HomeMy WebLinkAboutFile Documents.533 W Hallam St.0031-2022-BCHO (3)
DRAINAGE REPORT FOR
DUNCAN SKIHAUS
533 W. HALLAM STREET, ASPEN, CO
I hereby affirm that this report and the accompanying drawings for the analysis of 533 W.
Hallam Street, Aspen, Colorado was prepared under my direct supervision for the owners
thereof in accordance with the provisions of the City of Aspen Urban Runoff Management
Plan with proposed variances and 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.
Permit #: 0285.2018.ARBK
Permit Submittal: April 15, 2020
Change Order #1: August 6, 2020
Change Order #2: August 3, 2022
Rick Barth, P.E. 36749
Prepared by
118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004
970.945.5948 fax
Reviewed by Engineering
08/12/2022 3:11:02 PM
"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.
Aspen, Colorado August 3, 2022
Drainage Report for Major Design i
DRAINAGE REPORT FOR
DUNCAN SKIHAUS
533 W. HALLAM STREET, ASPEN, CO
REVIEWED BY
RICK BARTH
SGM Project # 2018-292.001
I:\2018\2018-292-DuncanSkihaus\001\E-Reports\SGM\Dun-Drainage-Report\docs\Duncan-
Drainage-Report.docx
Aspen, Colorado August 3, 2022
Drainage Report for Major Design ii
TABLE OF CONTENTS
1.0 Existing Site 3
1.1 Description of Existing Site 3
1.2 Description of Existing Drainage 3
2.0 Proposed Project 3
3.0 Proposed Basins 4
3.1 Basin A (sub-basins 1 & 2) 4
Sub-basin 1 5
Sub-basin 2 5
3.2 Basin B (sub-basins 3, 6, & 7) 5
Sub-basin 3 6
Sub-basin 6 6
Sub-basin 7 7
3.3 Basin C (sub-basin 4) 7
3.4 Basin D (sub-basin 5) 8
4.0 Peak Flow Methodology 8
5.0 Water Quality Methodology 9
6.0 Maintenance 10
6.1 Grass Buffer Maintenance 10
6.2 Pervious Pavers 11
6.3 Sump and Pump 11
6.4 Rain Garden 12
7.0 Conclusion 12
LIST OF APPENDICES
Appendix A – Proposed Drainage Routing Schematic
Appendix B – Tree Canopy Credit Map
Appendix C – Peak Flow Calculations – Proposed Conditions
Appendix D – Water Quality Capture Volume Calculations
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 3
1.0 Existing Site
1.1 Description of Existing Site
The physical address of the project is 533 W. Hallam Street, Aspen, Colorado and it is
located on parcel no. 2735-124-32-001. This parcel is approximately 0.14 acres and is
owned by Duncan Skihaus LLC. Current zoning of the property is medium-density
residential (R-6). The existing structures on the parcel are one house and a small shed. The
project focuses on renovating the house while preserving the historic portion of it. The
existing house is 1897 square feet and has two stories with a garage located near the Block
29 alley.
1.2 Description of Existing Drainage
Existing storm water runs off several areas of the existing house either as sheet flow or
through gutters and downspouts. On the west and north sides of the site, runoff sheet flows
over grass to the curb and gutters along W. Hallam & 5th Streets, respectively. Runoff on the
east side of the building currently enters the neighboring property. Storm water on the south
side of the property enters the alley. No existing storm water detention or treatment BMPs
have been found on the site.
A soils report was performed for the site by HP Kumar on June 19, 2018. The results found
the soil to be Type B with a moderate infiltration rate of 2 to 3 minutes per inch.
There is no existing offsite runoff that enters the property. The property exists within the
Aspen Mountain Drainage Basin and will not require detention beyond the water quality
capture volume (WQCV). Existing peak flow calculations have not been provided for this
report because detention will not be required, and runoff will not cross any other properties
before discharging to the City of Aspen (COA) storm sewer system.
2.0 Proposed Project
The primary purpose of this project is to demolish the non-historic portion of the existing
house and build a new addition. Other improvements include the addition of onsite snowmelt
pavement and improving storm water runoff quality. The proposed project is considered a
major project as defined by the COA Urban Runoff Management Plan (URMP) because
more than 1000 square feet and 25% of the site will be disturbed.
The proposed residential development with have a footprint of 3160 square feet. The
historical portion of the property, which has a footprint of 900 square feet, will be moved
north but will remain intact. The main building will be located on the southern portion of the
lot with a footprint of 2260 square feet.
Storm water quality improvements consist of 3295 square feet of grass buffer, 278.5 square
feet of no-infiltration pervious gravel and 6.5 cubic feet of rain gardens. The grass buffer will
be located on the western and northern portion of the lot. The no-infiltration pervious gravel
will be located on the eastern central portion of the lot. A rain garden is located on the
northeast portion of the lot. No storm water generated on the site will flow across the eastern
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 4
property line onto the neighboring residential lot due to the placement of a curb on the
shoring wall extended slightly above grade. Runoff from the eastern side of the lot will flow
north to W. Hallam St. through grading.
3.0 Proposed Basins
The proposed site has 7 sub-basins, as shown in Appendix A. To clearly describe these
sub-basins, they have been grouped into 4 drainage basins based on shared discharge
locations and water quality treatment. A summary of the basins can be seen below in Table
1 and detailed calculations for Table 1 can be found in Appendix C. Storm water runoff will
be routed to designated BMPs for water quality treatment through a combination of grading
and gutters with downspouts. All proposed storm water runoff will discharge from the site as
sheet flow to W. Hallam & N. 5th Streets. Basins are detailed in the sections that follow, with
tables showing that the required WQCV is treated by BMPs within the basin. The Available
WQCV (cf) is calculated as the WQCV provided (in cubic feet) based on grass buffer areas,
tree canopy credits, proposed rain garden volume, and pervious gravel sections.
Table 1 – Basin Summary
3.1 Basin A (sub-basins 1 & 2)
Basin A is located on the north side of the site generally between the property line and the
Duncan Skihaus historic building with additional drainage from the front of the historic roof.
Sub-basins 1 and 2 comprise Basin A. This basin is 1356 square feet of which 488 square
feet is impervious. Tree canopy credit is calculated per section 8.4.1 of the COA URMP, a
detailed calculation is shown in Appendix C. A 473 square foot grass buffer and a 6.5 cubic
foot rain garden will be used to treat the remaining WQCV. See Appendix A for the
proposed location of BMPs in this basin.
Change
Order #2
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 5
Table 2
Basin A
Basin Area (sf) 1356
% Impervious 36%
Required WQCV (cf) 8.39
Credits
Grass Buffer Area (sf) 473
Tree Canopy Credit Area (sf) 190
No-infiltration Paver Vol. (cf) 0
Rain Garden Vol. (cf) 6.5
Results
Effective Impervious Area (sf) -176
Effective % Imperviousness -13%
Net WQCV (cf) -6.5
Available WQCV (cf) 14.9
Sub-basin 1
Sub-basin 1 is located towards north-east corner of the property and has a total and
impervious area of 684 and 254 square feet, respectively. This corresponds to 37%
impervious area for the sub-basin. This area is mostly a grassed area with the north-eastern
portion of the roof included. Storm water will be routed from the roof to a rain garden via roof
drainage and gutter downspouts then west to the reverse trench drain and finally, curb and
gutter on W. Hallam St. 4” underdrains will be routed to a sump and pump in sub-basin 1.
From there the sump and pump will evacuate collected storm water utilizing a reverse trench
drain
Sub-basin 2
Sub-basin 2 is located at the very north-west corner of the property and has a total and
impervious area of 672 and 234 square feet, respectively. This corresponds to 35%
impervious area for the sub-basin. This area is mostly a grassed area with the north-western
portion of the roof included. Storm water will be routed from the roof to a 150 square foot
grass buffer via sheet flow and then north to the curb and gutter on W. Hallam St. Grass
buffer is at or above a 1:1 ratio to impervious area, after removing the tree canopy credit.
3.2 Basin B (sub-basins 3, 6, & 7)
Basin B is located on the eastern boundary of the site, with a “leg” that reaches the northern
boundary of the site and will collect runoff from the back side of the roof of the historic house
and the north end of the proposed structure. Sub-basins 3, 6, and 7 comprise Basin B.
Storm water runoff will flow from roof areas toward the east property boundary over no-
infiltration pervious gravel and then over the front lawn near W. Hallam Street. This basin is
2537 square feet of which 1921 square feet is impervious. No-infiltration pervious gravel,
and tree canopy credit will be used to treat the remaining WQCV. The proposed no-
infiltration pervious gravel is 278.5 square feet. See Appendix A for the proposed location of
BMPs in this basin. The no-infiltration pervious gravel is based off the no-infiltration pervious
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 6
paver detail found in figure 8.31 of the COA URMP. Since the landscape architect preferred
a gravel surface in this area and the pavers provide no treatment, they are not being used.
After water infiltrates through the gravel and enters the underdrain, it is discharged by a
reverse trench drain in sub-basin 1 to ensure runoff is leaving the site in the same manner it
did historically.
Table 3
Basin B
Basin Area (sf) 2537
% Impervious 76%
Required WQCV (cf) 33.3
Credits
Grass Buffer Area (sf) 0
Tree Canopy Credit Area (sf) 15.6
No-infiltration Paver Vol. (cf) (gravel) 46.4
Rain Garden Vol. (cf) 0
Results
Effective Impervious Area (sf) 1905
Effective % Imperviousness 75%
Net WQCV (cf) -13.5
Available WQCV (cf) 46.8
Sub-basin 3
Sub-basin 3 is located along the eastern portion of the property, toward W. Hallam Street,
and has a total and impervious area of 1262 and 970 square feet, respectively. This
corresponds to 77% impervious area for the sub-basin. All storm water in sub-basin 3 will be
routed by grading, downspouts and gutters to 126.5 square feet of no-infiltration pervious
gravel. Remaining WQCV is treated by tree canopy credits. The no-infiltration pervious
gravel in sub-basin 3 results in roughly 4.44 cubic feet of excess available WQCV that can
accommodate any untreated runoff from sub-basin 6 (see Section 3.2.2 ). Runoff collected
in the hydrodrain hydroduct system for the roof patios will be directed to an underdrain
system and routed to a sump and pump in the crawlspace. This sump and pump will push
water to a drain outlet in sub-basin 3 upon which it will hit a splash block and distribute the
flow over Mexican beach pebble landscaping of sub-basin 3.
Sub-basin 6
Sub-basin 6 is located on the east-central portion of the property and has a total and
impervious area of 442 and 327 square feet, respectively. This corresponds to 74%
impervious area for the sub-basin. All storm water in sub-basin 6 will be routed by grading,
downspouts and gutters to 112.2 square feet of no-infiltration pervious gravel. This results in
roughly 13.1 cubic feet of excess available WQCV that can accommodate untreated runoff
from sub-basin 7 (see Section 3.2.3).
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 7
Sub-basin 7
Sub-basin 7 is located on the southern portion of the property and extends from the western
most extent to the eastern. Sub-basin 7 has a total and impervious area of 834 and 624
square feet, respectively. This corresponds to 75% impervious area for the sub-basin. This
sub-basin is adjacent to the alley along the southern property boundary and has a garage.
Runoff generated will not be conveyed through the alley, except for the concrete garage
apron. The concrete garage apron will have a micropile wall below and will not
accommodate an underdrain to route water towards the rain garden in sub-basin 1.
Additionally, the utilities entering from the southeast corner would pose a conflict for the
underdrain routing. The remaining runoff from sub-basin 7 will be routed by grading,
downspouts and gutters to 39.8 square feet of no-infiltration pervious gravel to the east of
the residence. Drainage and grading will allow roughly 4.01 cubic feet of untreated runoff in
this basin will sheet flow into sub-basin 6, where there is excess available treatment volume
provided by the no-infiltration pervious gravel.
3.3 Basin C (sub-basin 4)
Basin C is located on the north-west side of the site and will collect runoff from the back side
of the roof for the historic building and the north end of the proposed structure. Basin C
consists only of sub-basin 4. Storm water will be routed from both roofed areas across the
paver patio and directed to the grass buffers and then sheet flow to 5th Street. Runoff from
non-pervious landscaping will sheet flow across grass buffer then to the curb and gutter on
5th St. This basin is 796 square feet of which 539 square feet is impervious. Storm water
runoff will be treated by the reduction in impervious area provided by a 137 square foot tree
canopy credit and 198 square feet of grass buffer. Grass buffer is at or above a 1:1 ratio to
impervious area, after removing the tree canopy credit. Roof runoff that is unable to be
treated by grass buffer and canopy credit will be routed via 4” PVC to a flo-well wqcv basin
with a drain grate inlet that acts as an overflow. See Appendix A for the proposed location of
BMPs in this basin. Water runoff is leaving the site in the same manner it did historically.
Table 4
Basin C
Basin Area (sf) 796
% Impervious 68%
Required WQCV (cf) 9.0
Credits
Grass Buffer Area (sf) 198
Tree Canopy Credit Area (sf) 137
Flo-well WQ Vol. (cf) 6.3
Rain Garden Vol. (cf) 0.0
Results
Effective Impervious Area (sf) -204
Effective % Imperviousness -26%
Net WQCV (cf) -2.47
Available WQCV (cf) 11.4
Change
Order #2
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 8
3.4 Basin D (sub-basin 5)
Basin D is located along the west-central portion of the site, comprised solely of sub-basin 5.
Storm water will be routed from the roof to a full-infiltration rain garden. A portion of the roof
runoff will be directed to a raised planter rain garden from one of the downspouts. Runoff
from non-pervious landscaping will sheet flow across grass buffer then to the curb and
gutter on 5th St. This basin is 1312 square feet of which 882 square feet is impervious Storm
water runoff will be treated with 459 square feet of grass buffer and 129 square feet of tree
canopy credit. Grass buffer is at or above a 1:1 ratio to impervious area, after removing the
tree canopy credit. See Appendix A for the proposed location of BMPs in this basin. Water
runoff is leaving the site in the same manner it did historically.
Table 5
Basin D
Basin Area (sf) 1312
% Impervious 67%
Required WQCV (cf) 14.7
Credits
Grass Buffer Area (sf) 459
Tree Canopy Credit Area (sf) 129
No-infiltration Paver Vol. (cf) 0.0
Rain Garden Vol. (cf) 6.9
Results
Effective Impervious Area (sf) 295
Effective % Imperviousness 22%
Net WQCV (cf) -1.2
Available WQCV (cf) 15.8
4.0 Peak Flow Methodology
Proposed drainage flow rates were determined using the rational method as explained in
section 3.4 of the URMP and rainfall characteristics as explained in section 2 of the URMP.
The rational method calculations for peak flow can be seen in Appendix D. The following
tables list the parameters that were used, and the results determined for proposed
conditions.
Table 6 Watershed Characteristics
Imperviousness
Overland
Flow Time
(min)
Channel
Flow Time
(min)
Time of
Concentration
(min)
Max
Watershed
Length (ft)
Watershed
Slope (ft/ft) C5
63.9% N/A N/A 7.53 100 0.026 0.53
Table 7 Proposed Conditions for 10-Year Storm
C10 – Hydro Soil I10 (in/hr) A - (acres) Q10 (cfs)
Change
Order #2
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 9
Group B Contributing Area
0.57 3.36 0.14 0.27
Table 8 Proposed Conditions for 100-Year Storm
C100 – Hydro Soil
Group B I100 (in/hr) A - (acres)
Contributing Area Q100 (cfs)
0.73 5.37 0.14 0.54
For the calculation of proposed peak flows, the entire property was modeled as one
catchment. This assumption is justified by the relatively small area of the property (the
Rational Method is valid to 90 acres and this property is 0.14 acres). From Figure 3.1 of the
URMP, the soil can be classified as NRCS Hydrologic Soil Group B. The intensity for the 10-
year and 100-year storm were determined using Equation 2-1 of the URMP.
5.0 Water Quality Methodology
The Required WQCV (cf) for each basin on the property was estimated from Figure 8.13 of
the URMP based on the basin imperviousness and basin area. Then WQCV credits were
estimated based on the proposed areas for grass buffers, pervious pavers, and preserved
tree canopy (see further discussion of tree canopy credit below). All credits were deducted
from the impervious area of the basin and the effective impervious area were computed,
based on the instruction of section 8.4.1 in the URMP. From effective impervious area, the
effective imperviousness was calculated and an Effective WQCV (cf) was estimated from
Figure 8.13. Rain gardens and no-infiltration gravel (porosity = 0.333) were assumed 0.5 ft
deep and their capture volumes were deducted from the Effective WQCV (cf) to find the Net
WQCV (cf). If Net WQCV (cf) was found to be negative, it implies there is surplus treatment
capacity in the basin, and a positive Net WQCV (cf) implies there is a shortage of treatment
capacity in the basin.
The Available WQCV is defined in this report to be the difference between the Required
WQCV (cf) and the Net WQCV (cf). If Available WQCV (cf) is greater than the Required
WQCV (cf), this implies there is a surplus treatment capacity within the basin, and if
Available WQCV (cf) is less than the Required WQCV (cf), this implies a shortage of
treatment within the basin.
The Tree Canopy Credit was calculated following the recommendations in section 8.4.1 of
the URMP. Detailed calculations for the Tree Canopy Credit can be found in Appendix C. In
addition, a map showing the designated trees and their canopy areas is shown in Appendix
B. Runoff from all impervious areas will be treated by directing the runoff over a grass buffer,
no-infiltration pavers (gravel), and a rain garden. All proposed runoff will leave the property
as sheet flow and will flow over grass lawn located in the right of way before entering the
curb and gutter system.
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 10
6.0 Maintenance
6.1 Grass Buffer Maintenance
The following maintenance recommendations were found in the City of Aspen URMP.
Mowing recommendations for grass buffers that are included in the Urban Drainage Flood
Control District USDCM: Volume 3 Stormwater Quality are provided in the figure below.
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 11
6.2 Pervious Pavers
The following maintenance recommendations were found in the Urban Drainage Flood Control
District USDCM: Volume 3 Storm water Quality (page 6-15 to 6-17) and will be used as a guide
for the maintenance of the proposed pervious pavers. An inspection of the condition and
observation of infiltration should be completed annually. The pavers will lose infiltration
capabilities over time and it is recommended to use ASTM C1707 Standard Test Method for
Infiltration Rate of In Place Pervious Concrete to measure the infiltration rate of the pavers. A
regenerative air or vacuumed sweeper should be used twice annually and after any significant
site work, like landscaping, is done. However, the timing of vacuum sweeping is variable based
on site conditions and biannual vacuuming may be more often than needed. Sand should never
be applied to the pavers for snow removal as this will decrease the permeability of the pavers.
The proposed pavers will have snowmelt capabilities, but if snow removal is necessary,
mechanical snow removal should be done. If the surface of the paver system ever becomes
completely clogged, remove and replacement of the first ½ to 1 inch of infill. The use of a push
broom is recommended for the replacement of infill.
6.3 Sump and Pump
Sump and pump must be inspected and maintained yearly to remove sediment and debris that
is washed into them. Minimum inspection and maintenance requirements include the following:
Aspen, Colorado August 3, 2022
Drainage Report for Major Design 12
• Inspect sump and pump annually and after every storm exceeding 0.5 inches.
• Locate sump lid and cut an outline in sod to replace sod patch after maintenance.
• Remove the pump from the sump and clean it and inspect it.
• Dispose of sediment, debris/trash, and any other waste material removed from sump at
suitable disposal sites and in compliance with local, state, and federal waste
regulations.
• Check the owner’s manual to see if the pump bearings require grease or oil. If you do
not have the manual, look it up on the internet using the manufacturer and model
number of your pump or contact the manufacturer.
• Replace the pump in the sump and check that it is connected to power.
• Dump a bucket of water into the sump to raise the float and make sure the pump turns
on, adjusting the float if required.
6.4 Rain Garden
The rain garden is to be filled above the filter material with Mexican beach pebbles on the south
and long fibered mulch to the north. These filler materials were chosen to integrate with the
surrounding landscaping materials and to provide free drainage for the rain garden filter
materials below. Annual inspection of the beach pebbles and mulch should be taken to ensure
that the material is not clogged and inhibiting drainage. Inspection after large storms should
also be taken to ensure that the rain garden is draining as it should. Replacement and/or
cleaning of the material should be performed when it is observed that the rain garden is
performing less than optimally in its existing condition.
7.0 Conclusion
The proposed BMPs (grass buffer, no-infiltration pervious gravel, and rain gardens) are
expected to treat 100% of the required WQCV at the 533 W. Hallam Street project with room
to spare. With proper maintenance and installation, these BMPs will meet the criteria
outlined in the City of Aspen’s current Urban Runoff Management Plan.
Aspen, Colorado August 3, 2022
Drainage Report for Major Design Appendix
Appendix C
Peak Flow Calculations for Project Site – Proposed Conditions
• Watershed Area, 𝐴
𝑨=𝟎𝟎𝟎𝟎 𝒄𝒓𝟎=𝟎.𝟎𝟎 𝒂𝒄𝒓𝒄𝒓
• Percent Imperviousness, �ℎ
�ℎ=𝐴𝑖𝑙𝑜
𝐴𝑡𝑜𝑡𝑎𝑙
× 100
�ℎ=488 𝑐𝑟2 +1921 𝑐𝑟2 +539 𝑐𝑟2 +882 𝑐𝑟2
6000 𝑐𝑟2 × 100
𝒉=𝟎𝟎.𝟗 %
• Maximum Watershed Length, 𝐿0
𝑳𝟎=𝟎𝟎𝟎 𝒄𝒓,extent of property from South to North
• Watershed Slope, 𝑆0
𝑆0 =𝐸𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑟𝑙𝑟𝑟� 𝑐𝑙𝑐−𝐸𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑙𝑙𝑟𝑟� 𝑐𝑙𝑐
𝐿0
𝑆0 =7915.14 𝑐𝑟−7912.54 𝑐𝑟
100 𝑐𝑟
𝑺𝟎=𝟎.𝟎× 𝟎𝟎−𝟎
• Runoff Coefficients, 𝐶5, 𝐶10, & 𝐶100
Expressions for the 5-, 10-, and 100-year storm return period runoff coefficients can
be found in Table 6-4 in the Urban Drainage and Flood Control District (UDFCD)
Urban Storm Drainage Criteria Manual (USDCM)
𝐶5 =0.86�ℎ0.33 =0.5283
𝐶10 =0.81�ℎ+0.057 =0.5746
𝐶100 =0.47�ℎ+0.426 =0.7263
• Computed Time of Concentration, 𝑆𝑐
𝑆𝑐=𝑆𝑜+𝑆𝑓
where…
𝑆𝑜=0.395(1.1−𝐶5√𝐾0 )
𝑅0
0.33 (Overland flow time, Equation 3-4 in COA URMP)
𝑆𝑓=𝐾𝑓
60𝐾√𝑅𝑓
(Channelized flow time, Equation 3-5 in COA URMP)
And since there’s no channelized flow, 𝑆𝑓=0 so…
𝑆𝐶=𝑆𝑜=0.395(1.1 −0.5283)√100 𝑐𝑟
(2.6 × 10−2)0.33
Aspen, Colorado August 3, 2022
Drainage Report for Major Design Appendix
𝑺𝑪=𝟎.𝟎𝟎 𝒎𝒉𝒎𝒓𝒓𝒄𝒓
Design Time of Concentration, 𝑆𝑐
The design time of concentration, 𝑆𝑐, is the lesser of the computed time of
concentration, 𝑆𝑐, and the regional time of concentration, 𝑆𝑅, which is given by
𝑆𝑅=10 +𝐿0 /180
=10 +(100 𝑐𝑟/180)
=10.6 𝑙�ℎ𝑙𝑟𝑟𝑐𝑟
so…
𝑺𝒄=𝟎.𝟎 𝒎𝒉𝒎𝒓𝒓𝒄𝒓
• Peak Runoff for 10-year Storm Return Period, 𝑃10
𝑃10 =𝐶10𝐼10𝐴
where 𝐶10 is the 10-year runoff coefficient, 𝐼10 is the 10-year storm rainfall intensity
(units of �ℎ𝑙/�𝑟), and 𝐴 is the watershed area in units of 𝑎𝑐𝑟𝑐𝑟
𝐼10 =88.8𝑃1
(10+𝑅𝑑)1.052 (Equation 2-1 in COA URMP)
and 𝑃1 is the 1-hr precipitation intensity for a 10-year storm
𝑃1 =0.77 �ℎ𝑙/�𝑟 (Table 2.2 of the COA URMP)
so…
𝐼10 =88.8(0.77)
(10 +7.5)1.052
𝑰𝟎𝟎=𝟎.𝟎𝟎𝒉𝒎
𝒉𝒓
then the 10-year peak runoff is given by..
𝑃10 =(0.5283)(3.36 �ℎ𝑙
�𝑟)(0.14 𝑎𝑐𝑟𝑐𝑟)
𝑸𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒓
• Peak Runoff for 100-year Storm Return Period, 𝑃100
𝑃100 =𝐶100 𝐼100𝐴
where 𝐶10 is the 100-year runoff coefficient, 𝐼10 is the 100-year storm rainfall
intensity (units of �ℎ𝑙/�𝑟), and 𝐴 is the watershed area in units of 𝑎𝑐𝑟𝑐𝑟
𝐼100 =88.8𝑃1
(10+𝑅𝑑)1.052 (Equation 2-1 in COA URMP)
and 𝑃1 is the 1-hr precipitation intensity for a 100-year storm
𝑃1 =1.23 �ℎ𝑙/�𝑟 (Table 2.2 of the COA URMP)
so…
Aspen, Colorado August 3, 2022
Drainage Report for Major Design Appendix
𝐼100 =88.8(1.23)
(10 +7.5)1.052
𝑰𝟎𝟎𝟎=𝟎.𝟎𝟎𝒉𝒎
𝒉𝒓
then the 100-year peak runoff is given by
𝑃100 =(0.7263)(5.37 �ℎ𝑙
�𝑟)(0.14 𝑎𝑐𝑟𝑐𝑟)
𝑸𝟎𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒓
7 (B)
834 10.76
Sub-basin (Basin)
Area WQCV
sqft cuft
5 (D)
1312 14.65
6 (B)
442 5.60
3 (B)
1262 16.93
2 (A)
672 4.05
1 (A)
684 4.33
4 (C)
796 8.97
Title:
Basins & WQCV
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2018-292.001
JAK
08.03.2022
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APPENDIX
A 2Of :
Graphic Scale
In Feet: 1" = 10'
0 5 10 20
7 (B)
834 10.76
Sub-basin (Basin)
Area WQCV
sqft cuft
5 (D)
1312 14.65
6 (B)
442 5.60
3 (B)
1262 16.93
2 (A)
672 4.05
1 (A)
684 4.33
4 (C)
796 8.97
Title:
Tree Canopy Credit
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Job No.
Drawn by:
Date:
File:
PE:QC:
2018-292.001
JAK
08.03.2022
RB
Dun-MajDrRep
RB
11
8
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APPENDIX
B 2Of :
Graphic Scale
In Feet: 1" = 10'
0 5 10 20
Input
Calculation
Sub Basin Basin
Name Total Area Imp.
Area
Imperv-
iousness
Req'd WQCV
(depth)
Req'd WQCV
(vol.)Decid. Area Conif. Area
Grass Buffer
Area
No Inf.
Paver Area
No Inf. Paver
Depth
No Inf.
Paver Void
Vol.
Flo-well
Volume
Rain
Garden
Vol.
Perv.
Paver
Ratio
Grass
Buffer
Ratio
Canopy
Credit
Grass Buffer
Credit
Eff. Imp.
Area
Eff. Imperv-
iousness
Eff. WQCV
(depth)
Net
WQCV
(vol.)
(#)(sq. ft.)(sq. ft.)(%)(in)(cu. ft.)(sq. ft.)(sq. ft.)(sq. ft.)(sq. ft.)(ft)(cu. ft.)(cu. ft.)(cu. ft.)(sq. ft.)(sq. ft.)(sq. ft.)(%)(in)(cu. ft.)
1 A 684 254 37%0.0761 4.33 309.09 0.00 298 0.0 0.00 0.0 0.00 6.50 2 1 46.4 298 -91 -13.3%0.0000 -6.50
2 A 672 234 35%0.0724 4.05 0.00 478.30 175 0.0 0.00 0.0 0.00 0.00 2 1 143.5 175 -85 -12.7%0.0000 0.00
3 B 1262 970 77%0.161 16.93 75.06 0.00 0 126.5 0.50 21.1 0.00 0.00 2 1 11.3 0 959 76%0.1582 -4.44
4 C 796 539 68%0.135 8.97 170.89 370.42 198 0.0 0.00 0.0 6.28 0.00 2 1 136.8 198 204 26%0.0575 -2.47
5 D 1312 882 67%0.134 14.65 857.63 0.00 459 0.0 0.00 0.0 0.00 6.86 2 1 128.6 459 295 22%0.0520 -1.18
6 B 442 327 74%0.152 5.60 0.00 0.00 0 112.2 0.50 18.7 0.00 0.00 2 1 0.0 0 327 74%0.1522 -13.10
7 B 834 624 75%0.155 10.76 29.23 0.00 0 39.8 0.50 6.6 0.00 0.00 2 1 4.4 0 620 74%0.1533 4.01
Total 6000 3830 64%0.9 65.3 1441.9 848.7 1130.2 278.5 1.5 46.4 6.3 13.4 470.9 1130 2228.5 35%0.57 -23.7
Sub Basin Basin
Name Total Area Imp.
Area
% Imperv-
iousness
Req'd WQCV
(depth)
Req. WQCV
(vol.)Decid. Area Conif. Area
Grass Buffer
Area
No Inf. Perv.
Paver Area
No Inf. Paver
Depth
No Inf.
Paver Void
Vol.
Flo-Well
Volume
Rain
Garden
Vol.
Perv.
Paver
Ratio
Grass
Buffer
Ratio
Canopy
Area Credit
Gras Buffer
Area Credit
Eff.
Imperv.
Area
Eff. Imperv-
iousness
Net WQCV
Depth (in)
Net
WQCV
(cu. ft.)
Available
WQCV
(cu. ft.)
1,2 A 1356 488 36%0.074 8.39 309 478 473 0.00 0.00 0.00 0.00 6.50 2 1 190 473.5 -176 -13.0%0.000 -6.5 14.9
3,6,7 B 2537 1921 76%0.157 33.27 104 0 0 278.5 0.50 46.4 0.00 0.00 2 1 15.6 0.0 1905 75%0.156 -13.5 46.8
4 C 796 539 68%0.135 8.97 171 370 198 0.00 0.00 0.00 6.28 0.00 2 1 136.8 197.9 204 26%0.057 -2.5 11.4
5 D 1312 882 67%0.134 14.65 858 0 459 0.00 0.00 0.00 0.00 6.86 2 1 129 458.8 295 22%0.052 -1.2 15.8
6000 3830 64%0.5 65 1442 849 1130 278 0.5 46.4 6.3 13.4 471 1130 2229 28%0.26 -23.7 89.0
Basin WQCV Calculations Duncan
Legend