HomeMy WebLinkAboutFile Documents.527 S West End St.0009.2018 (13).ARBKDRAINAGE REPORT
FOR
527 SOUTH WEST END STREET
CITY OF ASPEN, COLORADO
PREPARED FOR:
Winterhaven Condominium Association
Aspen, CO 81611
PREPARED BY:
High Country Engineering, Inc.
1517 Blake Avenue, Suite 101
Glenwood Springs, CO 81601
(970) 945-8676
February 4, 2014
Revised: December 5, 2017
HCE JOB NUMBER: 2141005.01
01/10/2018
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TABLE OF CONTENTS
SECTION PAGE
I. LOCATION AND EXISTING BASIN DESCRIPTION 3
II. PROPOSED PROJECT DESCRIPTION AND DRAINAGE BASINS 3
III. CONCLUSION 5
EXHIBITS:
1. Vicinity Map (8.5”x11”)
2. SCS Soils Map (8.5”x11”)
3. FEMA Map (11”x17”)
4. Grading Plan (24”x36”)
Appendices
Hydrologic Computations
Proposed Conditions for WQCV
Hydraulic Computations
Rain Garden Calculations
Aspen Charts and Figures
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I. LOCATION AND EXISTING BASIN DESCRIPTION
A. Location
The proposed project at the Winterhaven Condominiums is located at 527 South West End
Street, City of Aspen, County of Pitkin, State of Colorado. The proposed site is located at the
northwest corner of the Waters Avenue and West End intersection in Aspen, Colorado. A
Vicinity Map has been included as Exhibit #1.
B. Description of Existing Property
The proposed site is approximately 5,998.69 square feet (0.138 acres). The existing lot
consists of a condominium, landscaped yard, treed areas, asphalt parking lot, carport and
concrete walkways. The site is bordered by a private property to the west and south, East
Dean Street to the north and West End Street to the east. The site drains from the south to the
north and offsite to East Dean Street.
Within the area of study, there is one soil type as designated by the United States Department
of Agriculture Soil Conservation Service in cooperation with Colorado Agricultural
Experiment Station. The soil type that encompasses the site is designated as soil 107, Uracca,
Moist-Mergel Complex, having 1%-6% slopes. The Soil Conservation Service describes the
soil as deep and well drained with moderate to moderately rapid permeability, and a low water
capacity. The soil is defined as hydrologic group ‘B’. A brief description of the soil has been
included on the soils map (Exhibit #2).
II. PROPOSED PROJECT DESCRIPTION AND DRAINAGE BASINS
A. Criteria
This drainage study was prepared in conformance with the City of Aspen, Colorado Urban
Runoff Management Plan (URMP), dated December of 2014 and the revised sections dated
thereafter. More than 200 square feet and less than 1,000 square feet of area will be disturbed
with the proposed construction; therefore the site is viewed as a Minor Project per the URMP.
The site will be treated as an urban minor project, so the Water Quality Capture Volume
(WQCV) for the disturbed area will be necessary per the URMP. Since the site is classified as
a Minor project, no onsite detention is required for the Minor and Major storm events.
Detailed onsite drainage calculations were not performed since they were not required. Only
area and WQCV calculations were performed.
The WQCV will be determined for the proposed construction at the north east corner of the
condominiums 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 in watershed inches x 1 ft/12 in x area (acres)
x 43,560 ft2/acre.
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B. Description of Proposed Project
The proposed construction at 527 South West End Street includes the addition of a roof over
the entry way into the condominium complex, entry way remodel and reconstruction of the
existing sidewalk leading up to the entry. No new impervious area is being added to the site.
The general grading and drainage patterns and receiving system of the site, including the areas
of disturbance, will remain as existing since the improvements are being added over existing
features and no major grading will occur. The soil type will not change except in areas of bio
retention facilities since the infiltration media will need to be per the URMP standards.
C. Flood Description
The proposed site is located on FEMA’s Flood Insurance Rate Map (FIRM) No.
08097C0203C which has an effective date of June 4, 1987. The site is located in Zone X. This
zone is described as areas determined to be outside 100-year and 500-year floodplains. Refer
to Exhibit #3 for the FEMA map.
D. Drainage Issues
There are no major drainage ways, facilities, or easements within the property boundary, nor
are there any irrigation facilities that will impact the proposed construction. There are no
major issues that affect the proposed improvements to the site.
E. Drainage Basin Description and Drainage Alterations
The proposed site’s existing drainage pattern is from the south to the north and offsite East
Dean Street. The portion of Dean Street that the site drains to is the parking lot for multiple
multi-family complexes. The storm water is ultimately routed to the City of Aspen buried
storm water drainage system via curb and gutter system along West End Street and South
Original Street.
F. Proposed WQCV Concept
The proposed site calls for the construction of a roof over the entry way into the condominium
complex, remodeled entry way and a reconstructed sidewalk. No new impervious area is
being added to the site. Only slight alterations will be made to the building’s footprint and
exterior. The majority of the site will remain unaffected by the proposed construction since
the construction will occur over existing impervious areas. Runoff from the proposed area of
construction will be conveyed to the proposed bio retention pond to cleanse the runoff to
prevent pollutant travel offsite per the URMP.
The proposed site has one proposed onsite drainage basin for calculation of the WQCV. Areas
that remain undisturbed by the construction were not analyzed for this report since the project
falls under the Minor Design category. The area used for the determination of the WQCV can
be seen on sheet GR1 in the appendices. Table 1, below, is a summary of the required WQCV
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for the proposed basin.
Table 1. Proposed WQCV Table
BASIN
AREA
(S.F.)
IMPERVIOUS
AREA (SF)
PERCENT
IMPERVIOUS
WQCV (WATERSHED
INCHES)
REQUIRED
WQCV (CF)
PR-1 304.13 304.13 100.00 0.255 6.46
G. Specific Details
The UDFCD’s Rain Garden was used to determine the size of the pond for basin PR-1. The
detention facility will provide water quality for the areas of disturbance thus preventing
pollutants from exiting the site. The basin’s imperviousness was used to determine the
WQCV in watershed inches on Figure 8.13.
III. CONCLUSION
A. Compliance with Standards
This drainage report has been prepared in accordance with City of Aspen Regulations. The
proposed rain garden will detain the WQCV before releasing to the surrounding soil around
the condominiums. Only the WQCV was analyzed for this report.
B. Drainage Concept
The proposed drainage design will be effective in controlling any adverse downstream impacts
on landowners or structures by capturing the WQCV from the proposed addition. The site has
many low impact designs to help reduce runoff from the site.
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527 WEST END STREETDRAINAGE REPORTBY: LLCHECKED BY: RDN11/21/2017BASIN AREA (S.F.) AREA (ACRE) IMPERVIOUS AREA (SF) PERCENT IMPERVIOUSWQCV (WATERSHED INCHES)REQUIRED WQCV (CF)PR-1 304.13 0.01 304.13 100.000.255 6.4601/10/2018
01/10/2018
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia =100.0 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100)i = 1.000
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.40 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area) Area = 304 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV =cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 =0.30 in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER =cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER =6.5 cu ft
(Only if a different WQCV Design Volume is desired)
2. Basin Geometry
A) WQCV Depth (12-inch maximum)DWQCV =4in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 0.00 ft / ft
(Use "0" if rain garden has vertical walls)
C) Mimimum Flat Surface Area AMin =4sq ft
D) Actual Flat Surface Area AActual =22.00 sq ft
E) Area at Design Depth (Top Surface Area)ATop =22 sq ft
F) Rain Garden Total Volume VT=7.33 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
4. Underdrain System
A) Are underdrains provided?
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y =N/A ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 =N/A cu ft
iii) Orifice Diameter, 3/8" Minimum DO =N/A in
Design Procedure Form: Rain Garden (RG)
LL
HCE
December 5, 2017
527 SOUTH WEST END ST
PR-1
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
PR-1.xlsx, RG 12/12/2017, 1:01 PM
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Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
6. Inlet / Outlet Control
A) Inlet Control
7. Vegetation
8. Irrigation
A) Will the rain garden be irrigated?
Notes:
Design Procedure Form: Rain Garden (RG)
LL
HCE
December 5, 2017
527 SOUTH WEST END ST
PR-1
Choose One
Choose One
Choose One
Sheet Flow- No Energy Dissipation Required
Concentrated Flow- Energy Dissipation Provided
Plantings
Seed (Plan for frequent weed control)
Sand Grown or Other High Infiltration Sod
Choose One
YES
NO
YES
NO
PR-1.xlsx, RG 12/12/2017, 1:01 PM
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ASPEN CHARTS AND
FIGURES
01/10/2018
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 60Intensity (inch/hr)Duration in Minutes
Rainfall IDF for Aspen, Colorado
2‐yr 5‐yr 10‐yr 25‐yr 50‐yr 100‐yr
01/10/2018
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).
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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
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City of Aspen Urban Runoff Management Plan Chapter 8 – Water Quality 8-33 Rev 8/2009 Figure 8.13 Aspen Water Quality Capture Volume 01/10/2018
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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