleable and Suspended Solids 56,000 lbs.
Biochemical Oxygen Demand (GODS) 560 lbs.
Chemical Oxygen Demand 1,300 lbs.
Kjeldahl Nitrogen 88 lbs.
Phosphates 44 lbs.
Total Coliform Bacteria 400 x 1010
Heavy metals are other pollutants which are seldom measured, but which
flow into streams and which are then allowed to circulate throughout the
.environment.
The hypothetical city of 10,000 described in Table V-1 would generate
heavy metals in quantities as shown in Tabel V-2.
TABLE V-2
TYPICAL HEAVY METALS FROM A CITY OF 10,000 PEOPLE
FROM A 0.5-INCH STORM IN ONE HOUR 113)
Heavy Metal Pollutant Pollutant in Runoff in One Hour
Zinc 26 lbs.
Copper 8 lbs.
Lead 23 lbs.
Nickel 2 lbs.
Mercury 29 lbs.
Estimates of pollutants in typical separate storm runoff, in terms of
concentration, for a metropolitan area (Cleveland) are presented below.
-"
v-3
TABLE V-3
RUNOFF CHARACTERISTICS OF SEPARATE STORM RUNOFF (14)
(Values in Milligrams per Liter)
Urban Area with Dense Urban Area with
Pollutant 25 Percent Imperviousness 55 Percent Imperviousness
Suspended Solids 300 500
GODS 20 30
COD 150 200
Total Volatile Solids I10 140
Suspended Volatile Solids 80 105
Phosphorous as P 0.7 O.5
Total Nitrogen 3.1 2.2
Chlorides 160 166
The amount of constituen t pollutants in runoff v aries proportionately
with the time interval from the last runoff. This is important in Aspen
where the cold winter weather often precludes- runoff for several months.
A typical relationship of pollutants in runoff versus time is presented
in Figure V-I following.
ASPEN RUNOFF POLLUTANTS
The pollutants contained in snowmelt and storm runoff from the Aspen
study area have been estimated from field testing during the late winter and
early spring, from field observation, and from comparisons made with other
urban areas.
FIELD TESTS
The results from field tests are summarized in Table V-4 of this section
of the report.
Test sampling for BOD performed in February showed concentrations
of 60 and 125 mg 1 for the Center (Garmisch) and Mfll Street storm
V-4
O
v eoD
O COD
O
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S COD's
I,0
O,
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0
~ 0.3
r
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TIME SINCE UST RAINFALL (doyi)
FIGURE V-I Increase of DOD and COD concentrations
in Solids Samples with Increased
Elapsed Time since last Rainfall (12)
-5
v
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V-6
sewers. Using these concentratlons for the entire month runoff results
in a load of 5 to 11 pounds per acre.
May sampling indicated concentrations of 22 and 34 mg/1 for the
same storm sewers, resulting In unit loads of 4 to 6 pounds per acre.
Test sampling for COD showed similar results with COD concentrations
of 370 and 650 mg/1 for Mill and Center (Garmisch) storm sewers
which would result in loads of 33 to 581 pounds per acre for those
months.
The tests for total solids made in February and March of 1973 showed
concentratlons of 2470 and 906 mg/1. These samples were from the
central area storm sewers as presented in Table V-4. The amount of
total solids per acre represented by these concentrations are 240
pounds per acre and 360 pounds per acre.
Average Monthly Pollution
The adverse impact of pollutants on receiving waters can be
measured by both monthly and annual averages, and by those shock loads
which reach the stream during heavy runoff periods.
To compute monthly an~5annual average values a review of data from
a study at Tulsa, Oklahoma was performed, Thls studied measured
In detail, runoff flows and pollutants from 15 basins for over a year.
Each of these basins had a basically homogeneous character and related
certain levels of pollutant loads to land use. The approach taken was
to first identify basins that compared to Aspen basins with respect
to streets, lawns, housing and other factors. To compensate for climato-
logical effects a brief analysis of precipitation to Tulsa was compared to
Aspen precipitation. The final result was a number of equations which
give pollutant loads for a given area of land use and monthly precipl-
tatlon. These equations are given below:
BOD,COD, TS = pollutant (pounds per acre)
P Precipitation
fast Central West Outlying
BOD = 0.830 P + 0.15 = 0.888 P + 1.04 = 0.675 P - 0.057; = 0.3555P-0.080
COD = 6.380 P + 1.186 = 11.2.34 P + 3.133 = 2.137 P + 0.707; = 1.70 P-0.37
TS =19.89 P + 7.93 = 39.22 P + 36,25 = 16.71 P - 8.54 = 19.07 P-4.17
These equations were used to calculate Table V-5 which gives probable
pollutant loads for Aspen. These values are thought to be reliable
because studies have shown pollutants to be highly correlated with .land
use (greater than 95%), and that the comparison of precipitation further
improves the resulting pollutant estimates. The BOD and COD loads during
the colder winter months may be even higher due to the effects of a
generally longer period between runoff events as indicated in Ftgure V-f,
but the extent of which Is difficult to determine because of the direct
effect of cold on the pollution sources,
V-7
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v-8
The mean annual runoff from the Aspen study area represents the
average of the greatest runoff of each year of record ranging from very
small values to the very large ones. Statistically, the mean annual
runoff has a chance of occurring once each 2.3 years from a storm occurrence.
Snowmelt runoff peaks are less Lhan eyulvelent frequency storm runoff
peaks.
In Aspen, it is expected that the mean annual storm would carry the
pollutants into the Roaring Fork River as shown In Table V-6.
TABLE V-6
PROBABLE ASPEN POLLUTANT LOAD TO ROARING FORK RIVER
IN ONE HOUR FROM.RUNOFF RESULTING
FROM A MEAN ANNUAL STORM
HAVING A FREQUENCY OF OCCURRENCE OF ONCE
EACH 2.3 YEARS-,
Pollutant Pollutants in 1 H
our Concentration
Settleable and Suspended Solids 34 000 ibs
, 1562 mg/Liter
GODS 340 ibs 125 mg/Liter
COD 780 ibs 286 mg/Liter
Kjeldahl Nitrog en 53 ibs 19. 4 mg/Liter
Phosphates 26 ibs
956
mg/Liter
Total Coliform Bacteria 240x1010 88x 1010
;Designing treatment for the 2.3-year frequency storm runoff is
expected to Include 80 to 90 percent of those pollutants which
would be treated by designing for the 100-year frequency storm
runoff. Based on 6000 equivalent population, 8 AC-ft runoff.
INSTITUTIONAL REDUCTION OF RUNOFF POLLUTION
A clean and orderly city, not In the process of growth and develop-
ment construction, will generate less pollutants than a typical city.
There are numerous potential steps of an instltulonal/re ulatlve
nature which can be used_to reduce the pollution load carried to
receiving waters. These Include:
a. Upstream detention ponding to reduce flow rate and, hence erosive
nature and carrying capacity. This includes ponding on roofs,
parking lots and in basins.'
V-9
b. Control of land use, requiring planting and elimination of barren
or easily erodible surfaces and special care in construction areas.
c. Limiting of the over-use of fertilizers, and encouraging better
application techniques.
d. A hi h de ree of street cleaning using brush and vacuum trucks.
In addition, r~~ ents can a encouraged to sweep the sidewa s,
curbs, and gutters in front of their residences or places of business
~as is often done in Austrian and Swiss communities.
e, Initiation of viable flood plain regulations which are enforced,
and prohibition of stream bank filling or cutting,
f, Care in use of street sandin materials, elimination of salting
during winter months,
RECEIVING STREAM QUALITY
The Colorado Game, Fish, and Parks Department undg~took field
studies of streams in the Aspen area in 1967 and 1968. The intent
of the work was to determine whether or not the Milling Area on Castle
Creek was having an adverse impact on the high quality of fishing
streams of the Roaring Fork basin, and to gather basic data regarding
water quality to establish bench mark conditions for future studies.
The results of the studies are presented in Figures V-2 through V-5.
The locations of the measuring points are as follows:
2
3
4
5
8
9.
Roaring Fork just above Difficult Campground.
Roaring Fork just above Highway 82 Bridge on the east side of town.
Roaring Fork just below the Aspen Sewage Plant.
Roaring Fork at the confluence with Hunter Creek.
Castle Creek at Highway 82 Bridge.
Maroon Creek at Highway 82 Bridge.
Snowmass Creek at Highway 82 Bridge.
r
V-10
LOCATION OF iMEASURING PpINTg
1. DiffauM CamP9-and, above Roaring Fork RIVM
2. Above EaN Hghiray 82 bridy~ over Roarirq Fak River
3. Roaring Fark River b~bw A~ Sewage PIarM
4. Roaring Fak River and Huller Creek oanfluence
Q Cagle Creek of Hiphwgy 82 Midq~
8. Maroon Cook of Hphway 82 bridge
9. Snownase Creek at Highway 82 bridge
60
60
50
50
40
40 ~
30 30
20
1234589 1234589 1234589 1234589 0
~D-eq eumnr fall winMr
TEMPERATURE
WRIOMT- M~LAUONLIN ENOINEEgs
iZ4E0 ALCOTT ET. DENVEq, COLO. BOp11
V-12
400
400 'I
350 350
300 300
250 250
200 ~ 200
f
S
150 S 150
100 100
~ 50
0 0
1234589 1.234588 1234589 1234589
spring summa fall winter
DISSOLVED SOLIDS
WAIOMT-M~LAU0~ILIN ENOINEEA9
84p0 ALCOTT ST. DENVEFI, COLO. 808'1
7
.7
.6
.6
c
c
.5
.5
t
.4 0
a
4
.3
.3
.2
.I .2
1
1234589 1234589 1234589 1234589
spring summer fall winter
CHLORIDE
Wg10FIT-MCLAUONLIN ENOINEEgB
24E0 ALCOTT ST. DENVER, COLO. B021'i
SECTION VI
DESIGN CRITERIA
The planning and design of urban drainage systems must be
based on well conceived criteria consistent with the project objectives.
In this section of the report the planning and design criteria for
the hydrologic/hydraulic aspetts of the system are presented, along
with the pollution abatement portions of the plan.
HYDROLOGIC/HYDRAULIC CRITERIA
Major drainage is the cornerstone of an urban storm runoff system,
The major drainage system will operate whether or not it has been
planned and designed, and whether or not urban development is wisely
located in respect to it. Thus, the major drainage must be given
high priority when considering drainage improvements.
The major system for the 100-year runoff often includes many
features such as natural and artificial channels, long underground
conduit out a s, streets, property ine drainage easements and
other water carrying routes. It is closely allied to, but separate
from, the initial drainage system consisting of storm sewers, curbs
and gutters, and drainageways, A good major system can reduce or
eliminate the cost of an underground storm sewer system. An ill
conceived major system can make a storm sewer system very costly.
The 2 or 5-year runoff can flow in the major system, but only a portion
of the 100-year runoff will flow in the initial drainage system.
Routing of the outfall is usually a matter of following the
natural valley thalweg. In these cases the routing is a more straight-
forward matter, and essentially it need only be defined by mapping. In
many urbanized areas, however, there is no thalweg, or the thalweg
has been filled and built upon. In these cases it is necessary to
determine many factors before an optimum route can be chosen,
planning study are those contained in the Denver R gjonal
.Governments Urban Storm Drainage Criteria Manual, Volumes
criteria utilized for this storm drainage pilot
o,~ I ~f
and 2.
VI-2
1. The frequency of occurrence of storms and runoff for design purposes
should be:
a. _Initial Svstem Me
once in 2.3 years
b. Major System.
iual occurrence having a frequency of
of occurrence of once in 100 years.
2. Criteria for planning procedures, storm sewers, major drainage,
structures, inlets, storage, irrigation ditches, flood proofing
and runoff computations will be in accordance with the Urban Storm
Draina a Criteria Manual of the Urban Drainage and Flood Control
District an t e Denver Regional Council of Governments.
3. Criteria for snowmelt runoff is based on practices of the U. S. forest
Service and field observations of Aspen snowmelt characteristics.
Specific-planning and design criteria applicable to the Aspen study area
are:
1. Grass Lined Artificial Channels. Design should generally be based on
channels having naturaFwvaterway hydraulic and appearance characteris-
tics. Criteria include:
a. Design velocity between 7 and 8 fps.
b. Design depth of up to 4 feet except where top width to depth ratio
exceeds 30, and then not more than 5 feet.
c. Side slopes generally of 4:1, but not steeper than 3:1.
d. Centerline curvature of twice the top width but_not less than
100 feet unless special hydraulic considerations are taken.
e. Freeboard of from 1 to 2 feet.
f. Hydraulic roughness in accordance with DRCOG criteria.
g. Grass cover of crested wheat grass or equal, or sod in park areas.
h. Use overflow type channel wherever feasible, with center area
carrying approximately the initial runoff.
i. Use trickle channels for low flows with capacities of about one
percent of design flow, but not to exceed 20 cfs. Underdrain
piping may be used for short reaches where conditions dictate.
j. Design top width to range between 10 and 30 times the design depth.
vl-3
2. Natural Channels. Natural waterways to be used wherever feasible.
a. Channel and overbank capacity must be adequate for design runoff.
b._ Design velocities as computed should not generally exceed critical
velocity as indicated by backwater curves computations. Exceptions
permitted in short reaches where variances are reasonable environmentally.
c. The flood plain limits must be defined,
d. Filling of flood plain fringes should not be allowed in order to
maintain channel ~r~.~no ~_-_~:~,._
e. Hydraulic roughness should be representative of reasonably maintained
channel conditions.
f. Drops or check dams should be used to control water surface profile
slope to control erosion after urbanization.
g. Perform backwater curve computations based upon existing conditions
with assumption that flood plain limits will be revised as bridges are
replaced or new roadways are constructed.
3. Riprapped Artificial Channels. Higher velocity artificial channels will be
used with discretion, and only then to accommodate existing development for
minimum reaches. Provide for potential vandalism to riprap.
4. Bridges, New bridge openings will be based u on a roximatel one foot of
backwater effect. About one foot of clearance should be maintained between
the design water surface and t e ow mem er o ri e. ri ges in ity [o
meet City standards for width. County bridges will meet existing roadway
requirements.
5. Box Culverts. Box culverts will be used in lieu of bridges where feasible.
Backwater effect of culverts to be consistent with channel requirements and
adjacent topography. Energy dissipators must be used downstream of culverts
wherever flow is supercritical from channel standpoint.
6. Irri ation Ditches. All irrigation ditches and laterals will be provided for,
though laterals will not be shown on master plans. Use on-grade flumes where
feasible to reduce silt and maintenance problems. Capacity of crossings will
be limited to cause storm runoff in ditches to enter waterways. Irrigation ditch
crossings will be designed to obviate picking up of overflow from waterways.
7• Drops and Checks. Channel drops will be used as needed to control the hydraulic
grade line and velocity to within reasonable slopes and limits. Drops will
be of riprap, or riprap in gabions, be of pleasing appearance, have gravel
filter blankets under the rock to control undercutting, and be properly notched
for the trickle channel. Design must account for potential vandalism. Checks
will be end-protected [o eliminate side cutting.
vl-4
8, Energy Dissipators. These will usually consist of natural rock
impact blocks with suitable riprap or gebJon protected bottom and
side slopes, Design will account for potential vandalism,
9. Flood Plain Definition. Natural waterways haviny adequate capacity
far the design runoff will be flood plain zoned to protect their
capacity from encroachment.
10. Detention Storage. Storage will be utilized wherever practical
•to reduce peak flow rates.
Il, Sedimentation. Collect debris and sediment in suitable basins above
the town. The design criteria anticipate the regular removal of
sediment and debris for use as construction materials.
12. Multiple Use. The master planning of the waterways will be done in
a manner compatible with the following multiple uses and auxiliary
benefits.
a. Greenbelt and urban open space.
b. Controlled groundwater table.
c. Controlled rising groundwater table after urbanization.
d. Reduced street maintenance cost.
e. Reduced street construction costs.
f. Improved movement of traffic.
g. Improved public health environment.
h. Lower cost open 'space and mall areas.
i. lower cost park areas and more recreational opportunities.
j. Improved quality of receiving streams.
POLLUTION ABATEMENT CRITERIA
1, The criteria for pollution abatement will include collecting and
treating ows up to t e pea rate o runo generate by the
mean annua precipitation 2 3-year frequency The snowmelr peak
rate of runoff is not a constraint on a daily ba is, Larger f ows
should be treated where practical.
2. Discharge to the receiving stream will be consistent with the
Water Quality Standards of the State of Colorado for a Class A ~ B
stream as a fined by the water quality standards at the end of this
section, Although these standards were promollgated relative to
treated sanitary sewage effluent, it~is rational to assume that
storm runoff quantities should be at least as good,
3. Effluent standards to be achieved by the I
p an are as follows for
the period through 1983: Note that these are _equal to State Standards
for treated sanitary sewage (although the State Standards do not
specifically apply to SRO .
Date
1973
July 1, 1975
July 1, 1978
SS GODS
30 mg/I 30 mg/1
25 mg/I 25 mg/1
20 mg/1 20 mg/1
Turbidity Color
30 JTU 30 units
25 JTU 25 units
20 JTU 20 units
VI-5
4. Effluent standards to be achieved by 1983 and 1985 are not yet
defined; however, the abbreviated following national goals express
a Federal intent to enforce strict effluent standards, as presented
in PL 92-500.
a. Eliminate discharge of pollutants by 1985.
b. Provide for recreation in and on the water by 1983.
c. Discharge of toxic pollutants in toxic amounts would be prohibited,
5. Excess capacity of biological treatment plants during off-peak
hours would be utilized to no greater a degree than 50 percent (to
allow for operational safety factors).
6. Land treatment application rates would be limited to a rate to
equal annual plant nitrogen uptake Qlus the estimated loss to the
atmosphere ranging from 20 to 50 percent of the total nitrogen
applied,
The "Design Criteria for Sewage Treatment Water Quality Standards"
of the Colorado Water Pollution Control Comm(ssion which presents current
detailed standards follows.
VI-6
DESIGN CRITERIA FOR SEWAGE TREATMENT
WATER QUALITY STANDARDS
(ADOPTED 8Y THE COLC~,gDO STATE WATER POLLUTION CONTROL COMMISSION)
April 13, 1971
BASIC STANDARDS APPLICABLE TO ALL WATERS OF THE STATE:
A. All wastes capable of treatment or control prior to discharge into any
waters of the state, shall receive secondary treatment with disinfection
or its industrial waste equivalent, as determined by the State Water
Pollution Control Commission. Lesser degrees of treatment or control
may be permitted only where it can be demonstrated that the standards
applicable to the classified use of the water can be attained, Greater
degrees of treatment or control will be required where it can be
demonstrated that it is necessary to comply with the standards appli-
cable to the classified use of the water.
B. Free from substances attributable to municipal, domestic, or industrial
wastes, or other controllable sources that will either settle to form
unsightly, putrescent, or odorous bottom deposits, or will interfere
with the classified use of the water.
C. Free from unsightly floating debris, oil, grease, scum, and other
floating material attributable to municipal, domestic, or industrial
wastes, or other controllable sources.
D. Free from materials attributable to municipal, domestic or industrial
wastes, or other controllable sources that will produce objectionable
odor, color, taste, or turbidity in the water, or objectionable aquatic
life wh(ch may result in eutrophication or other conditions that inter-
fere with the classified use of the water.
E. Free from high temperatures, biocides, toxic, or other deleterious sub-
stances attributable to municipal, domestic, or industrial wastes, or
other controllable sources in levels, concentrations, or combinations
sufficient to be harmful to human or animal life,
F. Radioactive materials attributable to municipal, industrial or other
controllable sources will be minimum concentrations that are physically
and economically feasible to achieve. In no case shall such materials
in the stream exceed the limits established in the current edition of
the U. S. Public Health Service Drinking Water Standards or the limits
approved by the Federal Radiation Council, or, in the absence of any
limits specified by the U. 5. Publlc Health Service or the Federal
Radiation Council, I/30 of the 168-hour-week values for other radio-
active substances specified in the National Bureau of Standards Handbook
69. ,
VI-7
II. ADDITIONAL WATER QUALITY STANDARDS FOk HODIES OF WATER THAT HAVE BEEN
CLASSIFIED FOR ANY OF THE FOLLOWING USES:
CLASS A.
The following standards shall apply to water withdrawn for treatment
as a potable supply:
a. Bacteria: Wastes or substances from controllable sources shall
not be discharged into these waters in amounts which will cause
the number of organisms of the fecal coliform group, as determined
by either multiple tube fermentation or membrane filter techniques,
to exceed n log mean of 1000 per 100 milliliters or exceed 2000
per 100 milliliters in more than IO/,, of the samples collected in
any 30 day period.
b. Dissolved Oxvgen: Dissolved oxygen shall not be less than 4
milligrams per liter.
c. pH: The pH shall be maintained between 6.0 and 9.0.
d. Taste and Odor: Free from materials attributable to municipal,
domestic, or industrial wastes, or other controllable sources
that will produce taste or odor in the water.
e. Dissolved Solids: Total dissolved solids, annual volume weighted
average, should be less than 500 milligrams per liter.
f. Selected Chemical Constituents: The following substances shall
not be present in such amounts as to exceed the specified concen-
trations in a potable water supply according to the mandatory
requirements of the latest edition of the U. S. Public Health
Service Drinking Water Standards:
Substance
Concentration - mq/1
Arsenic - - - - - - - - - - - - - - 0.05
Barium - - - - - - - - - - - - - - I.00
Cadmium - - - - - - - - - - - - - - 0.01
Chromium (Hexavalent)- - - - - - - - O,pS
Cyanide _ _ _ _ _ _ _ _ _ _ _ _ _ _ 0.20
Lead - - - - - - - - - - -- - - - - 0.05
Selenium - - - - - - - - - - - - - - 0.01
Silver - - - - - - - - - - - - - - 0.05
CLASS B.
The following standards shall apply to waters classified for fish and
wildlife (Cold Water Fishery);
a, Bacteria; Wastes or substances from controllable sources shall nor
be discharged into these w~l:ers in amounts which will cause the
number of organisms of the fecal coliform group, as determined by
either multiple tube fermentation or membrane filter techniques, t~
exceed a log mean of 1000 per 100 milliliter's or exceed 2000 per 100
milliliters in more than 10'% of the samples collected in any 30 day
period,
r
1
VI-8
b: Dissolved Oxvgen: In cold water fisheries, the dissolved oxygen
content shall in no case go below 6 milligrams per liter.
c• ~: pH shall he maintained between 6.5 and $,5. No controllable pH
change wi}I b~° permitted whir.h will interfere w(th fish and aquatic.
life,
d. Turbidity: No turbidity shall ex L t in concentrations that will
Impair natural and developed fisheries.
e.' Temperature; In cold water fisheries the temperatures shall not
excee-d j0'Yp. No controllable temperature change will be permit[ed-
which will interfere with the spawning and other aspects of fish life.
f. Toxic__ M~terlal: Free from biocides, toxic, or other deleterious
substances attributable to municipal, domestic, or industrial wastes,
or other controllable sources in levels, concentrations, or combinations
sufficient to be harmful to aquatic life.
9. Other_ Materiel: Free from materials attributable to municipal, domestic,
or industrial wastes, or other controllable sources that will produce
off-flavor in the flesh of fish.
2. The following s[anclards shall apply to waters classified for fish and
wildlife (Warm Water Fishery);
a. Bacteria; Wastes ,or substances from controllable sources shall not
be discharged into these waters in amounts which will cause the number
of organisms of the fecal coliform group, as determined by either
multiple tube fermentation or membrane filter techniques, to exceed a
log mean of 1000 per 100 milliliters or exceed 2000 per 100 milliliters
in more than 10'/ of the samples collected in any 30 day period.
b. Dissolved Oxygen: In warm water fisheries, dissolved oxygen content
shall in no case go below 5 milligrams per liter.
c c~han eHwilllbebe maintained between 6.5 and 8,5. No r_ontrollable pH
9 permitted which will interfere with fish and aquatic
life,
d. Turbidity: No turbidity shall exist in concentrations that will impair
natural and developed fisheries.
e. Temperature; In warm water fisheries the temperatures shall not exceed
90° F: No controllable temperature change will be permitted which will
interfere with spawning and other aspects of fish life.
f. Toxic Materiel; Free from biocides, toxic, or other deleterious sub-
stances attributable to municipal, domestic, or industrial wastes, or
other controllable sources in levels, concentrations, or cornbina[ions
sufficient to be harmful to aquatic life,
g. Other Material; Free from materials attributable to municipal, domestic,
or industrial wastes, or other controllable sources that will produce
off-flavor in the flesh of fish.
vl-9
3. The following sta.dards shall apply to recreational waters classified
for body contact sports such as, but not limited to, swimming and
water skiing.
a. Bacteria: Total coliform bacteria shall not exceed 1,000 per
100 milliliters as a monthly average (either MPN or MF count);
nor exceed this nwTher in more than 20% of the samples examined
during any month; nor e;;;eed 2,400 per 100 milliliters in a single
sample. In addition, the fecal coliform count shall not exceed
100 per 100 milliliters, and the fecal streptococcus count shall
not exceed 20 per 100 milliliters, both of these limits to be an
average of five (5) consecutive samples within a month.
b. H~•. pH shall be maintained between 6.5 and 8.5.
Note: Limits on temperature change in fisheries have not been established due
to lack of historicial temperature data and lack of conclusive temperature
change criteria for the aquatic biota of waters of the state, These factual
data are being collected, however, to serve as a basis for setting limits.
In the meantime, the following tentative criteria will be used as administra-
tive policy;
In cold water fisheries an abrupt change in temperature must be
avoided and the normal pattern of diurnal and seasonal fluctuations
must be preserved. The maximum allowable temperature increase due
to waste discharges in streams and in the epi)imnion of lakes shall
be 2oF. No warming waste discharge shall be permitted to the
hypolimnion of lakes.
In warm water fisheries an abrupt change in temperature must he
avoided and the normal pattern of diurnal and seasonal changes must
be preserved. The maximum e311owable temperature increase due to
waste discharges in streams will be 5°F; in the epilimnion of lakes
the maximum increase will be 3°F. No warming waste discharge Shall
be permitted in the hypolimnion of lakes.
In temperature measurement, allowance shall be made for a mixing
zone, provisions shall be made for adequate mixing and no thermal
barrier to migration and free movement of aquatic biota shall be
permitted ,in any waters of the state.
CLASS C.
The following standards shall apply to waters classified for industrial
uses:
a. Dissolved Oxygen: Dissolved oxygen content shall not go below
3 milligrams per liter,
b. ~H: pH shall be maintained between 5.0 and 9.0.
c. Turbidity; No turbidity shall exist in concentrations that will
interfere with established levels of treatment.
d. Temperature; The temperature shall not exceed 90°F.
VI-10
CLASS D.
The following standards shall apply to waters classified for irrigation:
a. Total Dissolved Solids (Salt) Concentration: A time-weighted monthly
mean at a moniCoring station which exceeds the time-weighted monthly
mean for a base perioh established by the Commission by more than two
standard deviations shall be subject to review by the Commission.
b. Sodium Adsorption Ratio: A time-weighted monthly mean at a monitor-
ing station which exceeds the time-weighted monthly mean for a base
period established by the Commission by more than two standard devia-
tions shall be subject to review by the Commission.
c. Toxic Material; Free from biocides, toxic, or other deleterious
substances attributable to municipal, domestic, industrial wastes,
or other controllable sources in concentrations or combinations which
are harmful to crop life.
2. The following standards shall apply to waters classified for livestock
watering:
a. Soluble Salts; The soluble salts shall not exceed 3,000 milligrams
per liter.
SECTION VII
TREATMENT OF URBAN RUNOFF
As developed In Sections V 6 VI, treatment of snowmelt and storm
runoff at Aspen should include the followln !effects:
- - __-g'-_.___.._.
Reductlon of settleable and suspended solids.
Reductlon of floating oils and debris.
Reductlon of organic material (BOD).
No significant increase In temperature.
No significant lncreas,e in total dissolved solids.
Conceptual process designs resulted in several alternate schemes
which could be practicable at Aspen. ° -°~
1, Route the runoff flows to the existing Aspen Sanitation
i District sanitary sewage treatment plant.
2. Route the runoff flows to the existing Metro treatment plant.
3. After plain sedimentation, accomplish treatment thru storage
and then application to land, via the methods of spray
Irrigation, overland flow, or infiltration.
4. Construct separate mechanical type runoff treatment facilities.
ASPEN SANITATION DISTRICT PLANT (ASD)
The older ASD plant is at the foot of Mill Street. It is a
trickling filter type secondary pant, with the recent addition of
an activated sludge basin In series. The plant produces a good quality
secondary effluent when limited to a constant load of about 0,5 million
gallons per day (MGD),
Since present joint system operation results In a constant maximum
load to this plant, there is no convenient opportunity to treat storm
runoff at this time,
The concept of routing all sanitary flows to the new Metro plant
and converting the ASD plant for storm runoff only was evaluated, However
the pl~~,processes do not lend themselves to intermittent storm runoff
treatment. It is concluded that near-fut~u re plans should not utilize
this plant for runo treatment.
The plant site is ideal for the location of a runoff detention pond,
It is later planned that the plant wiil_be phased out as a sanitary
sewage treatment facility (at the end of the useful life of the existing
bfoiogical treatment units), At Ehat time, a detailed evaluation
should be made reconverting parts. of the plant to a chemical-physical
vll-2
runoff treatment plant. The plant would then treat, and discharge
directly to the Roaring Fork, all flows which could not be diverted
to the Metro Plant.
ASPEN METRO PLANT
The Metro Plant is a secondary activated sludge type sanitary
sewage treatment facility built Jointly by the Aspen and Aspen Metro
Sanitation Districts. The original plant had a design capacity_of
0.8 MGD and an ex ansion to 2.0 MGD is now under construction. Ultimately,
the plant is planned to have a EoEal capacity of .0 MGD, or more.
,v~___
Although not an "advanced" treatment plant no nutrieni removal processes),
at design loads, this plant has consistently removed in excess of
95% of the BOD and suspended solids contaminants. No indications of
stream degradation, as a result of sanitation effluent, have been observed
below the Metro plant.
The same processes that threat sanitary sewage are largely applicable
to typical storm runoff flows. It therefore is obvious, that efficiently
economical treatment of storm runoff could be obtained by using
one treatment fagiltty -- particularly if this facility already had unused
excess capacity.
Because of flow pattern and quality characteristics of sanitary
and storm sewage we have developed a concept whereby storm and
sanitary sewage can both be effectively treated at the Metro plant.
Essentially no ad itlona pant capita costs are required, although
goerattonal costs will increase as function of the amount of storm flow
treated. The concept is described following.
Sanitary sewage is putrescisble and decomposes rapidly. It then
cannot be conveniently stored and is treated a[ irregular rates as it
is generated and received at the plant. Thus the Metro plant has a
nominal 2.0 MGD capacity; however, it has a peak hydraulic capacity of
4.0 MGD. Except for biological system limitations, (relating to BOD
load) it could treat 4.0 MGD coming a[ a uniform rate. Storm runoff,
on the other hand with a lower early BOD, does not decompose as rapidly
and does not bear the adverse psychological stigma of sanitary sewage
ponds. It can therefore be economically stored in open ponds and then
created at lower constant rates. Storm runoff rates are .much more variable
than sanitary sewage production rates -- flows vary from zero to one
20 times the average rate. Almost all storm runoff treatment schemes
must incorporate detention storage preceeding the treatment facilities.
The plan lnvolves_providing detention stora a ponds for the design
,storm runoff. The water would then be released to the Metro plant at
low, controlled, rates only at times when sanitary flows were less than.
plant capacity. Sanitary flows vary daily. Theoretically, even during
the peak design day 2.0 MG of storm runoff could be treated. Since flow
rate control cannot be perfect, and to allow for reasonable operational
safety factors, we have assumed that only 50%, or 1 MGD of this unused
sanitary capacity would-be used far storm rnnnff Additional capacity is
available because of the seasonal variation In flows typical of Aspen.
Vil-3
Figure VII-a,b illustrates
capacity concept. Table VII-1
given month.
the variable flow rate - available
gives the available capacity for any
The capacity of the Roaring Fork trunk sewer, and other trunks,
is similar to that of the plant. Line hydraulic capacities were
reviewed and were found not to be limiting. The lower reaches of the
Roaring Fork trunk have a hydraulic capacity of greater than 8.0 MGD;
however, capacity reduces upstream so that storm runoff coming capacity of
each reach must be checked.
The treatment of storm runoff could later effect the design (cost) of
advanced waste treatment facilities when these are later added at the
Metro plant, since flow levelling would be a normally used tool, even
when treating only sanitary sewage.
It can be concluded that all (relatively) highly contaminated storm
runoff which requires BOD removal, can be most economically treated
using the existing available capacity of the sanitary sewer trunks and
Metro treatment plant. Detention storage and contolled-flow outlets must
preceed admission to the sanitary system. It is assumed that suitable
arrangements can be made with the Sanitation Districts to accept the
storm runoff for treatment.
LAND APPLICATION.
Low density areas, such as West Aspen should produce storm runoff
of better quality. Although quality projection is not now an exact
science, it is expected that many qualities in these areas will be
marginal - too contaminated to make direct discharge to streams desirable;
yet of good quality to permit storage of sedimentation followed by land
application (irrigation) to a golf course without secondary treatment.
Direct overflow to streams, after sedimentation, may also be possible
in some cases.
Land treatment involves the application of effluent to the surface
of the ground so that it infiltrates through the soil zone where it is
cleansed.
The sp-ray application rate on golf course land would probabl
approximate one MGD per acre per year. However, own pasture an
approximately two MGD would generally be suitable.
For permeable open space land where conflicting uses were not
a problem, the spray application rate of 3 to 4 MG per acre per year
would typically be used.
In cases where detention storage capacity was limited, the actual
annual application rate might be reduced because of hydraulic storage
and timing constraints.
Removal effeciencies with the land treatment method would be
expected to be In the 99 percent range for all poi}'utants, except
l.o
75
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vii
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UNIT DAILY SEWAGE FLOW CURVE
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May J J A S 0 N D J F M A May
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2420 ALCOTT ST. DENVER, COLO. 80211
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VII-6
chlorides and sulphates, and total dissolved solids. In general, land r
reatment can a consi ered the equivalent of advanced waste treatment
so that no future other treatment requirements would be expected.
-Land application is considered a method where Irrigation of green area
s a slmu taneous raqutrement.
SEPERATE PLANTS
Separate mechanical type plants are obviously costly to
build and operate. Processes used must be physical or chemical as
¢tological processes cannot be efficiently maintained with h ghTy
variable loads.
No separate storm runoff treatment plants are proposed at
this t me. In the future, it is anticipated that It may become
necessary to treat higher flow rates (flows fn excess of the design
storm). In this case it may be practical to construct storm runoff
plants, such as chemical coagulation - sedlmentatton processes, to
treat waters which would otherwise might flow directly untreated
to the streams. Separate, simple plants might also be located below
sedimentation ponds in areas such as West Aspen, where land application
became an impractical alternative.
+' 'i
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SECTION VIII
MASTER PLAN DESCRIPTION
The master plan for urban runoff management, including treatment
for water po ution control measures, is described on the attached
drawings I through 5• Drawing 1 shows the entire study area subdivided
into tributary drainage basins plus mountain tributary areas. Drawings
2 through 5 represent current topographic mapping at a scale of 1 inch
equals 200 feet, and with five-foot contour intervals. These drawings
2 through 5 present the master plan for urban runoff management in
[he Aspen study area. The drawings are laid out to cover the study
area from east to west, with drawing number 2 being [he most southerly
area and also the farthest upstream along the Roaring Fork River.
MOUNTAIN TRIBUTARY BASINS
The master plan for urban runoff management is based upon keeping the
~.__ _ _
rountain runoff separated from the urbanized runof f there ore re-
c u ing the necessity of its treatment.
The drainage fro Basin lA flowing down Spar Gulch is transported
directly to the Roarin~c -Park-River via the Spar Gulch/~itst Aspen flood-
w~. This open channel/greenway would be desioned for the 100-vear
runoff conditions with appropriate freeboard and with emer ency over-
flows situated so that t~-Ioods greater than the 100-year magnitude ~
would follow the historic natural routing into the City.
The mountain tributary basin 3A, which flows in Pioneer Gulch and
Vallejo Gulch, is shown following the historic flood routing with all
criteria based upon the 100-year interval runoff. A detention basin
would be situated at about elevation 025 feet at a location at Mill
Street extended. The detention storage would significantly reduce the
peak flow of the 100-year runoff and collect sediment and debris with
the overflow from [he sedimentation pond being carried into the pro-
posed Mill Street Mall by a narrow decorative structured channel to
eliminate potential problems with erosion. Gabion drops would be con-
sidered satisfactory, though many options are possible. At Dean Avenue
this water would enter the formalized Mall and be carried to the Roaring
Fork River, utilizing the Mill Street Mall as a floodway. Low flows
from Basin 3A would be routed through the Mall to provide a constant
flow of water for decorative and esthetic purposes. In addition, the low
flow from the Durant Mine would be transported to the corner of Durant
Avenue and Galena Street where it would enter the Mall/park area for
decorative purposes. Static pressure of the Durant Mine water would
provide opportunities for fountains and/or water falls as might be
r,,'
I;
desired by the planners. Figure VIII-I represents these concepts schematically.
That runoff tributary to Aspen from Basin 5A will be stored and
allowed to infiltrate under controlled conditions for aquifer recharge
purposes as is now the case to a limited extent.
viii e
I
MILL STREET ABOVE MALL I
;;`
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Curbing to pre- ~, y~~ Mall to carry regula ted flows from
vent polluted ,rr `• ~~1, Mountain, Durant ine, and in- ~
QZ 3 runoff ~~,~~ ~\ ~,I,, frequent major flows.
from Ma 1 1 ~ s-e ~'.
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Mall major drainage for
~: Polluted low flows~(Q~) Q100, with decorative
[o storm sewer trickle channel for its
~ esthetic flow.
Storm sewer for ~''
~ _ ~ local drainage ~
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MALL FOR MILL OR GALENA STREETS ~,;.
Figure VIII-1
WRIGHT-McLAUGNLIN ENGINEERS
masonry walled channel s[reet+inflow~~~'~r-""-
for low flows and --'
modified Q100
VIII-3
DENSE URBAN RUNOFF
The plan for managing the runoff from the denser urban areas is
illustrated on sheets 2 and 3• A minlnwm of nrw :corm sewers I, Indi-
cated. This tributary area Is rrpre~~cntcd by Basins IB, 2, 3B, 4 and
5B. This portion of [he master plan is based upon collecting and
treating at least the mean annual runoff peak which has a frequency
of occu~ rrence of approximately once each 2.3 years. In some instances
where Ghe storm sewers are adequately sized or where water can be ade-
quately transported via street flow, larger peak runoffs would be
collected and treated up to the capacity of the detention/sedimentation
basins as shown on the drawings.
New storm sewers on East Hopkins Avenue and East Cooper Avenue i
provide interception of flows for connection to the exrstrng Orrgrnal Street
36-inch storm sewer. The discharoe to the Roaring Fork River at Original
and Main Streets would be abandoned and all runoff would be transported
[o the Aspen storm runoff storage fonds in the vicinity of the old '
a
trickling filter plan[.
%pyWW ~.N .''
Situated under the Mill Street Mall is an extension of the -inch
Mill Street storm sewer, which would carry polluted urban runoff under
the Mall, keeping it separated from the clean mountain runoff which wool-
\ be used for decorative purposes on the surface. While the surface
>fl_ws are discharged directly to the Roaring Fork without treatment, all
runoff up to the capacity of the Mill Street storm sewer would be sto`re3
and 'then dischafged to the Roaring Fork outfall line of the Aspen Metro-
politan Sanitation District.
Wagner Park would be synergistically used to temporarily detain
storm runoff which would occur on an infrequent basis at times when the
Mill Street major drainageway reached its capacity. This combined use
of Wagner Park would not interfere with its primary function.
Paepcke Park, situated on Center Street, would provide local tem-
porary shallow flood storage capacity and would be utilized only during
those times that the local street and storm sewer system is overtaxed.
WEST ASPEN BASINS
Basins 6A and 6C include fairly low density urban development, and
runoff from these areas would be transported to proposed detention storage
areas situated'on the Aspen Institute land ;urban runoff from these
basins being redirected away from Hallam Lake to eliminate nutrient
contributions which affect eutrophication. If the nutrients contained
in the runoff were later found to be desirable for Hallam Lake, the
runoff could be redirected to the lake. Therefore, a choice exists here.
Runoff from these basins, after detention storage, would be subject to
sedimentation process as with primary treatment and discharge to the
Roaring Fork River. However, a significant opportunity exists for
_ _ _ _ _.
VIII-4
utilization of the runoff for irrigation purposes, and this oppor- ill
tunny should be fully explored by the land o~~mers. It should be
Holed Ilinl the ninoFf front Basin: ~,A and F,f, could als~~be routed to
Ihr Ronl ln~l Ital. rnillnll nl Ilir, A•.I~en M~Irnpnlltan ,anitatlon Dltlrlct,
OUTLYING BASINS. ~'
The outlying basins are shown on Drawings 4 and 5 and here the ~'`
mod e, of collection and treatment varies significantly from the more ~
heavily urbanized areas. In these basins covering 66, 6D, 7A and 4
through to Basin 10 the means of collection is via road side drainage ~''
ditches improved irrigation ditches and some new swales. However, in I'=
areas not yet developed it is proposed that the development design be
based on the on-site storage and on-site treatment principle indicating +l:
infiltration of the runoff into the underlying formations via land
spreading. In such areas there will be some runoff during intense
storms, and the plan directs its attention to this runoff with a
centralized system of open channels and detention ponds as shown. In
general these ponds would be constructed in an environmentally desireable
manner to provide open space and permanent pools where desirable.
Generally the detained water would be treated by spray irrigation, over-
land runoff or by rapid infiltration. The Aspen golf course provides
an excellent centrally located open space area for treatment of the
runoff using the existing irrigation sprinkling system. It should also
be noted that opportunities exist for this entire area runoff to be '.
transmitted to the Aspen Metropolitan Sanitation District plant
utilizing existing and proposed trunk lines of the District.
CAPACITY OF ASPEN METROPOLITAN PLANT
The characteristic sanitary sewage flow daily hydrographs lend
themselves well to excess capacity for treatment of urban runoff.
An investigation of the Aspen Metropolitan plant shows that the future
development runoff as indicated in the master plan can be handled by
utilizing no more than 50 percent of the excess capacity of the present '
plant. With sufficient storage, as previously designed, the average i_
dail flow rate to the Metro Plant from Basins 18, 2A, 3B, 4A and 5B ~`z,
would be approximately 0.2 MGD. If Basins 6A and 6C were added to
the Metro plant, the additional design flow rate to be treated would
be 0.07 MGD. These flows represent daily snowmelt flows which would
basically be routed to the treatment facilities on a daily basis during
the heavy snowmelt periods. However, heavy rainfall runoff flows
would exceed these figures and use available plant capacity, f
An option exists for the utilization of the old trickling filter
lp ant for the treatment of storm runoff; however, it will probably be ',
dazed before the end of the decade and the site converted to an open
space/detention storage basl~or urban runoff prior to its discharge
to the Roaring Fork outfall. It 'should be noted that the xricklinq
filler plant would not lend itself well to runoff treatment because of
the difficulties of maintaining propoer trickling filter conditions
during "drouth" periods. '
VIII-5
Treatment by surface infiltration and application to land for
irrigation purposes is a reasonable opportunity which would provide
a high level of treatment and recharge of ground waters. Many oppor-
tunities exist in the Aspen area for land treatment which fulfills a
recycling objective in both terms of water and nutrients.
L. d, ;'~
AGREEMENT FOR TREATMENT ~~ ~'
'The City of gspen would need to reach an agreement with the Aspen
and Aspen Metropolitan Sanitation District in regard to utilization of
off-peak capacity in the trunk lines as well as the treatment plant
for urban runoff. No new facilities will be required by the Sanitation
District; however, it will be necessary for the City to install metering
and control devices at the point of storm runoff entry and the trunk
lines. The negotiations between the City of Aspen and the Sanitation
Districts could follow any one of a number of potential routes; however,
for the purposes of this study it would be presumed that the Aspen
Metropolitan Sanitation District would charge a basic stands fee (readi-
ness to serve charge) and a unit rate per thousand gallons treated.
It is believed that the volume of water to be treated would average
about 60 million allons per year, assuming treatment for the ense
urban area only.
Historically, sanitary engineers across the country and in govern-
mental agencies have been working towards separation of storm water
from sanitary sewage. The plan proposed here does not conflict
with this long standing philosophy because the combination would be made
under highly controlled conditions so as not to adversely affect the
treatment plant and to eliminate the usual problem of overflows of
combined sewage. Here overflows of combined sewage could no[ occur.
COSTS AND PHASING
Implementation of the Aspen Urban Runoff Management Plan should
proceed in a phased-project type approach for several reasons.
1. The basic nature of this project is that as a specific improvement
is made in any given area, the storm drainage pollution situation
will improve, the extent of which may alter other improvements.
2. A phased program usually allows more workable cost strategy. Detail
design of important items are available first.
3. The members of the community and other influential bodies are able
participate in subtle details of the program.
4. Resulting construction projects would be of an acceptable size that
would not seriously disrupt the community and local environment.
The intent of the following discussion is to describe the suggested
phasing program and briefly describe why the particular items within
VIII-6
that phase were chosen. The range of alternatives from which this plan
resulted is extensive, and variations can be reviewed with the city as
desired.
Phase I. The analysis made of pollution loads indicates clearly
that the _yast and center portions of Aspen contribute the heaviest
(in .the order of 60% to 70%) of the total load from the study area,
depending on the particular type of pollutant. Thus, this area represents
a logical starting basin. In order to treat this heaviest portion
of storm drainage pollution from the Aspen study area, the various storm
drainage facilities in the east and center portion of Aspen must be
interconnected and Aspen Mountain runoff kept separate from the storm
drainage facilities. °~°'
Separation of the mountain draina e requires construction of three
major draina a floodwa s for Basins lA, 3A and 5A. The first of these,
Spar Gulch/ bst Aspen floodway, was choseh as the best alternate because
it was quite clear that construction of a major drainage outfall to
the Roaring Fork River by any other route would entail the construction
of large and very expensive storm sewers through Aspen streets. The
alternate of large size storm sewers was not chosen because it does not
provide any other benefits to the community. Similarly for the
Pioneer/Vallejo Mall drainageway, large diameter storm sewers
were not chosen because 'o their expense and lack of usefulness to the
community. Rather, a small decorative rock-masonry wall channel was
chosen for Mill Street above the mall because it would allow the
continued use of Mill Street for mornal uses but provide a fairly
inexpensive way of carrying the initial and a greater portion of the
major drainage runoff from Aspen Mountain 3A. The proposed mall along
Mill Street (or Galena Street) provides a relatively inexpensive way
of keeping the local drainage from polluting mountain runoff and providing
a major drainage floodway. If the mall scheme could not be implemented,
a larger sized "hard" lined channel or storm sewer would have to be
provided because of the flatter street grades through town. This might
cost in the order of an additional $60,000. The West Aspen detention
system should be constructed with total detention capacity as indicated;
otherwise, a separate storm sewer would have to be constructed to bypass
runoff from Basin 5A to the Roaring Fork River.
The outfalls of existing storm sewers would be reconstructed to
connect to a system of grass lined surface swales that would inter-
connect to the Aspen storm runoff storage ponds. This sytem of surface
swales provides a more inexpensive system than a 100`'/„ storm sewer
transmission system an a so provides an intercepting barrier frrm Inral
drainage overflows in Aspen. The Aspen storm runoff storage ponds would
be connected to the Aspen Metro Roaring Fork trunk line by an auto-
matic system that would relate to the sewage flows to the plant.
The Mill Street storm sewer should be extended in conjunction
with the major drainageway construction associated with the mall flood-
way and to prevent local runoff from entering the floodway. Phase I costs
represent approximately 50% of the total project cost yet will treat 60%
to 70% of the indicated pollution loads.
VIII-7
Phase II. Phase II basically conslsls of improviny Aspen's
initial drainage system end inltlnting the first step of treatment
for the West Aspen areas.
The Original and Center Street storm sewer Improvements would
consist of replacing and adding new Inlets. These would use the avail-
able capacity of the existing storm sewers which is Qresently_bei~
wasted because nearly all of the existing inlets are inadequate. Some
relatively minor storm sewer improvements are anticipated to drain sump
areas and an area tributary to Hopkins Avenue which presently drains-:
directly to the Roaring Fork. Paepcke Park presently functions as a
detention area during high runoff periods and it Is anticipated that
this would continue, alleviating the need for a large major drainage
outfall to the Roaring Fork. With the runoff from the mountain
slgnfficantly curtailed by the West Aspen detention system, P`pcke
Park will provide the key to major drainage for Basin 58 The Park
would not be slgnfficantly altered other than to lower curb elevations,
some regrading from these points Into the lower area of the park
and a storm sewer outlet to Center Street. This system would also
help to prevent flooding of local residences.
The Lake and Gillisple Street area would be improved by building
larger roadside swales and higher capacity street culverts. The various
roadside swales would lead to greenway swales which would travel to
detention ponds In the Aspen InsEltufe area. It is important to
realize that roadside swales, besides being significantly less expends
sive, prevent heavy runo po ut~on as in cate n severa stu ies.
It is qu to poss e t en t at once these swales and the detention ponds
are built, the resulting storm runoff quality will be sign iflcantly
improved as to obviate the need for further treatment.
Phase III. Once the effectiveness of the Aspen Institute detention
ponds is determined, it is conceivable that further treatment may be
desired. There are several treatment possibilities available as
mentioned earlier; however, the most practical appear to be connection
[o the Aspen Metro trunk lines or lmplementatlon of land treatment. The
treatment costs indicated In Phase III would probably be of a
similar magnitude for either alternate. However, we have indicated
the cost of connection to the Aspen Metro trunk Ilnes on the basis of
easier lmplementatlon and that the Aspen Institute may not desire
irrigation for the natural vegetation on the site.
Drainage improvements for the Red Butte area would be constructed
in this phase. This would consist largely of construction of roadside
swales, grass-lined collection swales and detention basins. The
treatment concept o this area would be similar to West Aspen; that Is,
to construct the various detention facilities and analyze the results
of the detention pond effluent over a period of time after theft
construction to determine if further treatment is necessary.
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VIII-d
Phase IV. Assuming that the Red Butte area required further
treatment than provided by the detention system, several treatment
alternatives are available; however, the most likely being the use
of land treatment by spray irrigating the runoff on the golf course
and surrounding areas. This would provide a more than satisfactory
treatment level for this area.
_ _ -------__ _ __ _ _ _ _ ~r,
URBAN RUNOFF MANAGEMENT PLAN
PRELIMINARY COST ESTIMATE
PHASE 1
MAJOR DRAINAG/_E ,L
cg51
Spar Gulch/West Aspen Floodway
Includes 2,200 feet of channel excavation and grading,
a 1.0 acre foot sedimentation basin, an overflow structure,
10 drop structures, and a bypass structure for the Durant Mine
water flow. -
Pioneer/Valleio/Mall Floodway
Includes a 4 acre foot detention basin, outlet controls
and an overflow structure, a rock masonry channel along Mill
Street to the mall, rough excavation for the mall portion of
the floodway, a large culvert under main street, channel ex-
cavation from the mall to the river, an outfall structure and
other structures to prevent contamination of mountain runoff
by local drainage.
West Aspen Detention System
Includes a series of detention ponds with a capacity of
5.7 acre feet, 1,600 feet of channel excavation and grading,
an overflow structure and a drop structure into Castle Creek
for the small channel furthest west.
Phase I MAJOR DRAINAGE CONSTRUCTION COST
VIII-9
529,900
578,100
530,700
$138,700
COLLECTION AND TREATMENT
Collection Swales
Includes 3,200 feet of channel excavation and grading
leading from the outfalls of the improved storm sewer mainlines.
£ ,: r
Center Street Storm Sewer
Includes readjustment or abandonment of the existing 36"
CMP outfall and construction of a surface Swale that would be
capable of flowing into the Aspen Storm Runoff Storage Ponds.
Original Street Storm Sewer
Includes construction of a storm sewer from Main Street
and Original Street to the collection Swale system
S16,oo0
$25,200 ~ -'-'-
$28,200
VIII-10
_. t _ ,
Mill Street Culver[ l
Includes construction of a culvert under Mill StreeC
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or the collection Swale system.
5 4,800
Aspen Storm Runoff Storage Ponds
Includes excavation and grading of 6.6 acre feet
capacity in a system of connected ponds.
Controls and Trunk line Connection
Includes the automatic metering and control devices
,
structure and connection to the Roaring Fork Trunk Sewer.
$ 22,000
Overflows
a
Includes two stratigically located overflows which w111
direct lar
er m
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agn
tu
e flood flows. ~:
S 8,000
Phase I COLLECTION AND TREATMENT CONSTRUCTION COST
$114,800
k
INITIAL DRAINAGE
Mill Street Storm Sewer
Includes construction of a storm sewer under the entire
length of the M
ll !
a
and the various inlets and :other facilities
such
as cross pan construction to properly direct storm run-
off and ~ ~_
prevent contamination of mountain runoff,
$ 81
200 j
I
f _. 1... , ~
-
_..
Storm Inlets
Includes four inlets for the Original Street Storm Sewer
Extensi
b ~'
on to
e located at Main and Spring Streets.
$ 4,000
Phase I INITIAL DRAINAGE CONSTRUCTION COSTS
$ 85,200
PHASE I BASE COST
$338,700
Add 30% for contingencles, Engineering and legal fees
,
land acquisition- costs, administrative and miscellaneous
costs
$l0 600
PHASE I TOTAL ESTIMATED PROJECT COST $440
300
(based on 1973 dollar values) ,
i
I
~;
~:
~: ;
VIII-11
PHASE II
INITIAL DRAINAGE
Original Street Storm Sewer Improvements
Includes 1,050 feet of storm sewer extensions, 17
new inlets ~ and cross pan work
Center Street Storm Sewer Improvements
Includes 18 new inlets and storm sewer extension
into Paepcke Park and a small amount of reshaping in
Paepcke Park to allow detention storage for large magni-
tude storms ranging near the 100-year storm
Lake Street Area Drainage Improvements
Includes 3,200 feet of Roadside Swale, 2,400 feet of
grass lined channel to and from detention basins, 10 cul-
verts, 6 acre feet detention storage and an outfall struct-
ure to the Roaring Fork.
Gillespi Street Area Drainage Improvements
Includes 2,100 feet of roadside Swale, 2,500 feet of
grass lined channel to and from detention basins, 5 acre
feet of detention storage, 8 culverts and an outfall structure
to the Roaring Fork.
Phase II INITIAL DRAINAGE CONSTRUCTION COSTS
Add 30% for contingencies, Engineering and legal fees,
land acquisition ousts, administrative and miscellaneous
costs
PHASE II TOTAL ESTIMATED PROJECT COSTS
(based on 1973 dollar values)
S 37,100
$ 24,400
S 51,700
S 43,100
5156,300
$ 46,900
$203,200
VIII-12
PHASE III
TREATMENT
West Aspen
Includes the possible connection to the various
trunk sewers with control facilities and i in
p p 9' (Other ~`
alternatives possible for similar cost). ,,,;
$31,600
INITIAL DRAINAGE
-.~..._.___ _..__ 'I''"
t,.
Red Buttc Area is
Includes 7,400 feet of roadside Swale construction ;
or improvement, 6,200 feet of other collection and trans- y
mission swales to and from detention areas, 35 acre feet
of detention storage, )0 sets of culverts, an enlarge-
ment of the culvert under Highway 8';, and four outfalls
for the receiving streams. $133,$00
PHASE III CONSTRUCTION COSTS $165,400
Add 30% for_conttngencies, Engineering and legal fees, ~`'
land acquisitiontosts, administrative and miscell-
aneous costs- $ 49,600
PHASE III TOTAL ESTIMATED PROJECT COSTS $215,000 I's
ase on o ar va ues
Phase IV costs cannot be reliably estimated at this time but would
Include treatment of storm runoff In the Red Butte Area. ';',~i
SECTION IX
INSTITUTIONAL RECOMMENDATIONS
$Pe ctlsa P~, w- N
important aspects of urban storrti drainage management have to do with
reasonable regulations and policies dealing with reduction of runoff, con-
trol of pollution sources, and land use regulations. These are generally
categorized as institutional methods to implement better urban water
resources management. There are three general categories of regulations
which relate to the objectives of this report. They are:
a. Land use, including flood plain regulations, and land zoning to create
open space and to provide for proper drainage in new developments;
b. Urban drainage ordinances dealing with upstream ponding on both exist-
~ing and proposed development; and
c. Pollution control ordinances.
Regulations, however, will not achieve the desirable goals of reducing
runoff and reducing pollutants carried into the waterways. More important
than the regulations is the environmental ethic which needs to be ingrained
into the citizens, and this is generally accomplished through example, through
the schools, and through an effective educational campaign,community-wide.
LAND USE
A flood plain regulation creating overlay zoning which restricts
development and building in the flood plain and which restricts excavation
and filling in the flood plain is an effective and widely accepted use of
police powers to protect the health, safety and welfare of the general
public.
An example flood plain regulation was prepared by Wright-McLaughlin
Engineers in 1969• This flood plain regulation is suitable for adoption
in Aspen and in Pitkin County. The example flood plain regulation is
included in the Appendix. Modifications to the regulation can readily
be made to direct it specifically towards Aspen's individual characteristics
and problems.
UPSTREAM PONDING
One of the most useful tools in controlling urban drainage rates of
runoff is through the use of short-term detention ponding situated w ere
the rainfall occurs, that is, as close to the inception of runoff as possible.
Study and analysis by municipal administrators, engineers and attorneys
have shown that the provision of urban storm drainage facilities is a
service to the citizens in much the same manner as is a sewerage system.
I
jB
4y `
1., . 1
is '
i
':5
lx-2
A legal basis exists for charging residents of an urban area for this
service on the basis of the amount of runoff generated. Therefore,
an ordinance creating an urban drainage and flood control municipal
agency which is funded by a service charge on all properties which
generate runoff in excess of the historic amount of runoff can be
adopted to provide credit for creation of detention storage on private
lands. Thus an ordinance of this nature can generate monies to pay for
urban drainage and flood control facilities and at the same time
stimulate voluntary on-site detention storage on roofs and parking lots,
as well as grassed areas. If the incentive given to create upstream
ponding was totally successful, very little or no money would be
generated, but this would be acceptable because very little or no
special urban drainage facilities would be required either.
Wright-McLaughlin Engineers has proposed such an ordinate to various
municipalities over the last four years with the support of legal
professionals and planners.
The cities of Denver and Boulder are ready to adopt service
charges far drainage. Boulder's will be based on the amount of
runoff generated.
POLLUTION CONTROL REGULATIONS
Pollution control regulations can take various forms and cover a
wide variety of matters related to litter, sidewalk sweeping, animals,
etc. Oftentimes these can be considered as nuisance regulations,
which is unfortunate. If the goals can be achieved through a
citizenry having a strong environmental conscience, it would be
better than having regulations, though certain controls on new
construction erosion, dumping of oil and grease, use of insecticides
-and herbicides, are significant enough to warrant specific regulations.
Tabulated below by subject headings are potential matters worthy
of consideration regarding local anti-pollution regulations.
GENERAL
Anti-litter
Handbills
Vending
Carnivals/Fairs
Confetti
Dumping, particularly in
catch basins & waterways
Sidewalk sweeping
Bonfire burning
Animals
PRIVATE
Vacant Lots
Parking Lots/Garages
"Drive In" Establishments
Storage and Disposition of garbage
Cleanliness
Erosion Control
Sources of drifting dirt
Stockpiling
Animals, manure
Markets, food processing
Weed control
Garden refuse
t
IX-3
CONSTRUCTION
Eros(on Control
Stockpiling
Handling excavated material
Cleanup-backfill
Site cleanliness
House moving
Refuse
Dumping into waterways
Special activities
VEHICULAR SPILLAGE
011
Grease
Gas
Antifreezes and coolants
Garbage
Refuse
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REFERENCES
1. Urban Storm Drainage Criteria Manual, 4lrlght-McLaughlin Engineers,
Denver, Colorado, Vols. 1 and 2, March 1969.'
2. Magnitude and Frequency of Floods in the United States, Part 9.
Colorado River Basin, Geological Survey Water Supply Paper 1683,
U. S. Department of the Interior, 1966.
3. Roarin Fork Greenwa, and Trails Plan, Aspen, Colorado, Laboratory
o Mountain Ecology for Man, Department of Environmental, Population
and Organismic Biology, University of Colorado, Boulder, Colorado,
April, 1973.
4. Wolle, M. S., Stampede to Timberline, Sage Books, Denver, 1949
5. Rainfall Intensit -Duration - Fre uenc Curves for Selected Stations
in t e United States A aska Hawaiian Is ands an Puerto ico
echn~ca aper No. 25, Cooperative tudies Section, U, Department.
of Commerce, Weather, Bureau, December, 1955.
6. Rainfall Fre uenc Atlas of the United States for Durations from
30 Minutes to 2 Hours and Return Periods from I to 100 years,
Technical Paper No. 40, Cooperative Studies Section, U. 5. Department
of Commerce, January, 1963.
7. Chow, V. T „ Ph. D. (Editor), Handbook of Applied Hydrology, McGraw-
Hill, Inc., 1964
8. Snow-H drolo , Corps of Engineers, U. S. Army, Portland, Oregon,
30 June 95 .
9. Flood Plain Information. Roaring Fork River and Castle and Hunter
Creeks. Aspen, Colorado, Department of the Army, Sacramento District
Corps of Engineers, June, 1973.
10. Com ilation of Records of Surface Waters of the United States thro
9 0, Part 9, Co orado River Basin, Geological Survey Water-Supply
Paper 1313, U, S. Department of the Interior, 1964,
ll, Water Resources Data for Colorado, Part I Surface Water Records,
United States Department o the Interior Geological. Survey, 1961
through 1971.
12. Water Pollution Aspects of Street Surface Contaminants, Office of
esearc an onitor~ng, nvironmenta rotection Agency,
November, 1972, i
~.
R-2
13. Condon, F. J. "Treatment of Urban Runoff," APWA Reporter, March
1973.
14. Wastewater Mana ement Stud for Cleveland-Akron Metropolitan and
Three Rivers Watepshed Areas Dra t ,Wright McLaughlin Engineers and
Havens and Emerson, May, 973•
Vol. 1 Summary Report
Vol. 2 Land Treatment
Vol. 4 Municipal Waste Water and Stormwater Runoff,
Part 1
Vol. 5 Municipal Waste Water and Stormwater Runoff,
Part II
Vol. 6 Municipal Waste Water and Stormwater Runoff,
Part III
15. Storm Water Pollution from Urban Land Activit U.S. Department of
t e Inter or, Federa ater Qua ity min stration, July, 1970.
16. Bingham, D. A., Water Pollution Studies, Federal Aid in Fish
Restoration, Protect F-33-R-2, September, 1967.
Bingham, D. A., Water Pollution Studies, federal Aid in Fish
Restoration, Project F-33-R-3, June, 1968.
ALSO SEE
Chow, V. T., Ph. D. (Editor), Open-Channel Hydraulics, McGraw-Hill,
Inc., 1959.
Linsley, R. K. and Franzini, J. B., Water-Resources Engineering,
McGraw-Hill, Inc., 1964.
Poertner, H. G., Detention Storage of Urban Storm Water Runoff
from the APWA Reporter, Vol. 40, Ilo. 5, May, 1973•
Prellminar Feasibilit Re ort 'for Or
Metropo itan San Cation D strict, rig
r,
n Engineers,
Retention Basin Control of Combined Sewer Overflows, Environmental
Protect on gency, ater ua ity ce, ugust,~ 0.
Rotar Vibrator Fine Screenin of Combined Sewer Overflows, U. S.
Department o the Inter or, Federal Water Quality Administration,
March, 1970.
_.
4
A
1
F
,~ - -. ~ ~ r....--
R-3
Storm and Combined Sewer Demonstration Projects, U. S, Department of
[he Interior, Federal Water Pollution Control Administration,
January, 1970.
Storm Water Management Modei, Vol. 1 - Final Report, Environmental
Protection Agency, Water Quality Office, July, 1971.
Storm Water Mana ement Model, Vol. II - Verification and Testing
Env ri onmenta Protection Agency, Water Quality Office, August, 1971•
Urban Runoff Characteristics, Environmental Protection Agency, Water
Qua ity 0 ice, October, 1970.
Water Pollution As ects of Urban Runoff, U. 5. Department of the
Inter or Federa Water Pollution Control Administration, January,
1969.
-- - _
r~--T-----~-~-
DRAINAGE CRITERIA MANUAL POLICY APPENDIX
EXAMPLE FLOOU PLAIN REGULATION
urllvER REr.1oN
SECTION 1~0 STATUTORY AU TIIOIt11AT1011„ FINDIIIG OF FACT, STATEMENT OF
PURPOSE., AND TITLE
I,l Statutor)r Author+zat+on Thrs ordrnance for flood plain pro-
tection ,s adopted pursuant to the authorization contained
in Sections l39 60-1 and 1062.10 of the Colorado Revised
Statutes
lag Finding of Fact.' The uncontrolled use of the flood plains
and++watercourses++of the (C~ty) (Town) (County) of ~
Colorado., adversely affects the publ~c health, safety; con-
venience and general welfare of that (City) (Tom) (County),
In addition., extraordinary publ+c expenditures are required
for the projection of persons and property and for the relief
of distress in areas subject to periodic flooding, Filling,
construction.;, and certa+n other land use practices have been
determined to be major contributions to such effects. The
effects of a single fill or other project upon flood heights,
velocities„ or flood plain storage areas may be relatively
insignificant compared [o the combined effects of a number
of such projects which over a long period of time may dras-
tically increase the flood hazard, Without a competent analy-
sis of such projects., It is not possible to adequately ascertain
the effects of each flood plain use upon subsequent development
or the compatibility thereof with the long-range needs of the
commun+tyo
1,3 Statement of Purpose.. To promote the publ;c health„ safety,
and general welfare, to m.n+m~ze flood losses !n areas subject
to flood hazards., and to promote wise use of the~~rlood plains,
this flood plain zoning ordnance has been established with
the following purposes ,mended
1..31 To reduce the hazard of floods to life and property
through.
(I) Prohrb,t.ng certain uses ~rh:ch are dangerous to
life or property in t me of flood,
(2) Restricting uses o,h~ch would be hazardous to the
public health sn time of flood:
(3) Restricting uses which are pa•ticularly susceptible
to rlood damage. so as to alleviate hardship and
reduce demands fcr publ~c expend,[ures for relief
and protection.
3-I-69 KRw
DRCOG
~l
-- _----
- - _>
POLICY APPEfIDIX DRAINAGE CRITERIA MANUAL
1.3 Continued
(4) Requi-ring permitted flood plain uses, including
public facilities which serve such uses, to be pro- -
tected against floods by providing"flood proofing"
and general flood protection at the time of initial
construction,
1.32 To protect flood plain occupants from a flood which is
or may be caused by their am, Or other; land use and
which is or may be undertaken without full realization
of the danger„ through
(I) Regulating the manner in which structures designed
for human occupancy may be constructed so as [o
prevent danger to human life within such structures,
(2) Regulating [he method of construction of water supply '
and sanitation systems so as to prevent disease, con-
tamination and unsanitary conditions,
(3) Delineating and describing areas that could be
inundated by floods so as to protect individuals
from purchasing flood plain lands for purposes which i
are not in fact suitable,
1.33 To protect fhe public from the burden of extraordinary
financial expenditures for flood control and relief,
(I) Regulating all uses within the flood plain district
so as to produce a method of construction and a
pattern of development which will minimize [he
probability of damage Co property and loss of life
or injury to the inhabitants of the flood hazard
areas,
1.34 To protect the "storage capacity of flood plains" and
Co assure retention of sufficient "flocdway" area to
convey flood flows which can reasonably be expected to
occur by°
(I) Regulating filling, dumping, dredging, and alteration
of channels by deepening, widening, or relocating,
(2) Prohibiting unnecessary encroachments.
(3) Encouraging uses such as agriculture, recreation,
and parking.
1.35 To protect tfie hydraulic characteristics of the small
watercourses, including the gulches, sloughs, and artificial
water channels used for conveying flood waters, which make
up a portion of the urban major drainage system;
i
3-I-69 KRw _
DRCDG
DRAINAGE CRITERIA MANUAL
POLICY APPENDIX
1,3 Continued
(I) Regina n ng fl]ling, dump"ng. and channelization
so as to maintain natural storage capacity and
slow flow characrer+stics.
(2) Prohlblthng encroachment into the small watercourses
to mainkarn the+~ water carrying capacity,
(3) Encourag ng uses such as greenbelt, open space,
recreation. and rid,ng trails,
1,4 Title
SECTION 2,0 GENERAL PROV1510f1S
2,1 Jur!sdict~on The Jur~sd+ct-on of [he ordinance Includes all
a~-adjacent to any watercourse w+th n the (City) (Town)
(County) of Colorado., that would be inundated
by the "'100-yeas flood"for that watercourse as defined in the
Definitions, Section 11,16 of [h;s ordinance,
2,2 District Types„ The '"Flood Regulatory" District covers the 100'
year flood plain, Where deemed to be in the public interest by
the (City) (Town) (County) of
Promote prudent use of the floodplain, the FloodCRegula~orynd to
Dlstrlct may be subdivided into the "Floodway District" and the
'rFlood Storage D,~str+ci.."
The Flood Regulatory O~:sir:ct is def.ned by compuCing the 100-year
flood plain 1+mrts under ex~stang channe~ and flood plain conditions,
Subdivrs"on of the Flood Regu~ato•y D+str+ct into the Floodway
District and the Food Storage Dist•~ct must not cause a 100-
year flood water surface profile r~se of more than one foot
above that for the Food Regulatory D+str~ct,
The subdivision of the Flood Regulatory D+;trict and accompany-
ing hydraulic studies must be based upon all of the Flood Storage
District reach being f~lledp C,ea+ on of the Floodway District
and Flood Storage D,str+ct must be made only with the full under-
standing that such subd+v+s~on w l: tend to increase flood peaks
downstream, The Floodway D st-•ct and flood Storage District should
be added to the zon ng map for the Food Regulatory District,
z.3 Dis trio Boundaryes The boundaries of the Flood Regulatory
Dlstrlct., the Floodway D+st+~ct and the Food Storage Dlstrlct
shall be as they appear on the zoning map which is on file in
the office of the The boundary lines on
the map shall be deterrn ned by the use of the scale a
on the map, Where there +s a confli PPearing
lines illustrated on [he map and actualbftween the boundary
the dispute shall be sett! eld conditions.;
ed according [o ,Section
Mapping Disputes of this ord+nance, ~~3'
3-1-69 KRW
DRCOG
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POLICY APPENDIX
DRAINAGE CRITERIA MAIIUAL
2,4 Effect of Flood Plain Reculations. The regulations set forth in this
ordinance 'or the F ood Regulatory Uis[rict, the Floodway Distrlet,
and tlrc Flood Storage listrict shell apply to those lands within
[he 100-year flood plain mapped and deslgna[ed on the official
zoning map and shall be void and of no effect in areas not so
mapped and designated. The regulations of this ordinance shall
be construed as being supplementary to the regulations imposed
on the same lands by any underlying zoning ordinance, When
flood plain and underlying zoning ordinance regulations conflict
with one another, the most restrictive combination of such
regulations shall control.,
2.5 Com Hance: IJo"structure,"land or water, shall hereafter be
us~and no structure shall be located, extended, converted
or structurally altered without full compliance with the terms
of this ordinance and other applicable regulations,
2.6 Abrogation and Greater Restrictions
2.61 This ordinance supersedes provisions of any zoning
ordinance relatiny to flood plains, Ila~~ever, any under-
lying zoning ordinance shall remain in full force and
effect to the extent that its provisions are more
restrictive.
2,62 It is not otherwise intended by Chis ordinance [o repeal,
abrogate, or impair-any existing deed restrictions; ha,r-
ever, where this ordinance imposes greater restrictions,
[he provisions of this ordinance shall prevail,
2,7 Interpretation: In their interpretation and application, the
provisions of [his ordinance shall be held to be minimum require-
ments and shall be liberally construed in favor of the governing
body and shall not be deemed a limitation or repeal of any other
powers granted by Colorado Statutes,
2.8 Warning and Disclaimer of Liability; The degree of flood pro-
tection intended to be provided by [his ordinance is considered
reasonable for regulatory purposes and is based on engineering
and scientific methods of study. Larger floods r.iay occur on
occasions or the flood height may be increased by man-made or
natural causes, such as ice jams and bridge openings restricted
by debris. This ordinance does no t. imply that areas outside
flood plain zoning district boundaries or land uses permitted
within such districts will always be totally free from flooding
or flood damages. Nor shall this ordinance create a liability
on the part of or a cause of action against the (City) (Town)
(County) of or any officer or employee thereof
for any flood damages that may result fdr reliance on this
ordinance.
3-i-69 KRW
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DRAItJAGE CRITERIA MANUAL
POLICY APPEIJDIX
2.9 Severa~bilit a If any section, clause, provision or portion
o thf is ordinance is adjudged uncons[ttutional or invalid
by a court of competent jur+sdictionr the remainder of this
ordinance shall not be affected thereby.
SECTION 3,0 IJONCONFORMIIJG USES
3.1 The existing lawful use of a structure or premises which is
not in conformity w+th tl~c provisions of th,s ordinance may
be cont+nued subject to tl~e following condit!ons
3,11 tJo such use shall be expanded or enlarged except .n
conformity w+Ih the p~ovis+ons of this ord Hance.
3,12 No structural alteration, addit!on o' repair to any
nonconforming st~ucture over the +~fe of the structure
shall exceed Fifty (SOj percent of .ts assessed value
at the time of its becoming a nonconforming use
unless permanently changed [o a conforming use,
3.13 If such use is discontinued for torelve (12) consecutive
months„ any future use of the building and premises
shall conform to [his ordinance.
3.14 Uses or adjuncts thereof which are nuisances shall no[
be permitted to cunt+nue as nonconforming uses,
315 Any alteration.. addit,on or repair to any nonconforming
structure perritted pursuantto-Sect,on 3.12 of this
ordinance shall be protected by flood proofing measures
pursuant to Section 7.45(1)„ Flood Proofing of this
ordinance.
SECTIOFJ 4,0 FLOOD REGULATORY D1STP.ICT
4.1 Application, The provisions for th,s district apply to all
flood plains of watercourses +n the (C+tv' (Tom) (County) of
Colorado„ for which 100=year flood data
and corresponding elevations or profiles are available, and
which have been approved by the State Water Conservation Board,
4.2 Description of District. The Flood Regulatory District shall
include [he area delineated on the maps and profiles for the
100°year flood plain limits for the watercourses in the (City)
(Town) (County) of Colorado, signed by the
(City) (Town) (County v Engineer and approved
by the Colorado Water Conservation (3oard, and on file and
available in the Clerk and Recorder s Office of
County.
3'1-69 KRW
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POLICY APPENDIX
DRAINAGE CRITERIA 11ANUAL
4.3 S ecfal Provisions, The folly-ping regulations shall apply to
a uses wit in t e Flood Regulatory District, notwithstanding
that such uses may be specifically permitted under the terms
of this ordinance.
4.31 The flood protection elevation or height shall correspond
to a point one foot above the elevation or "flood profile"
shown on or attached to [he flood map for a particular
area.
4.32 No "structure (temporary or permanent)"; fill, including
fill for roads and levees; deposit; obstruction; storage
of materials; or other flood plain uses which acting
alone or in combination with existing or future flood
plain uses shall be permitted that adversely affects the
efficiency or the capacity of the floodway or increases
flood heights or adversely affects the "storage capacity
of the flood plains" based on the assumption that there
will be an "equal degree of encroachment" extending for
a signifPcant "reach" on both sides of the stream,
4.33 No flood plain uses shall adversely affect the efficiency
of or unduly restrict the capacity of the channels or
floodways of any tributaries to the main stream, drainage
ditches, or any other drainage facilities or systems.
4,4 Description of Uses
Permitted Uses„ The following open uses shall be permitted
within the Flood Regulatory District [o the extent [hat they
are not prohibited in a particular area by any underlying
zoning ordinance,
4.41 Agricultural uses such asp general farming, pasture,
truck farming, forestry, sod farming, and wild crop
harvesting;
4.42 Industrial-commercial uses such as: loading areas,
parking areas, airport landing strips, and storage
yards for equipment or machinery easily moved or not
subject to flood damage;
4,43 Public and private recreational uses not requiring
"permanent or temporary structures" designed for human
habitation such as: parks, swimming areas, golf courses,
driving ranges, picnic grounds, wildlife and nature
preserves, game farms, fish hatcheries, shooting preserves,
target ranges, trap and skeet ranges, and hunting, fishing
and hiking areas;
3-1-69 KRW
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-------r
_~._
DRAINAGE CRITERIA MANUAL
POLICY APPENDIX
4,4 Continued
4.44 Utility facilities such as :: flowage areas, transmission
linesr pipelines„ water monitoring devices, roadways,
and bridges„
4,5 Special Exceptions:. Any use enumerated in th+s section may be
permitted only upon application to the Zoning Administrator and
the issuance of a special exception permit by the Board of
Zoning Adjustment as provided in Section 7, 4; Special Exception
Permits of this ordinance.
4.51 "Structures" Accessory to Open Uses permitted in Section
Description of Uses, of this ordinance, whether
temporary or permanent may be permitted only upon a
de[erm'ination by the Board of Zon%ng Adjustment pursuant
to a finding under the procedure required by Section 7,4;
Special Exce [ion Permits of this ordinance that;
(I) Structures will not be des~gned for human habitation;
(2) Structures will have a low flood damage potential;
(3) The structure or structures, if permitted; will be
constructed and placed on the building site so as
Eo offer the minimum obstruction to the flow of
flood waters;
(a) Whenever possible, structures will be constructed
with the longitudinal axis parallel to the
direc U on of flow of flood waters, and
(b) So far as practicable,, structures w%11 be
placed so their longi w d,nal axes are
approximately on the same I~ne as those of
adjoining structures.
(4) Structures will be firmly anchored to prevent the
structure or building from floating away and thus
threatening to further restrict bridge openings
and other restricted sections of the stream or
river; and
(5) Service facilities such as electrical equipment
will be at or above the flood protectior. elevation
for the particular area-
4,52 Other Structures (Tem orar or Permanent for
may be permitted only upon a finding by the
Zoning Adjustment that:
3-1-69 KRW
ORCOG
~~
~~
POLICY APPENDIX DRAINAGE CRITERIA MANUAL
4.5 Continued
(1) Such structures shall comply with Section 4,51 (3),
(4), and (5) of this ordinance.
(2) The first floor, or basement floor of any structure
to be erected, constructed, reconstructed, or moved
on the flood plain shall be constructed on fill at
or above a point two (2) feet above the 100-year
flood elevation for the particular area and the
fill shall extend at such elevation at least fifteen
(15) feet beyond the limits of any structure or
building erected thereon.
4.53 Fills or De osition of Materials may be permitted only
upon a ending by [he Board o Zoning Adjustment that:
(1) Any fill or deposition of materials will comply
with the Section 4,3 Special Provisions, of [his
ordinance, and
(2) The fill or deposition of materials will have same
beneficial purpose and [he amount [hereof will not
be greater than is necessary to achieve that purpose,
as demonstrated by a plan submitted by the owner
showing the final dimensions of the proposed fill
or other material and the use to which [he filled
land will be put;
(3) The fill or deposition of materials does not
imprudently reduce the flood storage capacity of
the waterway unless a permit has been granted by
the (legislative body of [he City, Town, or County)
and the other requirements of this section are met;
and the fill or deposition of materials does not
encroach on that portion of the flood plain which
would have significant and perceptible flow during
the flood, and which for that reason would help
convey the flood waters. Any additional filling
reduces [he hydraulic capacity and requires
appropriate hydraulic studies and a review of the
urban impact of such reduction,
(4) The fill or other materials will be protected
against erosion by rip-rap, strong vegetative
cover or bulkheading-.
4.54 The stora a or rocessin of materials that are buoyant,
aroma e, exp osrve, or in times o boding, could be
injurious to human., animal, or plant life, shall be at or
above the flood protection elevation for the particular
3-I-69 KRw
DRCOG
__
POLICY APPENDIX DRAINAGE CRITERIA MANUAL
6.3 Continued
main stream or river; drainage ditches; or any other
drainage facilities or systems.
6.32 The first floor_or basemrnt opening of any building or
structura to be erected, cons[ru~ted, reconstructed,
altered, or moved in the Flood Storage District shall
be constructed on fill at or above a point two (2) feet
above the 100-year flood elevation for the particular
area and the fill shall extend at such elevation at
least fifteen (15) fee[ beyond [he limits of any structure
or building erected thereon,
6.33 The store a or rocessin of materials chat are buoyant,
ammo e, exp ostve, or in times o boding, could be
injurious to human, animal, or plant life, shall be at
or above a point two (2) feet above the 100-year flood
elevation for the particular area,
SECTION 7.0 ADMINISTRATION
7,1 Zonin Administrator: The Zoning Administrator (or other
o icer, appointed to administer the underlying zoning ordinance)
shall also administer the provisions of this ordinance in the
same manner as outer zoning matters are administered.
7.2 Zoning Permit: A zoning permit must be obtained from the l
Zoning Administrator before any new land use subject to the
provisions of this ordinance may be initiated.
7.3 Mapping Disputes, The following procedure shall be used by
the Board of Zoning Adjustment in deciding contested cases
in which the location of a district boundary is disputed;
(1) In all cases the person contesting the location of the
district boundary shall be given a reasonable opportunity
to present his case to the Board and to submit his own
technical evidence if he so desires. The Board. shall not
allow deviations from the boundary line as mapped unless
the evidence clearly and conclusively establishes that
the mapped location of the line is incorrect,
7.4 Special Exception Permits
7.41 Application for; Any use listed in this ordinance as
requiring a special exception permit may be allayed
only upon application to the Zoning Administrator and
issuance of a special exception permit by the Board of
Zoning Adjustment.
3-1-69 KRw
ORCOG
DRAINAGE CRITERIA MANUAL
7.4 Continued
POLICY APPENDIX
7.42 Procedure to be Followed in an Flood Re ulator District
F oodway District., or Flood Storage District by Board n
corn ny „u~ustment ~n rassing on Special Exception Permits;
Upon receiving an application for a special exception
permit involving the use of fill., construction of structures,
or storage of materials, the Board shall, prior to
rendering a decision thereon, obtain the opinion of the
Planning Director and:
(1) Require the apnlicant to submit,, at the time of
application., two copies of an aerial photograph,
or a,plan certified by a registered engineer;
competent in open channel hydraulics, which
accurately locates ttre flood plan proposal
with respect to the district limits, channel of
stream, existing flood plain developments, together
with all pertinent information such as the nature
of the proposal; legal description of the property,
fill limits and elevations; building floor eleva-
tions; and flood proofing measu*es,
{2) Require the applicant to furnish such of the
following additional information as is deemed
necessary by the Board for the evaluation of the
effects of the proposal upon flood flows and flood
plain storage and to render a decision on the
proposed flood plain use,
(a) A tyQical vall~ cross-secr~on showing the
channel of the stream, the flood plain adjoining
each side of the channel< cro=_s sectional area
to be occupied by the proposed development, and
high water information,
(b) Plan (surface view) showing elevations or
contours of the ground, pertinent structure,
fill or storage elevations; size, location and
spatial arrangement of all proposed and existing
structures on the site, location and elevations
of streets, water supply.. sanitary facilities,
and soil types and other pertinent information,
(c) Profile showing the slope of the bottom of [he
channel or thalweg of the stream.,
(d) Specifications for building construction and
materaals~{lood proofing," filling, dredging,
grading, channel improvement, storage of
materials, water supply., and sanitary facilities,
3-I-69 KRw
DRCOG
f
POLICY APPEtdDIX
7,4 Continued
DRAINAGE CRITERIA MANUAL
7.43 Factors u on which the decision of the Board of Zonin _
Ad1ustmen[ sha be based. The determination o the
Board on each spec a exception permit shall be based
on the effects of the proposed project with respect to
the objectives and purposes of this ordinance as stated
in the Statement of Pur ose, Sections 1.31, 1.32. 1.33,
1.34 an 5 o t is ordinance;
7.44 The Board shall ac[ on an application in the manner above
described within 60 days from receiving the application,
7,45 Conditions Attached to Special Exception Permits; Upon
consideration of the factors listed above and the purposes
of this ordinance„ the Board of Zoning Adjustment may
attach such conditions, in addition to those required
by special permits.„ as it deems necessary in furthering
the purposes of this ordinance. Such conditions may
include specifications fors without limitation because
of specific enumeration; modification of sewage disposal
and water supply facilities, modification of other
waste disposal methods and facilities, landscaping,
periods of operation, operational controls, sureties,
deed .restriction, and adequate flood proofing,
(1) Floodproofing. Special exceptions requiring flood
proofing measures such as the following shall be
designed consistent with the flood protection
elevation for the particular area as described in
the Social Provisions„ Section 4,31; and flood
velocities, o~ rc~ es and other factors associated
with the flood protection elevation, The Board
shall require that the applicant submit a plan
or document certified by a registered professional
engineer that the flood proofing measures are
consistent with the flood protection elevation
for the particular area,
(a) Anchorage to resist flotation and lateral
movement.
(b) Installation of watertight doors, bulkheads
and shutters.
(c) Reinforcement of walls to resist water pressures.
(d) Use of paints, membranes or mortars to reduce
seepage of water through walls,
~~
6e~";;
Fa~'.i
3-1-69 KRW
ORCCG
DRAINAGE CRITERIA MANUAL
7,4 Continued
POLICY APPENDIX
(e) Addition of mass or weight to structures to
resist flotation,
(f) Installation of pumps [o lower water levels
in structures.
(g) Construction of water supply and waste
treatment systems to prevent the entrance
of flood waters.
(h) Pumping facilities for subsurface drainage
systems For buildings to relieve external
foundation wall and basement floor pressures.
(i) Construct+on to resist rupture or collapse,
caused by water pressure or floating debris.
(j) Cutoff valves on sewer lines or the elimination
of gravity flow basement drains,
SECTION 8.0 CERTIFICATE OF COMPLIANCE
8,1 No vacant .land shall be occupied or used and no building
hereafter erectedn altered,; or moved on the flood plains of
any watercourse shall be occupied until a certificate of
compliance shall have been issued by the Zoning Administrator.
8,2 The Zoning Administrator shall request the applicant to submit
a certification by a registered professional engineer that the
finished fill and building floor elevations, flood proofing
measures, or other flood protection factors were accomplished
in compliance with the provisions of this ord Hance. The
Zoning Administrator shall within ten days after receipt of
such certification from the applicant issue a certificate of
compliance only if the building or premises and the proposed
use thereof conform with all the requirements of this ordinance'.
SECTION 9,0 ENFORCEMENT AND PENALTIES
9.1 Every structure, building, fill or development placed or
maintained within any flood plain n violation of this ordinance
is a public nuisance and the creation thereof may be enjoined
and maintenance thereof may be abated by action at suit of
the (City)(Town)(Coun[y) of ,the state or any
citizen thereof. Any person who places or maintains any
structure, build"ing„ fill or development within any flood
plain in violation of this ordinance may be fined not more
than $50 for each offenseo Each day during which such
violation exists is a separate offense.
3-1-69 IcRw
DRCOG
POLICY APPENDIX
SECTION 10.0 AMENDMENTS
DRAINAGE CRITERIA MANUAL
10,1 The (legislative body)of (City)(Town)(County) of "
Colorado, may from time to time, alter, supplement or change [ e
district boundaries and the regulations contained in this
ordinance in the manner provided by law.
10.11 Amendments to [his ordinance may be made on petition
of any interested party in accordance with the
provisions of the Colorado Revised Statutes.
1.0.12 The subJivisions of the Flood kegulatory Uistrict into
the Floodway Uistrict and Flood Storage Uistrict will
only be made by action of the (leyislative body) of
(City) (Town) (County) of Colorado,
SECTION 11,0 DEFIIJITIOIJS
Unless specifically defined below, words or phrases used in
this ordinance shall be interpreted so as to give them the
same meaning as they have at common law and to give this
ordinance its most reasonable application,
II.I Channel - a natural or artificial watercourse of perceptible
extent, with definite bed and banks [o confine and
conduct continuously or periodically flowing water,
.Channel flow thus is that water which is flowing within
the limits of the defined channel.
11,2 Encroachment Lines - are limits of obstruction to flood
ows. These lines are generally parallel to the stream,
The lines are established by assuming that the area
landward (outside) of the encroachment lines may be
ultimately developed in such a way that it will not be .
available to convey flood flows; The stream channel
and adjoining flood plains between these lines will be
maintained as open space and will be adequate to convey
the 100-year flood without adversely increasing flood
heights, such increase under any condition not exceed-
ing one foot,
II.3 L-qual De ree of Encroachment- is established by considering
the effect of encroachments on the hydraulic efficiency
of the flood plain along a significant reach of the
stream, on both sides.
11,4 Flood - water from a river, stream, watercourse, ocean,
a~ke or other body of standing water that temporarily
overflows or inundates adjacent lands and which may
affect other lands and activities through stage eleva-
tion, backwater, and/or increased ground water level,
is
_:
3-1-69 KRW
DRCOG
DRAINAGE CRITERIA MANUAL
ll Continued
POLICY APPENDIX
11.5 Flood Plain - the relatively flat or lowland area adjoining
a river, stream, watercourse, ocean, lake, or other body of
standing water which leas been ur may be covered temporarily
by flood water. I"or administrative purposes Chc flood plain
may be defined as the arua that would be inundated by the
Standard Project Flood (Corps of Engineers) or the Maximum
Probable Flood (Tennessee Valley Authority).
11.6 Flood Itegula[ory Uistrict - that area represented by the
flood regulatory area which has been approved by the
Colorado Water Conservation Ooard and shown on [he zoning
map in the office of the
11.7 Flood Re ulatory Area - that portion of the flood plain sub-
ject to inundation by the 100-year flood Its width
is determined by the 100-year flood.. Its length or reach
is determined by natural bounds such as an ocean or lake,
or by structures such as a dam or bridge, or by political
or legal bounds.
11,8 Flood Storage Area - [hat portion of the regulatory area
that may serve as a temporary storage area for flood waters
from the 100•-year flood and that lies landward of the flood-
way.
11.9 Flood Storage District - that area represented by the flood
storage area which has been hydraulically defined and shown
on the zoning map in the office of the
It exists only after appropriate action of the legislative
body) of (City) (Town) (County) of , Colorado.
11.10 Floodway - that portion of the regulatory area required for
the reasonable passage or conveyance of the 100-year flood,
This is the area of significant depths and velocities and
due consideration should be given to effects of fill, loss
of cross sectional flow area, and resulting increased water
surface elevations.
11,11 Floodway District - [hat area represented by the floodway
-which has been hydraulically defined and shown on the
zoning map in the office of the It
exists only after appropriate action of the legislative
body) of (City) (Town) (County) of Colorado.
11.12 Flood Profile - a graph or a longitudinal profile showing
the relat o ship of the water surface elevation of a flood
event to location along a stream or river,
11.13 Fl_oo~d Proof~in$ - a combination of structural provisions,
c angeh s, or adjustments to properties and structures subject
3-I-69 KRW
DRCOG
~._
POLICY APPENDIX
11 Continued
DRAINAGE CRITERIA MANUAL
to flooding primarily for the reduction or elimination of
flood damages to properties, water and sanitary facilities,
structures, and contents of buildings in a flood hazard area.
11.14 Flood Protection Elevation - an elevation one foot above the
elevation or flood pro ile" of the 100-year flood under
existing channel and flood plain conditions. It is one foot
above the elevation of the flood for the Flood Regulatory
District as shown on the zoning map in the office of the
11.15 Flood Stage - for purposes of this ordinance the term is
used to mean the height or elevation of a flood as referred
to same datum For other purposes it is commonly used to
refer to the elevation a[ which a stream will overtop its
normal stage banks..
11.16 Hundred- ear Flood - is one tl~a[ teas a frequency of occurrence
0 one hundred 100) years determined from an analysis of
floods on a particular watercourse and other watercourses
in the same general region, I*. has about a one percent
chance of occurring in any given year.
11.17 Ordinary High Water Mark - the highest point on the bank
of a normal stage channel at which the water level has been
for a sufficient period of time to leave a definite mark.
11.18 Reach - a hydraulic engineering term to describe longitu-
dinal segments of a stream or river. A reach will generally
include the segment of the flood plain where flood heights
are primarily controlled by man-made or natural flood plain
obstructions or restrictions, In an urban area, the segment
of a stream or river between two consecutive bridge crossings
would most likely be a reach,
11.19 S[ora a Ga acit of a Flood Plain - the volume of space above
an area o lood plain and that can be occupied by flood
water of a given stage at a given time, regardless of whether
the water is moving, Storage capacity tends [o reduce down-
stream flood peaks.
11.20 Structure - anything constructed or erected, the use of which
requires a more or less permanent location on or in the
ground. Includes but is no[ limited to objects such as
buildings, factories, sheds, and cabins.
3-1-69 Icaw
DRCOG
DRAINAGE CRITERIA MANUAL
ll Continued
11.21 Structure Permanent
materials and n such
expected to last and
of time.
POLICY APPENDIX
- a structure which is built of such
a way Chat it would commonly be
remain useful for a substantial period
11.22 Structure Tem orar - a structure which is built of such
materia s and in such a way that it would commonly be
expected to have a relatively short useful life, or is
'built for a purpose that would commonly be expected to be
relatively short-term.
11,23 Watercourse - a channel, natural depression, slough,
arts cia channel, gulch, arroyo, stream, creek, pond,
reservoir, or lake in which storm runoff and flood water
flows etcher regularly or infrequently. This includes
major drainageways for carrying urban storm runoff.
3- I -69 IcRw
DRCOG
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~Y
DRAINAGE CRITERIA MANUAL
8 2 FLOOD 0 Of[q WAGE POUC!
AMAGE SURVEY FORM
SURVEY OF _
FLOOD DAMAGE NON-PUBLIC BUILDINGS District
Farm _ReSidence _Comme rcial _Ma nuf acturing Serial Number
Ri-ver Period of Flood Date of Crest
Bldg, No. of ~- Bldcs,
Clty Sta to Flood Zone Flood Stage
ft, on gage
Name of Owner gddrezs
Naive of Occupant Addy nss
~_-
T e ul Busint•s •: ~
-~~
Dame r _ nfr 5 stlme [e i5tricC 5[iTa[e
D i rec t -.-___.._ Sub-wCOI
_ _. _._. _ __ . Total Sub-total Total
BuilJing - foundation ~
Supe rs [ruc[ure $
Improvements
'
Decorations
Other $
Contents - Furnishings
Personal Effects
Equipment
Stock
Raw Materials or Supplies
'
Finished Products
Records
$ S
Miscellaneous
Minor Bldgs „ 'Contents
Cars, Trucks, etc,
Grounds and Improvements
Total Oi rec[ $
$
Indirect
Loss of Profits due [o
Interruption of Business $ 5
Increased Cost of Operations
Loss of Earnings by Employees
or Occupants
Cost of flood Fighting
Evacuations and Reoccupation
Total Indirect S
S
Other (5 pecify) -
Grand Total $
S
RE LE VANI OA TA
Value of Bld Value of Contents
No, o Persons A ected b~ Flood
Condition of Bld s, Good Felr Poor
No, of Da s out of Business Cse
Slze of Bld s. No
of Floors
,
Max, Ht. of Mater rom Ground at. Bld ,
Losses Prevented by Evacuation or Emergency Preparations N
b
um
er o Hours Morning o Flood Stage
No. of Da s Va ter in Basement On Fl rst floor On Second Floor
Dama a Oc curve Direct Over ow Sewer Bar. -u See a e
Ht. of floors Above or Below Cround Feet Bsmt. Ist 2nd rd
Pe rcenta a of Value of Contents b floors Bs mt. Ist 2nd 3rd
Pe rcenta a of To [al Losses to Bld and Contents b Floors Bsmt, st 2nd re
Mes _Stand-pipe _Flap-gates _Pumpi ng Facilities E ective in Reducing Extent o Damages
On Premises Yes No
Data Collected/Submitted by Title 't
Date
Signature
- ~`.
i
- - MISE ~,-
' _ i "(:~~
RUG-23-2000 14:57 FROM: 70:9709271351 P.002~003
Results of site investigation and instrumentation of the
Keno Gulch landslide/debris-flow source area, Aspen, U-S. Government Publications
Colorado
~'i)1'~'rResults of site investigation and instrumentation of the Keno Gulch
' landslideldebris-flow source area, Aspen, Colorado
by Alan F. Chleborad, William L. Ellis, Dave Kibler.
[microform] /
Author: Chleborad, A. F,
Publisher: [Denver, Colo,] ;
U.S. Dept. of the Interior, U.S. Geological Survey
1997.
Description: 17 leaves
ill., map ;
28 cm.
Series Titlg,:. Open-file report ;
97-717
Call Number: 119.76:97_717
Notes; Includes bibliographical references (leaves 16-17).
Shipping list no.: 99-0312-M.
Subject(s): Landslides
Colorado
Debris avalanches
Colorado
Other Entries: Ellis, William L.
Kibler, D. F.
1940-
,~~, =.~.{David F.),
Geological Survey (U.S.)
b..:,
Page 1 of 1
AUG-23-2000 14:57 FROM: 70:9709271351 P.OO3~OO3
L~~~~~'~
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Marked Remrda Title: Resuifs of Bile investigation and Instrumentation el the Keno Gulch
previous SearGles landslide/debris-flow source area, Aspen, Colorado
Ez2 Author(s): ~. "~_- - -... ... " ~ ~ : .. .__ '.. y ~: (David F.),
Optlonsn~ Publication: [Denver, Cob.] : U.S. Dept. of the Interior, U,S, Geological Survey :Branch of
EnelW EapnM1ol Frmrcxl. Information Servlce6 (dl6trib4tof],
Yoar: 1B97
Descdptloni 171aeves :ill., map ; 28 cm.
Language: Fslgllsh
Serlas: Open-flla report ; 97-717; Variation: U.S. Geological Survey open-file report ;; 97-
717.
SUBJECT(S) -
Descriptor.
Note(s): Shipping list no.: 99-D312-M./ Includes bl6liegraphical references (leaves 18-17)./
Reproduction: Microfiche. [Denver, Colo.: U.S- Geological Survey, 1998?] i
mlcrofirJle :negative. .
Class Descrp[: GPO No: 118.78:87-717; GOVDOC: 119.76:97-717; GPO Item No: 0624-H (MF)
Mere Corp AUth: _..._ _
Document Type: Book -
Accosslon No: OCLC: 41259615
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