HomeMy WebLinkAboutMaster Permit.701 Gibson Ave.0086-2021-BRES (4)
DRAINAGE REPORT
FOR
701 GIBSON STREET
CITY OF ASPEN, COLORADO
PARCEL ID: 273707345003
PREPARED FOR:
BLD Seed Architects
417 Main St.
Aspen, CO 81611
PREPARED BY:
High Country Engineering, Inc.
1517 Blake Avenue, Suite 101
Glenwood Springs, CO 81601
(970) 945-8676
May 4, 2021
Revised: October 29, 2021
HCE JOB NUMBER: 2201051.00
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TABLE OF CONTENTS
SECTION PAGE
I. GENERAL LOCATION AND HISTORIC DESCRIPTION 3
II. DRAINAGE STUDIES 5
III. DRAINAGE DESIGN CRITERIA 7
IV. DRAINAGE FACILITY DESIGN 10
V. CONCLUSION 14
VI. REFERENCES 15
EXHIBITS:
1. Historic Drainage Conditions
2. Proposed Drainage Conditions
3. Drainage Detail Sheet
4. CTL Thompson, Inc. Soils Report
Appendices
Hydrologic Computations
Historic Conditions
Proposed Conditions
Hydraulic Computations
Trench Drain Channel Calculations
Trench Drain Inlet Calculations
Weir Calculations
Pond Calculations
Pipe Calculations
Aspen Charts and Figures
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Engineers Certification
“I hereby affirm that this report and the accompanying plans for the reconstruction of garage
and house remodel at 701 Gibson 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
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I. GENERAL LOCATION AND DESCRIPTION
A. Location
The site is located at 701 Gibson Drive within the City of Aspen, County of Pitkin, State of
Colorado, south of the Gibson Avenue and Lone Pine Road Intersection. A Vicinity Map is
shown below.
FIGURE 1: VICINITY MAP OF 701 GIBSON
B. Description of Existing Property
The 701 Gibson property is approximately 46,800 square-feet (1.07 acres). The existing lot
consists of a home, basement, patios, second-story deck, driveway, garage slab, paths,
landscaping tennis court and numerous trees. The site is bordered by a private properties to the
east and west, Gibson Avenue to the north and North Spring Street to the south. The site
drains from the north to the south, across North Spring Street and into the Roaring Fork River
at the southwest end of the property. Existing grades range from approximately 2-percent to
65+ percent.
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C. Soils Description
HCE has reviewed the soil types from the soil conservation service mapping as shown below
for the 701 Gibson project site. The soil type in the area of disturbance is Uracca, Moist-
Mergel complex, 6% to 12% slopes. The makeup of the two soils include both Type A and
Type B Hydrologic Soil Groups (HSG). The Uracca, Moist is described as alluvial fans,
structural benches and valley sides and is a Type B Soil. The Mergel is descibed as structural
benches, valley sides and alluvial fans similar to the Uracca but is a Type A soil. HCE utilized
a Type B soil for drainage calculations.
FIGURE 2: SOIL INFORMATION FROM SOIL CONSERVATION SERVICE (Gypsum Area)
CTL Thompson, Inc. completed a Subgrade Investigation and Pavement Design report (Project
No. GS06573.000-135) on August 19, 2021. The report describes an exploratory pit excavated
to a depth of 5.5 feet in the area of the proposed driveway. The exploratory pit encountered
about 5 feet of existing fill comprised of silty gravel and cobble, underlain by natural silty
gravel and cobbles to the total explored depth of 5.5 feet. Free groundwater was not found in
the exploratory pit. According to the report, the soils at this site can be described HSG Type
B. The site specific soils report can be found in the appendices of this report.
II. DRAINAGE STUDIES
A. Major Drainage Way Planning and Influential Parameters
The upper portion of 701 Gibson Ave was reviewed for impact from FEMA flood mapping
and determined to be well clear of the Roaring Fork River flood plain.
The site is located near FEMA’s major drainage study of the area on its Flood Insurance Rate
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Map (FIRM) No. 08097C0354E which has an effective date of August 15, 2019. Although the
tennis court area of the site is near the floodplain the upper area of the site where modifications
are being proposed is well clear of the 100 or 500 year floodplains.
FIGURE 3: FEMA MAP SHOWING NO FLOODPLAIN IMPACT TO 701 GIBSON (RED MARKER)
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.
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 just outside of System 3 to the
north and the site will flow directly to the Roaring Fork River on the south side of the site.
This site was previously developed and per discussions with PJ with the City of Aspen
Engineering Department the proposed modifications will not impact the system that is
currently in place for the residence and calculations for the site are being prepared
independently as a separate water quality project addressing only the new disturbance. The
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existing system includes the residential structure drainage system and three drywells that
receive site drainage below the residence just above North Spring Street.
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 Spring
Street ROW after passing through the existing storm water infrastructure including three
drywells. The existing flow leaves the property to the south, then travels west along North
Spring Street and will sheet flow across the 701 Gibson Property to the Roaring Fork River.
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 but less than 25-percent of the overall site is
being disturbed, so water quality for the improvements will be necessary per the URMP. The
site detention is not being disturbed and is currently being handled by the existing site
drywells. The existing disturbed 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. The runoff for the proposed basin which includes the driveway paving will be routed
through a bioretention water quality pond just below the driveway. The remainder of the
disturbed area is site landscaping and conveys the flows to N. Spring Street via a series of
improved grass areas receiving storm water from sheet flow and the bioretention
areas/planters. These areas all provide treatment of the runoff 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.
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.
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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 Smuggler /Hunter Drainage Basin detention and water quality capture are
required. In this case the detention volume is currently being captured by the existing site
storm system for the existing residence and no modification of this system is being proposed.
We are proposing to add WQCV for the disturbed impervious area of the site. The concept
was discussed with the City of Aspen Engineering Department and was selected as the best
alternative, since the existing system was not being impacted or modified and contains three
area drywells to accommodate the existing stormwater detention.
The bioretention area was sized to handle the WQCV, but does 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 trench drain, piping 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 streets or structures that cause major site constraints for the drainage system
design. A utility corridor does affect the disturbed area by affecting grading and location of
structures to accommodate depth of cover and drainage structure options within the easement
area. 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 near proximity with adjacent properties
E. Easements and Irrigation Facilities
There are no major drainage ways, drainage easements or tracts located on the site. A utility
easement exists within the disturbed landscaping area from Gibson Avenue to down the Hill to
North Spring Street. Minor grading potentially impacts waterline and electric line cover and
adjustments were made to maintain the minimum cover for both utilities. Verification of
waterline depths prior to approval of the permit will be required to verify acceptable cover.
Irrigation facilities onsite will be added or repaired and should not affect the overall proposed
development.
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F. Low Impact Site Design
The bioretention area will be implemented to allow for the capture of the required WQCV per
the URMP code. Should the bioretention area exceed the WQCV capacity, runoff will reach
the City of Aspen drainage system and outfall directly to 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 the bioretention pond and improved grassed area and ground cover
for sheet flow, the majority of the disturbed 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 disturbed area will have
a decrease in impervious area by the removal of the existing gravel parking area.
4. Reduce runoff rates and volumes to more closely match natural conditions
Runoff rates and volumes are unchanged from the current condition. Stormwater
detention is unchanged, but additional water quality has been provided for the site.
Therefore, runoff rates have been reduced, as recommended, by implementing the
addition of a bioretention pond connected by vegetated grass areas. All of the new
impervious areas will drain to the bioretention area. The site overflow drains along
the North Spring Street ROW for approximately 100 feet and then through the
property to the Roaring Fork River.
5. Integrate storm water quality management and flood control
The bioretention area captures runoff onsite via storm pipe and are connected by
vegetated grass areas in the case of overflow. Downstream sheet flow on the steep
hillside accommodates overflow from the site basin. The proposed disturbed site
has been designed for water quality only, which does not provide flood control
detention, but the existing site drainage system for the residence does include three
drywells for detention that would have included the whole site and has the overflow
path from the drywells that will suitably convey runoff to the Roaring Fork River.
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6. Develop storm water quality facilities that enhance the site and environment.
The proposed water quality facilities enhance the site and the environment with a
bioretention area that will become part of the landscape.
7. Use a treatment train approach
The treatment train approach has been implemented by incorporating bioretention
areas that connect overflow to vegetated grassed areas.
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 bioretention area has been designed into the
landscaping away from the walking areas of the yard.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
The proposed construction calls for some interior remodel of the existing residence along with
driveway and yard landscaping improvements. The existing, short driveway that connects the
garage to Gibson Avenue is being improved to include stone pavers and is the source of the
change of impervious area for the site. The disturbed impervious areas for the site is being
piped to the bioretention pond and treated for WQCV. Runoff will be routed by sheet flow
and storm pipe through the new landscaping areas and bioretention pond. The bioretention
pond will have partial impervious membranes for the concrete walls, and will infiltrate into the
soils below the pond. Runoff, greater than WQCV, will leave the site in historical fashion to
the City of Aspen’s right of way and into the Roaring Fork River.
B. Historic Drainage Basins Descriptions
The proposed site’s historic drainage pattern is from the northeast to southwest (Gibson
Avenue to North Spring Street). The storm drainage ultimately enters the Roaring Fork River
after flowing a short distance along North Spring Street to the lower 701 Gibson property
through native vegetation to the Roaring Fork River. 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 #1) for
a map of existing basin layout.
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Historic Flow Path:
Runoff from basin EX-1 sheet flows southwest from the north 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.
Three existing drywells onsite provide stormwater detention. Stormwater discharges to the
drywells via pipe flow and a 24-inch inlet grate that tops one of the drywells. The proposed
project does not interfere with the existing drywells or their current capability to capture and
detain stormwater; therefore, stormwater detention for the site is provided as an existing
condition that will not be disturbed. The ability of the drywell inlet grade to capture runoff can
be increased with the proposed site grading during landscape improvements. The proposed
raise grade around the south side of the grate will help the inlet capture more stormwater
runoff.
Table 1. Historic Basin Characteristic
BASIN AREA,
ACRES C, 10YR I, 10YR Q10-YEAR,
CFS C, 100YR I, 100 YR Q100-YEAR,
CFS
EX-1 0.043 0.15 3.42 0.02 0.35 5.47 0.08
C. Proposed and Offsite Basin Descriptions
One proposed onsite basin and two offsite basins have been delineated in analyzing drainage at
701 Gibson.
The proposed onsite basin matches the existing disturbed basin. Proposed basin PR-1
encompasses the driveway and landscaping located north and west of the residence and along
the west property line southwest to the bioretention pond. From the northeast the storm flows
will sheet flow across the proposed driveway and into a slot drain that pipes directly to the
bioretention pond. Additional stormwater will sheet flow across a grass lawn south of the
driveway and into the bioretention pond. Stormwater overflows below the detention pond
area in the landscaping will sheet flow down the vegetated hillside. The landscape architects
have proposed improved grasses and landscape features that will help stabilize the hillside and
provide improved water quality to the stormwater flows. Design point one is located at the
bioretention pond southwest 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
overflow the pond weir into a riprap spreader to sheet flow across the lawn area and improved
landscaped areas down the hill to North Spring Street. At the bottom of the hill the stormwater
will flow along North Spring Street for approximately 100’ where the stormwater will flow
into the lower portion of the 701 Gibson site and flow through native vegetation to the Roaring
Fork River. The stormwater infrastructure was sized to adequately convey PR-1 and offsite
basin OS-1 design flows for a 100-yr event. Calculations for the storm pipe, weir, and trench
drain inlet and channel can be found in the appendices of this report.
Offsite basin OS-1 includes the proposed sidewalk, driveway and landscaping in the ROW
area south of Gibson Avenue. Basin OS-1 will drain onto the property and be captured by the
proposed stormwater infrastructure during major and minor storm events. The stormwater
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The existing drywell system can be used for
detention of the disturbed area if adequate
capacity/functionality is confirmed. Detention or
conveyance to the City's system is still required
for this project per the URMP.
What conveyance mechanism is proposed to
verify that runoff leaving the rain garden will
reach the drywells for detention? Based on the
grading plan it looks like the runoff will sheet
flow to the south, how will it enter the grate to
the drywell and not flow past? Is erosion of the
steep slope a concern as runoff leaves the
riprap?
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infrastructure has been sized to accommodate drainage from basin OS-1 up to a 100-year
event. The bioretention pond has also been sized with a WQCV that will treat the offsite
impervious areas of basin OS-1.
The second offsite basin OS-2 includes Gibson Avenue south of the road centerline and the
proposed gutter. The proposed curb and beveled edge along the sidewalk in the proposed
driveway will prevent OS-2 drainage from entering the property. During major storm events,
runoff may overtop the proposed beveled edge and flow down the proposed driveway. If
stormwater from basin OS-2 inundates the proposed drainage infrastructure, the runoff will
overflow at the southwest side of the proposed driveway and discharge as sheet flow south
down the lawn and steep hill as it did historically.
The proposed bioretention pond has been designed to provide water quality treatment for the
new impervious areas in basin PR-1 and from ROW area of basin OS-1. The combined new
impervious areas equal 1,928 square-feet with a total combined basin area of 3,188 square-feet
for an imperviousness of 60.5 percent. Using figure 8.13 of the URMP, the effective
imperviousness of 60.5 percent gives a WQCV of 0.116 watershed-inches. To achieve this
amount of WQCV the required flat area of the pond is 30.8 square-feet with a depth of 12
inches. The proposed pond has a flat area of 31.6 square-feet. 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 pond overflows a rectangular weir with a crest two
feet wide and 2.5 inches deep to Type VL riprap for erosion protection at the outlet. Type VL
riprap will have a median stone size (D50) of six inches.
The previously mentioned calculations can be found in the appendices of this report. Pond
section details can be found on Exhibit #3 drainage details.
See Exhibit #2 for the delineation of the proposed and offsite basins 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 0.044 0.47 3.88 0.08 0.57 6.20 0.15
OS-1 0.030 0.45 3.72 0.05 0.56 5.94 0.10
OS-2 0.141 0.92 3.96 0.51 0.96 6.33 0.85
Table 3. Proposed WQCV Table
BASIN
AREA
(S.F.)
NEW
IMPERVIOUS
AREA (SF)
EFFECTIVE
IMPERVIOUS (%)
WQCV
(watershed-
inches) WQCV (CF)
PR1+OS1 3,187.8 1,928 60.4 0.116 30.8
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D. Downstream Impacts
The proposed onsite grading and bioretention pond facility will have positive downstream
impacts during frequent storm events by capturing and treating the onsite and offsite 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 to the Roaring Fork River via North Spring Street and the lower portion of
the 701 Gibson property. 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 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 groundcover 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 to be are prepared for spring thaw flows.
The bioretention basin 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 riprap should be inspected annually and after every major storm. Replace rock or other
components that have become dislodged. Remove accumulated material including sediment,
trash and woody debris. Repair damages to geotextile fabric should they occur. If riprap stones
continually wash away, replace them with larger stones.
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.
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 proposed grass conveyance and within the
proposed bioretention basin.
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V. CONCLUSION
A. Compliance with Standards
This drainage report has been prepared in accordance with City of Aspen Regulations. The
proposed bio retention pond will capture and treat the proposed WQCV for the 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 pond.
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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.
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.
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APPENDICES
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EXHIBITS
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Center of Colorado
E;HIBIT
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COLORADO 811
CALL BEFORE
YOU DIG
Utility Notification
Center of Colorado
2201048.00
EXHIBIT
3
DA
R
I
N
E
Y
D
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N
B
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CI
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BD
B
ENGINEERING ADVICE MAY BE REQUIRED.
ENGINEERING ADVICE MAY BE REQUIRED.
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
Do any special soils
preparations need to
be considered for the
rain garden treatment
technique? Will
infiltration be
achievable?
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
11/02/2021
HYDROLOGIC
COMPUTATIONS
11/02/2021
HISTORIC CONDITIONS:
10 YEAR:
100 YEAR:
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
(
%
)
P
I
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 1
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF
0.15 7.21 EX-1 0.023.42
PIPE TRAVEL TIME
REMARKS
CHANNEL
STRUCTURE NO.
0.01
TOTAL RUNOFF
0.043
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
)
S
U
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 1
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF CHANNEL PIPE TRAVEL TIME
REMARKS
1 EX-1 0.043 0.35 7.2 0.01 5.47 0.08
11/02/2021
PROPOSED CONDITIONS:
2 YEAR:
10 YEAR:
100 YEAR:
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)
TOTAL RUNOFF TRAVEL TIMEPIPESTREET
REMARKS
0.02
0.37
5.92 0.010
0.02 2.37
DIRECT RUNOFF
2
0.04
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
OS-1
1
0.35
PR-1 5.280.044
0.030 2.27
Design Point 1
Design Piont 1
3 OS-2 0.141 0.88 5.00 0.124 2.42 0.30 Design Piont 3
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)
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE TRAVEL TIME
REMARKS
1 PR-1 0.044 0.47 5.28 0.02 3.88 0.08 Design Point 1
2 OS-1 0.030 0.45 5.92 0.013 3.72 0.05 Design Piont 1
3 OS-2 0.141 0.92 5.00 0.129 3.96 0.51 Design Piont 3
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)
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE TRAVEL TIME
REMARKS
1 PR-1 0.044 0.57 5.28 0.02 6.20 0.15 Design Point 1
2 OS-1 0.030 0.56 5.92 0.017 5.94 0.10 Design Piont 1
3 OS-2 0.141 0.96 5.00 0.135 6.33 0.85 Design Piont 311/02/2021
HYDRAULIC
COMPUTATIONS
11/02/2021
TRENCH DRAIN CALCULATIONS
11/02/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
Trench 10yr (PR1+OS1)
Rectangular
Bottom Width (ft) = 0.66
Total Depth (ft) = 0.66
Invert Elev (ft) = 7912.46
Slope (%) = 0.50
N-Value = 0.011
Calculations
Compute by: Known Q
Known Q (cfs) = 0.13
Highlighted
Depth (ft) = 0.11
Q (cfs) = 0.130
Area (sqft) = 0.07
Velocity (ft/s) = 1.79
Wetted Perim (ft) = 0.88
Crit Depth, Yc (ft) = 0.11
Top Width (ft) = 0.66
EGL (ft) = 0.16
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
7911.50 -0.96
7912.00 -0.46
7912.50 0.04
7913.00 0.54
7913.50 1.04
7914.00 1.54
Reach (ft)
11/02/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
Trench 100yr (PR1+OS1)
Rectangular
Bottom Width (ft) = 0.66
Total Depth (ft) = 0.66
Invert Elev (ft) = 7912.46
Slope (%) = 0.50
N-Value = 0.011
Calculations
Compute by: Known Q
Known Q (cfs) = 0.25
Highlighted
Depth (ft) = 0.18
Q (cfs) = 0.250
Area (sqft) = 0.12
Velocity (ft/s) = 2.10
Wetted Perim (ft) = 1.02
Crit Depth, Yc (ft) = 0.17
Top Width (ft) = 0.66
EGL (ft) = 0.25
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
7911.50 -0.96
7912.00 -0.46
7912.50 0.04
7913.00 0.54
7913.50 1.04
7914.00 1.54
Reach (ft)
11/02/2021
Inlet Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
Heel Resistant Brickslot with 50% Clogging @ 10yr (PR1+OS1)
Drop Grate Inlet
Location = Sag
Curb Length (ft) = -0-
Throat Height (in) = -0-
Grate Area (sqft) = 0.65
Grate Width (ft) = 24.61
Grate Length (ft) = 0.09
Gutter
Slope, Sw (ft/ft) = 0.010
Slope, Sx (ft/ft) = 0.010
Local Depr (in) = -0-
Gutter Width (ft) = 24.61
Gutter Slope (%) = -0-
Gutter n-value = -0-
Calculations
Compute by: Known Q
Q (cfs) = 0.13
Highlighted
Q Total (cfs) = 0.13
Q Capt (cfs) = 0.13
Q Bypass (cfs) = -0-
Depth at Inlet (in) = 0.11
Efficiency (%) = 100
Gutter Spread (ft) = 26.44
Gutter Vel (ft/s) = -0-
Bypass Spread (ft) = -0-
Bypass Depth (in) = -0-
11/02/2021
Inlet Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
Heel Resistant Brickslot with 50% Clogging @ 100yr (PR1+OS1)
Drop Grate Inlet
Location = Sag
Curb Length (ft) = -0-
Throat Height (in) = -0-
Grate Area (sqft) = 0.65
Grate Width (ft) = 24.61
Grate Length (ft) = 0.09
Gutter
Slope, Sw (ft/ft) = 0.010
Slope, Sx (ft/ft) = 0.010
Local Depr (in) = -0-
Gutter Width (ft) = 24.61
Gutter Slope (%) = -0-
Gutter n-value = -0-
Calculations
Compute by: Known Q
Q (cfs) = 0.25
Highlighted
Q Total (cfs) = 0.25
Q Capt (cfs) = 0.25
Q Bypass (cfs) = -0-
Depth at Inlet (in) = 0.17
Efficiency (%) = 100
Gutter Spread (ft) = 27.44
Gutter Vel (ft/s) = -0-
Bypass Spread (ft) = -0-
Bypass Depth (in) = -0-
11/02/2021
WEIR CALCULATIONS
11/02/2021
Weir Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
WEIR 10YR (PR1+OS1)
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 2.00
Total Depth (ft) = 0.21
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.13
Highlighted
Depth (ft) = 0.07
Q (cfs) = 0.130
Area (sqft) = 0.14
Velocity (ft/s) = 0.90
Top Width (ft) = 2.00
0 .5 1 1.5 2 2.5 3
Depth (ft) Depth (ft)WEIR 10YR (PR1+OS1)
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.11/02/2021
Weir Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
WEIR 100YR (PR1+OS1)
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 2.00
Total Depth (ft) = 0.21
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.25
Highlighted
Depth (ft) = 0.11
Q (cfs) = 0.250
Area (sqft) = 0.22
Velocity (ft/s) = 1.12
Top Width (ft) = 2.00
0 .5 1 1.5 2 2.5 3
Depth (ft) Depth (ft)WEIR 100YR (PR1+OS1)
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.11/02/2021
BIORETENTION CALCULATIONS
11/02/2021
POND WQCV DEPTH AND AREA CALCULATIONS
Basins PR-1 and OS-1:
Total Area: 1,901.3 + 1,286.5 = 3,187.8 𝑓𝑡ଶ
New Impervious Area: 1,176.6 + 751.4 = 1,928.0 𝑓𝑡ଶ
Imperviousness: ଵଽଶ଼௧ మ
ଷଵ଼.଼௧ మ × 100 = 60.5%
Treated as 100% Impervious WQCV (watershed-inches): 0.116
New Impervious Area WQCV (𝑓𝑡ଷ ): 0.116(𝑤𝑎𝑡𝑒𝑟𝑠ℎ𝑒𝑑−𝑖𝑛.)× ଵ௧
ଵଶ × 3187.8 𝑓𝑡ଶ = 30.8 𝑓𝑡ଷ
Bioretention Pond:
WQCV Depth Capacity = 1𝑓𝑡
Flat Area Required =ଷ.଼௧ య
ଵ.௧ = 30.8 𝑓𝑡ଶ
Flat Area Provided =𝟑𝟏.𝟔 𝒇𝒕𝟐
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This is misleading.
The calculation is
performed using
60.5%
imperviousness, as
calculated in the step
above, not 100%
impervious as stated
in this line. This
calculation is done
using 60.5%
impervious.
PIPE CALCULATIONS
11/02/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
4in Pipe 10yr (PR1+OS1)
Circular
Diameter (ft) = 0.33
Invert Elev (ft) = 7912.46
Slope (%) = 1.00
N-Value = 0.010
Calculations
Compute by: Known Q
Known Q (cfs) = 0.13
Highlighted
Depth (ft) = 0.18
Q (cfs) = 0.130
Area (sqft) = 0.05
Velocity (ft/s) = 2.72
Wetted Perim (ft) = 0.55
Crit Depth, Yc (ft) = 0.21
Top Width (ft) = 0.33
EGL (ft) = 0.29
0 1
Elev (ft)
7912.00
7912.25
7912.50
7912.75
7913.00
11/02/2021
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Oct 22 2021
4in Pipe 100yr (PR1+OS1)
Circular
Diameter (ft) = 0.33
Invert Elev (ft) = 7912.46
Slope (%) = 1.00
N-Value = 0.010
Calculations
Compute by: Known Q
Known Q (cfs) = 0.25
Highlighted
Depth (ft) = 0.29
Q (cfs) = 0.250
Area (sqft) = 0.08
Velocity (ft/s) = 3.14
Wetted Perim (ft) = 0.80
Crit Depth, Yc (ft) = 0.28
Top Width (ft) = 0.21
EGL (ft) = 0.44
0 1
Elev (ft)
7912.00
7912.25
7912.50
7912.75
7913.00
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Pipe is required to be
sized so that design
depth does not
exceed 80% of the
pipe diameter. Revise
to comply with section
4.8.
ASPEN CHARTS AND
FIGURES
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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
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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).
11/02/2021
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
11/02/2021
City of Aspen Urban Runoff Management Plan
Chapter 8 – Water Quality 8-33 Rev 8/2009
Figure 8.13 Aspen Water Quality Capture Volume
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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
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