HomeMy WebLinkAboutFile Documents.621 W Francis St.0008-2021-BRES (31)
DRAINAGE REPORT
FOR
621 WEST FRANCIS STREET
CITY OF ASPEN, COLORADO
PARCEL ID: 273512426801
PREPARED FOR:
JIM SHUFORD
621 West Francis Street
Aspen, CO 81611
PREPARED BY:
High Country Engineering, Inc.
1517 Blake Avenue, Suite 101
Glenwood Springs, CO 81601
(970) 945-8676
December 22, 2020
Revised April 22, 2021
HCE JOB NUMBER: 2201048.00
05/09/2021
Reviewed by Engineering
"It should be known that this review shall not
relieve the applicant of their responsibility to
comply with the requirements of the City of
Aspen. The review and approval by the City is
offered only to assist the applicant's
understanding of the applicable Engineering
requirements." The issuance of a permit based
on construction documents and other data shall
not prevent the City of Aspen from requiring the
correction of errors in the construction
documents and other data.
06/02/2021
Page 1
j:/sdskproj/217/1613.02/drainagestudy.doc
TABLE OF CONTENTS
SECTION PAGE
I. GENERAL LOCATION AND HISTORIC DESCRIPTION 3
II. DRAINAGE STUDIES 3
III. DRAINAGE DESIGN CRITERIA 4
IV. DRAINAGE FACILITY DESIGN 7
V. CONCLUSION 11
VI. REFERENCES 12
EXHIBITS:
1. Vicinity Map (8.5”x11”)
2. SCS Soils Map (8.5”x11”)
3. FEMA Map (11”x17”)
4. Flow Path to City System (8.5”x11”)
5. Historic Drainage Conditions (24”x36”)
6. Proposed Drainage Conditions (24”x36”)
7. Drainage Detail Sheet (24”x36”)
8. Soil Reports (H-P/Kumar)
Appendices
Hydrologic Computations
Historic Conditions
Proposed Conditions
Hydraulic Computations
Trench Drain Calculations
Swale Calculations
Weir Calculations
Pond Calculations
Aspen Charts and Figures
05/09/2021
Page 2
j:/sdskproj/217/1613.02/drainagestudy.doc
Engineers Certification
“I hereby affirm that this report and the accompanying plans for the reconstruction of garage
and house remodel at 621 W. Francis Street was prepared by me (or under my direct
supervision) for the owners thereof in accordance with the provisions of the City of Aspen
Urban Runoff Management Plan and approved variances are exceptions listed thereto. I
understand that it is the policy of the City of Aspen that the City of Aspen does not and will not
assume liability for drainage facilities designed by others.”
License No. __29975__________
Roger D. Neal, P.E.
Licensed Professional Engineer, State of Colorado
05/09/2021
Page 3
j:/sdskproj/217/1613.02/drainagestudy.doc
I. GENERAL LOCATION AND DESCRIPTION
A. Location
The site is located at 621 West Francis Street within the City of Aspen, County of Pitkin, State
of Colorado, the second lot east of North 6th Street along the south side of West Francis Street.
A Vicinity Map has been included as Exhibit #1.
B. Description of Existing Property
The proposed site is approximately 5,981 square-feet (0.14 acres). The existing lot consists of
a home, basement patio, second-story deck, driveway, garage slab, paths, landscaping and
numerous trees. The site is bordered by a private property to the east and west, a gravel alley
to the south, and West Francis Street to the north. The site drains from the south to the north
and offsite in West Francis Street then enters a storm inlet at the intersection of West Francis
Street and North 6th Street. Existing grades range from approximately 2-percent to 10-percent.
C. Soils Description
Kumar and Associates, Inc. on November 24, 2017, project number 20-7-692, completed a
site-specific geotechnical soil testing. The geotechnical study describes the site as having about
four feet of granular fill material, below the topsoil, overlying dense, silty sandy gravel with
cobbles down to the boring depth of 12.5 feet. There was no free water encountered in the
boring at the time of excavation, and the subsoils were slightly moist. The report recommends
bio-swales for the tested percolation rate should extend down into the natural granular soil
below the topsoil and fill soils. A percolation test conducted in the completed 4-inch diameter
borehole indicate suitable infiltration rates of 0.7-2.5 minutes per inch for bioretention. The
testing results have been attached to this report as exhibit #8. The site is also well above the
river elevation, and groundwater was not encountered to the borehole depth of over 12 feet.
The City of Aspen soils map locates this site in the Type “B” soils area. According to the
USDA Web Soil Survey, the property is within section 107 that consists of soil Type “B”; see
USDA Web Soil Survey exhibit #2.
II. DRAINAGE STUDIES
A. Major Drainage Way Planning and Influential Parameters
The site is located within FEMA’s major drainage study of the area on its Flood Insurance
Rate Map (FIRM) No. 08097C0354E which has an effective date of August 15, 2019. The
area of interest within the site is located in Zone X. This zone is described as areas determined
to be outside the 0.2% chance floodplain. Refer to Exhibit #3 for the FEMA map.
Mud flow was not analyzed for the site since the site is located outside of the Mud Flow Zone
as indicated in the Storm Drainage Master Plan for the City of Aspen, Colorado by WRC
Engineering, Inc. in November of 2001.
05/09/2021
Page 4
j:/sdskproj/217/1613.02/drainagestudy.doc
B. Previous Drainage Studies
Per the November 2001 study completed by WRC Engineering, Inc. titled, “Storm Drainage
Master Plan for the City of Aspen, Colorado,” the site is located within System 3. According
to the City of Aspen Master Drainage Plan figure ES-3, an existing 15-inch HDPE pipe travels
along West Francis Street, collecting stormwater from street area inlets, to an adjoining 36-
inch HDPE pipe at the intersection of North Garmisch Street and West Francis Street. This 36-
inch HDPE pipe conveys runoff to the outfall point.
C. Receiving System and Effects of Adjacent Drainage Issues
There are no major drainage issues with the adjacent properties that affect the site or that the
site affects. The existing site directly discharges a majority of the runoff to the north property
line. The existing flow leaves the property to the north, then travels west along the south side
of West Francis Street. The runoff will enter catch basins at the intersection of West Francis
Street and North 6th Street. Once captured in the City of Aspen catch basin, the runoff will be
conveyed with a 36” HDPE pipe to the outfall point at the Jennie Adair Wetlands.
III. DRAINAGE DESIGN CRITERIA
A. Criteria
This drainage study was prepared in conformance with the City of Aspen, Colorado Urban
Runoff Management Plan (URMP), dated April of 2010 and the revised sections dated
thereafter. More than 1,000 square feet of area will be disturbed with the proposed remodel
and exterior improvements; therefore, the site is viewed as a Major Project per the URMP.
More than 1,000 square feet are being disturbed and more than 25-percent of the overall site is
being disturbed, so water quality for the entire site will be necessary per the URMP. The
existing site was analyzed in its historic condition (i.e. no improvements).
Water Quality Capture Volume (WQCV) will be determined for the site that will undergo site
grading as per the URMP standards. The WQCV is defined as the treatment for up to the 80th
percentile runoff event, corresponding to between a 6-month to 1-year event. The WQCV was
determined using the equations and Figure 8.13 from Chapter 8 of the URMP. The WQCV
equation is: Volume (ft3) =WQCV (watershed-inches) x 1/12(ft./in) x area(acres) x 43,560
ft2/acre. A tree canopy credit outlined in the URMP has been utilized to determine the
required WQCV, as shown on the proposed drainage map exhibit #6. The runoff for the
proposed basins will be routed through a series of swales receiving storm water from sheet
flow and downspouts to two bioretention areas/planters, where the runoff will be treated for
WQCV.
B. Hydrologic Criteria
The hydrologic methods for this study are outlined in the URMP from the City of Aspen,
Colorado (April, 2010) and the Microsoft Excel spreadsheet for the Rational Method.
05/09/2021
Page 5
j:/sdskproj/217/1613.02/drainagestudy.doc
The rainfall amounts for each basin were obtained using Figure 2.1 “IDF Curves for Aspen,
Colorado” in the URMP publication from the City of Aspen, Colorado. Using these curves, the
rainfall intensity corresponding to the 2-yr, 1-hr storm 10-yr, 1-hr storm, and 100-yr, 1-hr
storm event were determined based on the time of concentration for each basin.
Figure 3.3 from the URMP was used to determine the runoff coefficients for the 2-year, 10-
year and 100-year storm events since the soils were determined to be type ‘B’ soils.
For areas within the Aspen Mountain Drainage Basin capable of discharging runoff into the
City’s system without impacting neighboring properties, detention is not required beyond
WQCV. The site meets the previously mentioned requirements, thus the volume for detention
was calculated for WQCV only, as outlined in the URMP. Exhibit 4 shows the flow path to the
City’s system.
The bioretention areas were sized to handle the WQCV, but do not detain the 10-year and 100-
year runoff per the URMP. Type ‘B’ soils were determined for the site per the NRCS Soil Map
for Aspen and confirmed by the USDA Web Soil Survey.
All charts and figures mentioned from the URMP are located in the last section of the
appendices under the “Aspen Charts/Figures” section.
C. Hydraulic Criteria
The swales, trench drain, and weir within the system have been calculated utilizing Hydroflow
Express with AutoCAD’s analyze system. All drainage features and structures have the ability
to carry tributary basin design flows anticipated in a major rain event. See basin descriptions
below for explanation.
D. Site Constraints
There are no utilities, streets or structures that cause major site constraints for the drainage
system design. Numerous trees have been considered in the drainage design to prevent impacts
as much as possible. A goal was to keep as much of the existing site undisturbed as possible
while improving the existing conditions. The site is in close proximity with adjacent
properties
E. Easements and Irrigation Facilities
There are no major drainage ways, drainage easements or tracts located on the site. A
transformer easement exists for the transformer located by the south property boundary.
Irrigation facilities onsite will be improved and should not affect the overall proposed
development.
05/09/2021
Page 6
j:/sdskproj/217/1613.02/drainagestudy.doc
F. Low Impact Site Design
Three bioretention areas will be implemented to allow for the capture of the required WQCV
per the URMP code. Should the bioretention areas exceed the WQCV capacity, runoff will
reach the City of Aspen drainage system and outfall in the Jennie Adair Wetlands where water
is absorbed and naturally treated before reaching the Roaring Fork River.
G. 9 Principles
The 9 Principles for storm water quality management were followed during the design process
to create the best storm water design and water quality management. The following is a
summary of compliance with the Storm Drainage Principles outlined in the City of Aspen
Urban Runoff Management Plan:
1. Consider storm water quality needs early in the design process
Storm water quality needs were considered early in the design process, as
recommended.
2. Use the entire site when planning for storm water quality treatment.
With the use of two bioretention ponds and vegetated swales, the majority of the
site is utilized for water quality treatment while attempting to cause the least
amount of disturbance possible.
3. Avoid unnecessary impervious area
Efforts were made to avoid unnecessary impervious areas in drainage design. Some
existing impervious areas will be redeveloped, but the site will have an overall
decrease in impervious area by the removal of an existing flagstone path that will be
replaced with gravel.
4. Reduce runoff rates and volumes to more closely match natural conditions
Runoff rates and volumes have been reduced, as recommended, by implementing a
series on bioretenion areas connected by vegetated swales. All impervious areas will
drain to at least one bioretention area. The lot overflow is approximately 100 feet
from the city storm sewer.
5. Integrate storm water quality management and flood control
Two proposed bioretention areas capture runoff onsite and are connected by
vegetated swales in the case of overflow. Downstream swales have been sized to
accommodate overflow from upstream basins. The south bioretention area will
overflow through a proposed, open-ended trench drain that will direct the excess
runoff to swale that’s conveys the stormwater to the north bioretention area. The
north bioretention area incorporates a weir to discharge overflow in a controlled
manner. The proposed site has been designed for water quality only, which does not
provide flood control detention, but has an overflow path to suitably convey runoff
into the City of Aspen’s drainage system.
05/09/2021
Page 7
j:/sdskproj/217/1613.02/drainagestudy.doc
6. Develop storm water quality facilities that enhance the site and environment.
The proposed water quality facilities enhance the site and the environment with
bioretention areas that will become part of the landscape.
7. Use a treatment train approach
The treatment train approach has been implemented by incorporating bioretention
areas connected by vegetated swales.
8. Design sustainable facilities that can be safely maintained
The proposed storm water quality facilities have been designed to be easily
accessible and safely maintained, as recommended.
9. Design and maintain facilities with public safety in mind
The proposed storm water quality facilities have been designed with public safety in
mind, as requested. For example, the south bioretention area has been designed with
a more gradual sloped area for access to the yard and the close proximity to foot
traffic.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
The proposed construction calls for the interior remodel of the existing residence with a small,
64 square-foot addition for storage units. The detached garage is being rebuilt after the existing
garage was demolished following severe fire damage. The existing garage foundation and slab
will be utilized for the rebuild. The existing, short driveway that connects the garage to the
alley will remain. The impervious areas for the entire site will be treated for WQCV by
bioretention areas. Runoff will be routed through sheet flow and drainage swales receiving
runoff from downspouts and hardscape. The bioretention ponds will have partial impervious
membranes, foundation waterproofing and the south pond will only infiltrate soils lower than
the existing foundation wall to prevent impacts on existing or proposed facilities. Runoff,
greater than WQCV, will leave the site in historical fashion to the City of Aspen’s drainage
system.
B. Historic Drainage Basins Descriptions
The proposed site’s historic drainage pattern is from the south to the northwest and offsite to
the West Francis Street right-of-way before being captured by the City of Aspen’s catch basin
and conveyed to the Jennie Adair wetlands. The existing site has been analyzed in its historic
conditions.
The historic site has been broken into one on-site basin. Refer to sheet EXDR (exhibit #5) for
a map of existing basin layout.
05/09/2021
Page 8
j:/sdskproj/217/1613.02/drainagestudy.doc
Historic Flow Path One:
Runoff from basin EX-1 sheet flows northwest from the south property boundary. Design
point one has been associated with the basin and is the location of the historic discharge point
from the site. Table 1 below is a summary of the existing basin information.
There are no negative impacts from the runoff to the adjacent properties because the runoff has
low discharge rates that sheet flow across permeable land and drains directly into the right-of-
way captured by the City of Aspen storm system. No runoff drains directly to any downstream
structures. Refer to Exhibit #5 in the appendices for the existing basin delineation and
information. Table 1, below, is a summary of the existing basin information.
Table 1. Historic Basin Characteristic
C. Proposed Basin Description
The proposed site has been separated into two proposed onsite drainage basins.
Proposed basin PR-1 encompasses the south and east portion of the site and consists of over
half of the residence including hardscape, the garage, the south landscape and back yard. The
runoff from the garage roof is captured by a vegetated swale that passes through an open ended
trench drain and discharges into a proposed bioretention area. A downspout from the east
portion of the remodeled roof and a trench drain in the walkway from the garage to residence
will discharge directly into the south bioretention pond. Runoff from the yard will sheet flow
into the previously mentioned pond. The pond has been designed with an impermeable
membrane in the areas nearest to the building and tree driplines. The pond area without the
impermeable membrane has screened rock to allow runoff to infiltrate beneath the existing
patio wall. Because of this deep rock layer needed to accomplish this, the impermeable liner
limits the potential impacts to surrounding trees. Design point one is located west of the trench
drain and is the proposed location of basin PR-1 overflow. Should the PR-1 bioretention over
exceed the required WQCV amount, runoff will backflow through the trench drain and
overflow into a shallow swale and travel along the west property line before entering the north
bioretention pond. The trench drain and swales were sized to adequately convey PR-1 design
flows. A weir flow calculation was performed to verify the trench drain has capacity discharge
100-yr design flows in the event of the south pond overflow. The previously mentioned
calculation controlled the trench drain sizing, because the trench grate captures a small portion
of basin PR-1 runoff. The PR-1 pond (south) was sized to have a required flat area of 163.9
square-feet with a depth of approximately 2.2 inches. The pond has a deep rock reservoir in the
portion of the pond that is without an impervious liner to allow storm water to infiltrate
beneath the foundation wall. The previously mentioned calculations can be found in the
BASIN AREA,
ACRES
C,
10YR I, 10YR Q10-YEAR,
CFS C, 100YR I, 100 YR Q100-YEAR,
CFS
EX-1 0.14 0.15 2.26 0.05 0.35 3.61 0.17
ONSITE
TOTAL 0.05 ONSITE
TOTAL 0.17
05/09/2021
Page 9
j:/sdskproj/217/1613.02/drainagestudy.doc
appendices of this report. Pond section details can be found on exhibit #7 drainage details.
Proposed basin PR-2 encompasses the west portion of the residence, side yard and front yard.
A drainage swale west of the existing residence receives runoff from downspouts and sheet
flow, and discharges into the north bioretention area. There is a swale in the front yard that
collects a small portion of site runoff and discharges into the north pond. This bioretention
area also directly receives sheet flow from the surrounding pervious areas, and overflow from
the south bioretention pond. Should the bioretention facility reach over capacity, the storm
drainage will spill over a weir into the right-of-way and sheet flow to West Francis Street.
Design point 2 is associated with the site overflow at the weir. The proposed weir will have a
36 inch long crest that is 2 inches deep. The flow depth over the crest during a 100-year event
was determined to be 1.5 inches deep. The north proposed bioretention pond has a flat area of
37.2 square-feet with a minimum depth of 3.7 inches. Pond and weir calculations can be found
in the appendices at the end of this report.
The tree canopy credit outlined in the URMP was used in the calculation of effective
impervious areas to determine WQCV for each basin. When coniferous and deciduous
canopies overlap, the tree with the larger diameter dripline was the area used.
See Exhibit #6 for the proposed basin delineation and tree canopy credit used in calculations.
Table 2 and Table 3, below, is a summary of the proposed (developed) basins hydrology and
WQCV, respectively.
Table 2. Proposed (Developed) Basin Characteristics
BASIN AREA,
ACRES
C,
10YR I, 10YR Q10-YEAR,
CFS C, 100YR I, 100 YR Q100-YEAR,
CFS
PR-1 0.086 0.40 3.85 0.13 0.52 6.14 0.28
PR-2 0.051 0.32 3.96 0.06 0.47 6.33 0.15
ONSITE
TOTAL 0.19 ONSITE
TOTAL 0.43
Table 3. Proposed WQCV Table
BASIN
AREA
(S.F.)
IMPERVIOUS
AREA (SF)
EFFECTIVE
IMPERVIOUS
(%)
WQCV
(watershed
-inches) WQCV (CF)
PR-1 3,754 2,165.7 50.1 0.096 30.0
PR-2 2,227 918 30.5 0.062 11.5
TOTAL 5,981 3,058 42.4 0.08 39.9
05/09/2021
Page 10
j:/sdskproj/217/1613.02/drainagestudy.doc
D. Downstream Impacts
The proposed onsite grading and detention facilities will have positive downstream impacts
during frequent storm events by capturing and treating the onsite WQCV. This will result in
less flow from the site during frequent storm events. There are no downstream facilities from
the site to be negatively impacted by the site’s improvements.
The onsite runoff will leave the site after cleansed in the water quality facilities thus preventing
the spread of pollutants downstream.
If the bioretention facilities surpass the WQCV and additional storage, the system will allow
runoff to overflow the north pond weir before entering West Francis Street, as it did
historically. Calculations for the proposed drainage facilities are included in the appendices of
this report under the Facility Calculations section.
E. Operation and Maintenance
The proposed drainage facilities are to be constructed in conformance with the City of Aspen
Urban Runoff Management Plan, dated April 2010 and revised thereafter.
The grass conveyance swales will be kept up with regular mowing with a bagger to keep fine
particles out of the upper layer of the system as much as possible. The swales will also need to
be inspected after heavy rainstorms to remove any large debris that may have collected and
they will be raked quarterly and during the fall months prior to snow fall to remove any built
up tree debris that may fall during those months so they are prepared for spring thaw flows.
The bio-retention basins will need to be inspected and maintained quarterly to make sure that
the reservoirs have not become clogged and that the reservoirs are functioning properly. Debris
and liter removal shall occur routinely.
The perforated pipe beneath the surface of the south bioretention pond will need to be
inspected and maintained quarterly for clogging and standing water. Clean outs can be used to
inspect pipes by cameras to determine the location of clogging or collapsed pipe. If clogging or
standing water is observed, thoroughly run a sewer snake through the pipe to unclog. Should
clogging or standing water remain, the growing media mix layer shall be removed and the
perforated pipe shall be exposed and cleaned. After cleaning the perforated pipe, discard old
growing media mix, wrap perforated pipe with filter fabric sock, and add new growing media
mix. It is not recommended to jet clear the perforated pipe to prevent the filter fabric from
tearing. Solid pipes may be jetted clear if clogged.
The owners or owner’s representative will be responsible for the maintenance and upkeep of
the drainage facilities. The property owner shall dispose of sediment and any other waste
material removed from a reservoir at suitable disposal sites and in compliance with local, state,
and federal waste regulations.
05/09/2021
Page 11
j:/sdskproj/217/1613.02/drainagestudy.doc
This project includes “Low Impact Site Design” to mimic the natural pre-development
hydraulic pattern. Storm water runoff is to be in contact with soils and plants prior to reaching
the City of Aspen right-of-way. The plants and soil are to act as filters to remove pollutants.
The proposed plants and soils are present along the lengths of proposed graded swales and
within the proposed bio-retention basins.
V. CONCLUSION
A. Compliance with Standards
This drainage report has been prepared in accordance with City of Aspen Regulations. The
proposed bio retention ponds will capture and treat the proposed WQCV for all impervious
areas added to the site.
B. Drainage Concept
The proposed drainage design will be effective in controlling any adverse downstream impacts
on landowners or structures. Water quality issues will be minimal as the runoff will be
intercepted and routed to the proposed bioretention ponds.
05/09/2021
Page 12
j:/sdskproj/217/1613.02/drainagestudy.doc
VI. REFERENCES
Autodesk, Inc. Hydraflow Express Extension for Autodesk AutoCAD Civil 3D. Version 10.5
City of Aspen, Colorado: Urban Runoff Management Plan. April 2010.
Kumar and Associates, Inc. “Percolation Test Results for Drywell or Bio-Swale Design, 621
West Francis Street, Aspen, Colorado” dated December 4, 2020, Job Number 20-7-692.
United States Department of Agriculture, Soil Conservation Service: Soil Survey of Aspen-
Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties, May 1992.
Urban Drainage and Flood Control District: Urban Storm Drainage Criteria Manual.
Volume III. August 2013. www.udfcd.org.
WRC Engineering, Inc. Storm Drainage Master Plan for the City of Aspen, Colorado.
November 2001.
05/09/2021
APPENDICES
05/09/2021
EXHIBITS
05/09/2021
scale:
page:
Project No:
drawn by:
checked by:
date:
file:
HIGH COUNTRY ENGINEERING, INC.
1517 BLAKE AVENUE, STE 101,
GLENWOOD SPRINGS, CO 81601
PHONE (970) 945-8676 FAX (970) 945-2555
www.hceng.com
EXHIBIT 1
2201048.00
1" = 1000'
FLOWPATH.DWG
12/21/20
RDN
BDB
VICINITY MAP
621 W. FRANCIS STREET
CITY OF ASPEN, COLORADO
JIM SHUFORD
05/09/2021
scale:
page:
Project No:
drawn by:
checked by:
date:
file:
HIGH COUNTRY ENGINEERING, INC.
1517 BLAKE AVENUE, STE 101,
GLENWOOD SPRINGS, CO 81601
PHONE (970) 945-8676 FAX (970) 945-2555
www.hceng.com
EXHIBIT 2
2201048.00
1" = 50'
FLOWPATH.DWG
12/21/20
RDN
BDB
SOILS MAP
USDA WEB SOIL SURVEY
621 W. FRANCIS STREET
CITY OF ASPEN, COLORADO
JIM SHUFORD
05/09/2021
BY
NO
.
DA
T
E
BY
PROJECT NO.
OR
5
3
4
-
0
6
7
0
0
I
N
M
E
T
R
O
D
E
N
V
E
R
UN
D
E
R
G
R
O
U
N
D
M
E
M
B
E
R
U
T
I
L
I
T
I
E
S
EX
C
A
V
A
T
E
F
O
R
T
H
E
M
A
R
K
I
N
G
O
F
BE
F
O
R
E
Y
O
U
D
I
G
,
G
R
A
D
E
,
O
R
CA
L
L
2
-
B
U
S
I
N
E
S
S
D
A
Y
S
I
N
A
D
V
A
N
C
E
1-
8
0
0
-
9
2
2
-
1
9
8
7
CE
N
T
E
R
O
F
C
O
L
O
R
A
D
O
CA
L
L
U
T
I
L
I
T
Y
N
O
T
I
F
I
C
A
T
I
O
N
RE
V
I
S
I
O
N
dr
a
w
n
b
y
:
ch
e
c
k
e
d
b
y
:
da
t
e
:
fi
l
e
:
HI
G
H
C
O
U
N
T
R
Y
E
N
G
I
N
E
E
R
I
N
G
,
I
N
C
.
PH
O
N
E
(
9
7
0
)
9
4
5
-
8
6
7
6
-
F
A
X
(
9
7
0
)
9
4
5
-
2
5
5
5
ww
w
.
h
c
e
n
g
.
c
o
m
15
1
7
B
L
A
K
E
A
V
E
N
U
E
,
S
T
E
1
0
1
,
GL
E
N
W
O
O
D
S
P
R
I
N
G
S
,
C
O
8
1
6
0
1
FE
M
A
.
d
w
g
12
/
2
2
/
2
0
RD
N
BD
B
FE
M
A
M
A
P
62
1
W
E
S
T
F
R
A
N
C
I
S
S
T
R
E
E
T
CI
T
Y
O
F
A
S
P
E
N
,
C
O
L
O
R
A
D
O
JI
M
S
H
U
F
O
R
D
3
EXHIBIT
2201048.00
05/09/2021
scale:
page:
Project No:
drawn by:
checked by:
date:
file:
HIGH COUNTRY ENGINEERING, INC.
1517 BLAKE AVENUE, STE 101,
GLENWOOD SPRINGS, CO 81601
PHONE (970) 945-8676 FAX (970) 945-2555
www.hceng.com
EXHIBIT 4
2201048.00
1" = 30'
FLOWPATH.DWG
11/16/20
RDN
BDB
CITY OF ASPEN STORM SYSTEM
SITE OVERFLOW PATH TO
621 W. FRANCIS STREET
CITY OF ASPEN, COLORADO
JIM SHUFORD
05/09/2021
BY
NO
.
DA
T
E
PROJECT NO.
RE
V
I
S
I
O
N
HI
G
H
C
O
U
N
T
R
Y
E
N
G
I
N
E
E
R
I
N
G
,
I
N
C
.
PH
O
N
E
(
9
7
0
)
9
4
5
-
8
6
7
6
-
F
A
X
(
9
7
0
)
9
4
5
-
2
5
5
5
ww
w
.
h
c
e
n
g
.
c
o
m
dr
a
w
n
b
y
:
ch
e
c
k
e
d
b
y
:
da
t
e
:
fi
l
e
:
15
1
7
B
L
A
K
E
A
V
E
N
U
E
,
S
T
E
1
0
1
,
GL
E
N
W
O
O
D
S
P
R
I
N
G
S
,
C
O
8
1
6
0
1
COLORADO 811
CALL BEFORE
YOU DIG
Utility Notification
Center of Colorado
2201048.00
EXHIBIT
5
JI
M
S
H
U
F
O
R
D
CI
T
Y
O
F
A
S
P
E
N
,
C
O
L
O
R
A
D
O
62
1
W
E
S
T
F
R
A
N
C
I
S
S
T
R
E
E
T
HI
S
T
O
R
I
C
D
R
A
I
N
A
G
E
BA
S
I
N
M
A
P
BD
B
RD
N
12
/
2
2
/
2
0
EX
D
R
05/09/2021
3 STORY2 UNITREEDS HOUSE CONDOMINIUM
DN
UP
DN
BY
NO
.
DA
T
E
PROJECT NO.
RE
V
I
S
I
O
N
HI
G
H
C
O
U
N
T
R
Y
E
N
G
I
N
E
E
R
I
N
G
,
I
N
C
.
PH
O
N
E
(
9
7
0
)
9
4
5
-
8
6
7
6
-
F
A
X
(
9
7
0
)
9
4
5
-
2
5
5
5
ww
w
.
h
c
e
n
g
.
c
o
m
dr
a
w
n
b
y
:
ch
e
c
k
e
d
b
y
:
da
t
e
:
fi
l
e
:
15
1
7
B
L
A
K
E
A
V
E
N
U
E
,
S
T
E
1
0
1
,
GL
E
N
W
O
O
D
S
P
R
I
N
G
S
,
C
O
8
1
6
0
1
COLORADO 811
CALL BEFORE
YOU DIG
Utility Notification
Center of Colorado
2201048.00
EXHIBIT
6
JI
M
S
H
U
F
O
R
D
CI
T
Y
O
F
A
S
P
E
N
,
C
O
L
O
R
A
D
O
62
1
W
E
S
T
F
R
A
N
C
I
S
S
T
R
E
E
T
PR
O
P
O
S
E
D
D
R
A
I
N
A
G
E
BA
S
I
N
M
A
P
BD
B
RD
N
12
/
2
2
/
2
0
PR
D
R
1
04
/
1
9
/
2
1
NE
W
A
R
C
H
I
T
E
C
T
U
R
A
L
B
A
C
K
G
R
O
U
N
D
S
05/09/2021
BY
NO
.
DA
T
E
PROJECT NO.
RE
V
I
S
I
O
N
HI
G
H
C
O
U
N
T
R
Y
E
N
G
I
N
E
E
R
I
N
G
,
I
N
C
.
PH
O
N
E
(
9
7
0
)
9
4
5
-
8
6
7
6
-
F
A
X
(
9
7
0
)
9
4
5
-
2
5
5
5
ww
w
.
h
c
e
n
g
.
c
o
m
dr
a
w
n
b
y
:
ch
e
c
k
e
d
b
y
:
da
t
e
:
fi
l
e
:
15
1
7
B
L
A
K
E
A
V
E
N
U
E
,
S
T
E
1
0
1
,
GL
E
N
W
O
O
D
S
P
R
I
N
G
S
,
C
O
8
1
6
0
1
COLORADO 811
CALL BEFORE
YOU DIG
Utility Notification
Center of Colorado
2201048.00
EXHIBIT
7
JI
M
S
H
U
F
O
R
D
CI
T
Y
O
F
A
S
P
E
N
,
C
O
L
O
R
A
D
O
62
1
W
E
S
T
F
R
A
N
C
I
S
S
T
R
E
E
T
DR
A
I
N
A
G
E
D
E
T
A
I
L
S
BD
B
RD
N
12
/
2
2
/
2
0
DE
T
-
0
1
05/09/2021
05/09/2021
05/09/2021
05/09/2021
05/09/2021
05/09/2021
05/09/2021
HYDROLOGIC
COMPUTATIONS
05/09/2021
HISTORIC CONDITIONS
05/09/2021
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 11-06-20 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
SL
O
P
E
(
%
)
CH
A
N
N
E
L
F
L
O
W
(C
F
S
)
DE
S
I
G
N
F
L
O
W
(C
F
S
)
SL
O
P
E
(
%
)
PI
P
E
S
I
Z
E
(
I
N
C
H
E
S
)
LE
N
G
T
H
(
F
T
)
VE
L
O
C
I
T
Y
(
F
P
S
)
Tt
(
M
I
N
)
(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)
DESIGN POINT 12.260.137
REMARKS
CHANNEL
EX-11 15.60.15 0.02 0.05
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF
STRUCTURE NO.
TOTAL RUNOFF
PROJECT: 621 W. FRANCIS STREET
JOB NO. 2201048.00
PIPE
DESIGN STORM: EXISTING 10 YEAR
TRAVEL TIME
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 11-06-20 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
SL
O
P
E
(
%
)
CH
A
N
N
E
L
F
L
O
W
(C
F
S
)
DE
S
I
G
N
F
L
O
W
(C
F
S
)
SL
O
P
E
(
%
)
PI
P
E
S
I
Z
E
(
I
N
C
H
E
S
)
LE
N
G
T
H
(
F
T
)
VE
L
O
C
I
T
Y
(
F
P
S
)
Tt
(
M
I
N
)
(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)
0.17 DESIGN POINT 1
TRAVEL TIME
REMARKS
1 EX-1 0.137 0.35 15.6 0.05 3.61
JOB NO. 2201048.00
PROJECT: 621 W. FRANCIS STREET
DESIGN STORM: EXISTING 100 YEAR
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF CHANNEL PIPE
05/09/2021
PROPOSED CONDITIONS
05/09/2021
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 11-06-20 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
SL
O
P
E
(
%
)
ST
R
E
E
T
F
L
O
W
(
C
F
S
)
DE
S
I
G
N
F
L
O
W
(
C
F
S
)
SL
O
P
E
(
%
)
PI
P
E
S
I
Z
E
(
I
N
C
H
E
S
)
LE
N
G
T
H
(
F
T
)
VE
L
O
C
I
T
Y
(
F
P
S
)
Tt
(
M
I
N
)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
Design Point 20.01 2.42 0.022 PR-2 0.051 0.18 5.00
TRAVEL TIMEPIPESTREETTOTAL RUNOFF
PROJECT: 621 W. FRANCIS STREET
JOB NO. 2201048.00
DIRECT RUNOFF
0.06
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
REMARKS
DESIGN STORM: PROPOSED 2 YEAR
0.30 Design Point 11 PR-1 5.420.086 0.03 2.35
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 11-06-20 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
SL
O
P
E
(
%
)
ST
R
E
E
T
F
L
O
W
(
C
F
S
)
DE
S
I
G
N
F
L
O
W
(
C
F
S
)
SL
O
P
E
(
%
)
PI
P
E
S
I
Z
E
(
I
N
C
H
E
S
)
LE
N
G
T
H
(
F
T
)
VE
L
O
C
I
T
Y
(
F
P
S
)
Tt
(
M
I
N
)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
JOB NO. 2201048.00
PROJECT: 621 W. FRANCIS STREET
DESIGN STORM: PROPOSED 10 YEAR
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE TRAVEL TIME
REMARKS
1 PR-1 0.086 0.40 5.42 0.03 3.85 0.13 Design Point 1
2 PR-2 0.051 0.32 5.00 Design Point 20.02 3.96 0.06
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 11-06-20 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
R
)
Q
(
C
F
S
)
SL
O
P
E
(
%
)
ST
R
E
E
T
F
L
O
W
(
C
F
S
)
DE
S
I
G
N
F
L
O
W
(
C
F
S
)
SL
O
P
E
(
%
)
PI
P
E
S
I
Z
E
(
I
N
C
H
E
S
)
LE
N
G
T
H
(
F
T
)
VE
L
O
C
I
T
Y
(
F
P
S
)
Tt
(
M
I
N
)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
JOB NO. 2201048.00
PROJECT: 621 W. FRANCIS STREET
DESIGN STORM: PROPOSED 100 YEAR
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE TRAVEL TIME
REMARKS
1 PR-1 0.086 0.52 5.42 0.04 6.14 0.28 Design Point 1
2 PR-2 0.051 0.47 5.00 Design Point 20.02 6.33 0.15
05/09/2021
HYDRAULIC
COMPUTATIONS
05/09/2021
SWALE CALCULATIONS
05/09/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Nov 13 2020
SWALE 2YR CAPACITY
Triangular
Side Slopes (z:1) = 3.00, 10.00
Total Depth (ft) = 0.33
Invert Elev (ft) = 7906.41
Slope (%) = 2.00
N-Value = 0.026
Calculations
Compute by: Known Q
Known Q (cfs) = 0.08
Highlighted
Depth (ft) = 0.11
Q (cfs) = 0.080
Area (sqft) = 0.08
Velocity (ft/s) = 1.02
Wetted Perim (ft) = 1.45
Crit Depth, Yc (ft) = 0.10
Top Width (ft) = 1.43
EGL (ft) = 0.13
0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
Elev (ft) Depth (ft)Section
7906.00 -0.41
7906.25 -0.16
7906.50 0.09
7906.75 0.34
7907.00 0.59
Reach (ft)
05/09/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Nov 13 2020
SWALE 100YR CAPACITY
Triangular
Side Slopes (z:1) = 3.00, 10.00
Total Depth (ft) = 0.33
Invert Elev (ft) = 7906.41
Slope (%) = 2.00
N-Value = 0.026
Calculations
Compute by: Known Q
Known Q (cfs) = 0.43
Highlighted
Depth (ft) = 0.20
Q (cfs) = 0.430
Area (sqft) = 0.26
Velocity (ft/s) = 1.65
Wetted Perim (ft) = 2.64
Crit Depth, Yc (ft) = 0.20
Top Width (ft) = 2.60
EGL (ft) = 0.24
0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
Elev (ft) Depth (ft)Section
7906.00 -0.41
7906.25 -0.16
7906.50 0.09
7906.75 0.34
7907.00 0.59
Reach (ft)
05/09/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Nov 6 2020
PR-1+PR-2 SWALE 10YR CAPACITY
Triangular
Side Slopes (z:1) = 3.00, 3.00
Total Depth (ft) = 0.50
Invert Elev (ft) = 7907.00
Slope (%) = 2.00
N-Value = 0.026
Calculations
Compute by: Known Q
Known Q (cfs) = 0.19
Highlighted
Depth (ft) = 0.20
Q (cfs) = 0.190
Area (sqft) = 0.12
Velocity (ft/s) = 1.58
Wetted Perim (ft) = 1.26
Crit Depth, Yc (ft) = 0.20
Top Width (ft) = 1.20
EGL (ft) = 0.24
0 .5 1 1.5 2 2.5 3 3.5 4
Elev (ft) Depth (ft)Section
7906.75 -0.25
7907.00 0.00
7907.25 0.25
7907.50 0.50
7907.75 0.75
7908.00 1.00
Reach (ft)
05/09/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Nov 6 2020
PR-1+PR-2 SWALE 100YR CAPACITY
Triangular
Side Slopes (z:1) = 3.00, 3.00
Total Depth (ft) = 0.50
Invert Elev (ft) = 7907.00
Slope (%) = 2.00
N-Value = 0.026
Calculations
Compute by: Known Q
Known Q (cfs) = 0.43
Highlighted
Depth (ft) = 0.27
Q (cfs) = 0.430
Area (sqft) = 0.22
Velocity (ft/s) = 1.97
Wetted Perim (ft) = 1.71
Crit Depth, Yc (ft) = 0.27
Top Width (ft) = 1.62
EGL (ft) = 0.33
0 .5 1 1.5 2 2.5 3 3.5 4
Elev (ft) Depth (ft)Section
7906.75 -0.25
7907.00 0.00
7907.25 0.25
7907.50 0.50
7907.75 0.75
7908.00 1.00
Reach (ft)
05/09/2021
TRENCH DRAIN CALCULATIONS
05/09/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Thursday, Dec 17 2020
TRENCH DRAIN CAPACITY AT 10YR EVENT
Rectangular
Bottom Width (ft) = 0.67
Total Depth (ft) = 0.26
Invert Elev (ft) = 7907.43
Slope (%) = 1.00
N-Value = 0.013
Calculations
Compute by: Known Q
Known Q (cfs) = 0.13
Highlighted
Depth (ft) = 0.10
Q (cfs) = 0.130
Area (sqft) = 0.07
Velocity (ft/s) = 1.94
Wetted Perim (ft) = 0.87
Crit Depth, Yc (ft) = 0.11
Top Width (ft) = 0.67
EGL (ft) = 0.16
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
7907.00 -0.43
7907.25 -0.18
7907.50 0.07
7907.75 0.32
7908.00 0.57
Reach (ft)
05/09/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Thursday, Dec 17 2020
TRENCH DRAIN CAPACITY AT 100YR EVENT
Rectangular
Bottom Width (ft) = 0.67
Total Depth (ft) = 0.26
Invert Elev (ft) = 7907.43
Slope (%) = 1.00
N-Value = 0.013
Calculations
Compute by: Known Q
Known Q (cfs) = 0.28
Highlighted
Depth (ft) = 0.17
Q (cfs) = 0.280
Area (sqft) = 0.11
Velocity (ft/s) = 2.46
Wetted Perim (ft) = 1.01
Crit Depth, Yc (ft) = 0.18
Top Width (ft) = 0.67
EGL (ft) = 0.26
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
7907.00 -0.43
7907.25 -0.18
7907.50 0.07
7907.75 0.32
7908.00 0.57
Reach (ft)
05/09/2021
Weir Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Dec 21 2020
WEIR FLOW CAPACITY AT TRENCH DRAIN FOR 100YR POND A OVERFLOW
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 0.67
Total Depth (ft) = 0.26
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Depth
Known Depth (ft) = 0.26
Highlighted
Depth (ft) = 0.26
Q (cfs) = 0.287
Area (sqft) = 0.17
Velocity (ft/s) = 1.68
Top Width (ft) = 0.67
0 .25 .5 .75 1 1.25
Depth (ft) Depth (ft)WEIR FLOW CAPACITY AT TRENCH DRAIN FOR 100YR POND A OVERFLOW
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.05/09/2021
WEIR CALCULATIONS
05/09/2021
Weir Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Thursday, Dec 17 2020
WEIR AT ENTIRE SITE 10YR
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 3.00
Total Depth (ft) = 0.17
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.19
Highlighted
Depth (ft) = 0.07
Q (cfs) = 0.190
Area (sqft) = 0.21
Velocity (ft/s) = 0.89
Top Width (ft) = 3.00
0 .5 1 1.5 2 2.5 3 3.5 4
Depth (ft) Depth (ft)WEIR AT ENTIRE SITE 10YR
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.05/09/2021
Weir Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Thursday, Dec 17 2020
WEIR AT ENTIRE SITE 100YR
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 3.00
Total Depth (ft) = 0.17
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.43
Highlighted
Depth (ft) = 0.12
Q (cfs) = 0.430
Area (sqft) = 0.37
Velocity (ft/s) = 1.17
Top Width (ft) = 3.00
0 .5 1 1.5 2 2.5 3 3.5 4
Depth (ft) Depth (ft)WEIR AT ENTIRE SITE 100YR
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.05/09/2021
BIORETENTION CALCULATIONS
05/09/2021
POND WQCV DEPTH AND AREA CALCULATIONS
Basin PR-1:
Total Area: 3,754 𝑓𝑡ଶ
Impervious Area: 2,165.7 𝑓𝑡ଶ
Coniferous Dripline Area: 1.1 𝑓𝑡ଶ
Deciduous Dripline Area: 1,891 𝑓𝑡ଶ
Effective Area: 2165.7 𝑓𝑡ଶ −(1.1 𝑓𝑡ଶ × 0.3)−(1891.3 𝑓𝑡ଶ × 0.15)= 1881.7 𝑓𝑡ଶ
Effective Imperviousness: ଵ଼଼ଵ.௧ మ
ଷହସ௧ మ × 100 = 50.1%
WQCV (watershed-inches): 0.096
WQCV (𝑓𝑡ଷ ): 0.096(𝑤𝑎𝑡𝑒𝑟𝑠ℎ𝑒𝑑−𝑖𝑛𝑐ℎ𝑒𝑠)× ቀ ଵ௧
ଵଶቁ × 3,754 𝑓𝑡ଶ = 30.0 𝑓𝑡ଷ
Northwest Pond:
WQCV Depth Capacity = 0.18𝑓𝑡
Flat Area Required =ଷ.௧ య
.ଵ଼ଷ௧ = 163.9 𝑓𝑡ଶ
Flat Area Provided =𝟏𝟔𝟑.𝟗 𝒇𝒕𝟐
Basin PR-2:
Total Area: 2,227 𝑓𝑡ଶ
Impervious Area: 918.4 𝑓𝑡ଶ
Coniferous Dripline Area: 435.1 𝑓𝑡ଶ
Deciduous Dripline Area: 718.2 𝑓𝑡ଶ
Effective Area: 918.4 𝑓𝑡ଶ −(435.1 𝑓𝑡ଶ × 0.3)−(718.2 𝑓𝑡ଶ × 0.15)= 680.1 𝑓𝑡ଶ
Effective Imperviousness: ଼.ଵ௧ మ
ଶଶଶ మ × 100 = 30.5%
WQCV (watershed-inches): 0.062
WQCV (𝑓𝑡ଷ ): 0.062(𝑤𝑎𝑡𝑒𝑟𝑠ℎ𝑒𝑑−𝑖𝑛𝑐ℎ𝑒𝑠)× ቀ ଵ௧
ଵଶቁ × 2,227 𝑓𝑡ଶ = 11.5 𝑓𝑡ଷ
Northeast Pond:
WQCV Depth Capacity = 0.31𝑓𝑡
Flat Area Required =ଵଵ.ହ௧ య
.ଷଵ௧ = 37.1 𝑓𝑡ଶ
Flat Area Provided =𝟑𝟕.𝟐 𝒇𝒕𝟐
05/09/2021
ASPEN CHARTS AND
FIGURES
05/09/2021
ASPEN CHARTS AND
FIGURES
05/09/2021
City of Aspen Urban Runoff Management Plan
Chapter 2 - Rainfall 2-4 Rev 9/2014
Note: Accuracy is more reliable at 5 minute increments.
Figure 2.1 IDF Curves for Aspen, Colorado
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30 35 40 45 50 55 60
In
t
e
n
s
i
t
y
(i
n
c
h
/
h
r
)
Duration in Minutes
Rainfall IDF for Aspen, Colorado
2‐yr 5‐yr 10‐yr 25‐yr 50‐yr 100‐yr
05/09/2021
City of Aspen Urban Runoff Management Plan
Chapter 2 - Rainfall 2-2 Rev 9/2014
into thunderstorms. Autumn in Aspen is usually dry and warm and during September daytime temperatures
can reach 70°F, but night temperatures can drop to freezing. Aspen is renowned for its warm winter sun.
Winter daytime temperatures typically range from 20 to 40°F in the City and from 10 to 30°F on the
mountain. Once the sun goes down, the temperature drops dramatically. Table 2.1 presents monthly
statistics for temperature, precipitation, snowfall, and snow depth in the Aspen area.
Table 2.1 Monthly Statistics for Temperature and Precipitation in Aspen
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Average Max. Temperature (F) 35 39 45 52 63 72 78 76 69 58 43 35 55.5
Average Min. Temperature (F) 9.1 12 20 26 35 41 47 46 39 30 19 9.7 27.7
Average Total Precipitation (in.) 1.7 2.1 2.7 2.5 2.1 1.4 1.8 1.6 2.1 2 2.6 1.9 24.37
Average Total Snowfall (in.) 25 27 28 20 7.8 1 0 0 1 11 28 25 173.8
Average Snow Depth (in.) 21 28 27 12 1 0 0 0 0 1 6 14
(Source: Station 050372 at Aspen 1 SW, Colorado)
2.3 Rainfall Depth, Duration, Frequency, and Intensity
The rainfall intensity-duration-frequency (IDF) curve is a statistical formula to describe the relationship
among the local rainfall characteristics and return periods. The IDF curve is used in the Rational Method
for peak runoff predictions of basins smaller than 90 acres. Based on the NOAA Atlas Volume 3, the
IDF curve for the City of Aspen can be derived according to the locality and elevation. The City of Aspen is
located at approximately 39°11′32″N and 106°49′28″W, at an elevation of approximately 8,100 feet.
Based on depth and duration data (Appendix B, Table 1), rainfall intensities can be calculated for various
frequencies. Rainfall intensity data, which form the basis of the Intensity-Duration-Frequency (IDF) curves
in Figure 2.1 are provided in Table 2.2.
Table 2.2 Rainfall Intensity-Duration-Frequency in Aspen, Colorado
Return Rainfall Intensity in inch/hr for Various Periods of Duration
Period 5-min 10-min 15-min 30-min 1-hr (P1) 2-hr 3-hr 6-hr 24-hr
2‐yr 2.06 1.51 1.23 0.77 0.47 0.28 0.21 0.13 0.06
5-yr 2.98 2.17 1.77 1.09 0.64 0.36 0.26 0.16 0.07
10-yr 3.72 2.72 2.22 1.35 0.77 0.43 0.30 0.18 0.08
25‐yr 4.75 3.47 2.82 1.71 0.95 0.53 0.36 0.21 0.09
50‐yr 5.53 4.05 3.30 1.98 1.09 0.60 0.41 0.24 0.11
100-yr 6.32 4.63 3.76 2.24 1.23 0.67 0.45 0.26 0.12
Using the data in Table 2.2 (derived from NOAA Atlas 14 Volume 8), the following equation was derived
that can be used to determine intensities not shown in the IDF table or curve:
052.1
1
)10(
8.88
dT
PI (Equation 2-1)
Where, I = rainfall intensity (inch/hr),
P1 = 1-hr rainfall depth (inches), and
Td = duration or time of concentration (minutes).
05/09/2021
City of Aspen Urban Runoff Management Plan
Chapter 3 - Runoff 3-6 Rev 10/2014
Figure 3.2 – Runoff Coefficients for NRCS Hydrologic Soil Group A
Figure 3.3 – Runoff Coefficients for NRCS Hydrologic Soil Group B
05/09/2021
City of Aspen Urban Runoff Management Plan
Chapter 8 – Water Quality 8-33 Rev 8/2009
Figure 8.13 Aspen Water Quality Capture Volume
05/09/2021
City of Aspen Urban Runoff Management Plan
Chapter 3 - Runoff 3-2 Rev 2/2010
Figure 3.1 Natural Resource Conservation Service (NRCS) Soil Map for Aspen
05/09/2021