HomeMy WebLinkAboutFile Documents.533 W Hallam St.0285.2018 (76).ARBK
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.
November 28, 2018
Prepared by
118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004
970.945.5948 fax
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design i
DRAINAGE REPORT FOR
DUNCAN SKIHAUS
533 W. HALLAM STREET, ASPEN, CO
REVIEWED BY
SGM Project # 2018-292.001
I:\2018\2018-292-DuncanSkihaus\001\E-Reports\SGM\Dun-Drainage-Report\docs\Duncan-
Drainage-Report.docx
11/28/18
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
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 6
3.3 Basin C (sub-basin 4) 7
3.4 Basin D (sub-basin 5) 7
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
7.0 Conclusion 12
LIST OF APPENDICES
Appendix A – Proposed Drainage Routing Schematic
Appendix B – Tree Canopy Credit Map
Appendix C – Water Quality Capture Volume Calculations
Appendix D – Peak Flow Calculations – Proposed Conditions
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
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.
There is no existing offsite runoff that enters the property. 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
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 150 square feet of grass buffer, 260 square
feet of no-infiltration pervious pavers, 315 square feet of no-infiltration pervious gravel and
6.5 cubic feet of rain gardens. The grass buffer will be located on the northern portion of the
lot. The no-infiltration pervious pavers will be located on the western central 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 property line onto the neighboring residential lot due to the
12/04/2018
Please discuss soil
types in Section 1.2
of the Drainage
Report
Report indicates 260 SF of pervious
pavers while drainage exhibit shows 216
SF.10/16/2019
533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 4
placement of a curb on the shoring wall exposed 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 proposed non-infiltrating pervious
paver void volume.
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 Error! Reference source not found.. A 150 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.
12/04/2018
It appears your Word template has a
broken reference in Section 3.1 of the
Drainage Report.
10/16/2019
533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 5
Table 2
Basin A
Basin Area (sf) 1356
% Impervious 36%
Required WQCV (cf) 8.39
Credits
Pervious Paver Area (sf) 0
Grass Buffer Area (sf) 0
Tree Canopy Credit Area (sf) 107
No-infiltration Paver Vol. (cf) 0.0
Rain Garden Vol. (cf) 6.5
Results
Effective Impervious Area (sf) 230.9
Effective % Imperviousness 17%
Net WQCV (cf) -1.7
Available WQCV (cf) 10.13
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 north to the curb and gutter on W. Hallam St.
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.
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
2744 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 315 square feet. See Appendix A for the proposed location of
BMPs in this basin. The no-infiltration pervious gravel is based off of the no-infiltration
pervious 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 slot drain, it is
12/04/2018
In Table 2 the grass buffer area is
omitted, however sub basin 2 utilizes a
grass buffer.
Impervious area to grass buffer
ratio must be 1:1. Refer to page
8-43 of the URMP.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 6
discharged by a reverse trench drain 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
Pervious Paver Area (sf) 0
Grass Buffer Area (sf) 0
Tree Canopy Credit Area (sf) 4.2
No-infiltration Paver Vol. (cf) 56.3
Rain Garden Vol. (cf) 0.0
Results
Effective Impervious Area (sf) 1917
Effective % Imperviousness 76%
Net WQCV (cf) -19.3
Available WQCV (cf) 52.6
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 66% impervious area for the sub-basin. All storm water in sub-basin 3 will be
routed by grading, downspouts and gutters to 168.3 square feet of no-infiltration pervious
gravel. Remaining WQCV is treated by tree canopy credits. The no-infiltration pervious
gravel in sub-basin 3 result in roughly 11.3 cubic feet of excess available WQCV that can
accommodate any untreated runoff from sub-basin 6 (see Section 3.2.2 ).
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 126.5 square feet of no-infiltration pervious gravel. This results in
roughly 15.5 cubic feet of excess available WQCV that can accommodate untreated runoff
from sub-basin 7 (see Section 3.2.3).
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 835 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. The runoff from sub-basin 7 will be
routed by grading, downspouts and gutters to 43.2 square feet of no-infiltration pervious
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 7
gravel. Drainage and grading will allow roughly 3.6 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 pavers.
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-basins 4. Storm water will be routed from both roofed areas to no-
infiltration pervious pavers and then to the curb and gutter on 5th St. This basin is 1312
square feet of which 720 square feet is impervious. Storm water runoff will be treated with
135 square feet of no-infiltration pervious pavers. Remaining WQCV is expected to be
treated by the reduction in impervious area provided by a 84.7 square foot tree canopy
credit. See Appendix A for the proposed location of BMPs in this basin. After water infiltrates
through the gravel and enters the slot drain, it is discharged by a reverse trench drain to
ensure runoff is leaving the site in the same manner it did historically.
Table 4
Basin C
Basin Area (sf) 796
% Impervious 33%
Required WQCV (cf) 4.56
Credits
Pervious Paver Area (sf) 0
Grass Buffer Area (sf) 0
Tree Canopy Credit Area (sf) 80.5
No-infiltration Paver Vol. (cf) 22.57
Rain Garden Vol. (cf) 0.0
Results
Effective Impervious Area (sf) 178
Effective % Imperviousness 22%
Net WQCV (cf) -19.1
Available WQCV (cf) 23.68
3.4 Basin D (sub-basin 5)
Basin D is located along the west-central portion of the, comprised solely of sub-basin 5.
Storm water will be routed from the roof to a grass buffer, rain garden and pervious pavers.
Runoff will then sheet flow to the curb and gutter on 5th St. This basin is 1312 square feet of
which 720 square feet is impervious. A grass buffer, pervious pavers, and a rain garden will
be used to treat runoff from the roof. The proposed grass buffer is 161 square feet and the
proposed pervious pavers cover 80.8 square feet. A rain garden is proposed to captured
5.68 cubic feet of runoff. Remaining WQCV is treated by the reduction in effective
impervious area provided by the 104 square foot tree canopy credit. See Appendix A for the
proposed location of BMPs in this basin.
12/04/2018
Please check all areas in the narrative
for Basin C and ensure they are
consistent with the rest of the Drainage
Report.
Please check all areas in the narrative
for Basin D and ensure they are
consistent with the rest of the Drainage
Report.
No rain garden is shown in
basin 5.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 8
Table 5
Basin D
Basin Area (sf) 1312
% Impervious 55%
Required WQCV (cf) 11.7
Credits
Pervious Paver Area (sf) 0
Grass Buffer Area (sf) 0
Tree Canopy Credit Area (sf) 104
No-infiltration Paver Vol. (cf) 13.47
Rain Garden Vol. (cf) 0
Results
Effective Impervious Area (sf) 616
Effective % Imperviousness 47%
Net WQCV (cf) -3.4
Available WQCV (cf) 15.1
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
56.5% N/A N/A 8.40 100 0.026 0.46
Table 7 Proposed Conditions for 10-Year Storm
C10 – Hydro Soil
Group B I10 (in/hr) A - (acres)
Contributing Area Q10 (cfs)
0.51 3.19 0.14 0.23
Table 8 Proposed Conditions for 100-Year Storm
C100 – Hydro Soil
Group B I100 (in/hr) A - (acres)
Contributing Area Q100 (cfs)
0.69 5.09 0.14 0.48
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
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 9
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. Then WQCV credits were estimated based
on the proposed areas for grass buffers, no-infiltration pervious pavers, and preserved tree
canopy. Then the effective impervious area and effective imperviousness were computed,
based on the instruction of section 8.4.1 in the URMP. From effective imperviousness, the
net WQCV required to treat was estimated from Figure 8.13. If net effective imperviousness
was less than zero, this meant that all Required WQCV (cf) was treated by grass buffer,
pervious paver area, and grass buffer
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 pervious pavers, 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.
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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.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
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.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design 12
7.0 Conclusion
The proposed BMPs (grass buffer, no-infiltration pervious pavers, 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.
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Title:
Basins & WQCV
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2018-292.001
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10.30.2018
AE
Dun-MajDrRep
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APPENDIX
A 2Of :
Graphic Scale
In Feet: 1" = 10'
0 5 10 20
12/04/2018
Show major Drainage Basins A, B, C
and D on the Drainage Map. Each
sub-basin should be labeled with its area
and WQCV. Also show roof downspouts
locations, and contours. Provide finished
floor and grade at foundation elevations
at all building corners or transitions. Also
provide lot corner elevations and any
grade break elevations critical to the
grading concept.
Hatching for no infiltration gravel is
different in the legend than shown on the
Drainage Map.
What area drains to the
raingarden? This drainage map
show Basin 1 routed north and
the underdrain from the gravel
bed routed under the rain
garden.
10/16/2019
12"18"
20"
10"
3"
2"
6"
18"
12"
12"
30"
Title:
Tree Canopy Credit
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Job No.
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File:
PE:QC:
2018-292.001
JB
10.30.2018
AE
Dun-MajDrRep
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APPENDIX
B 2Of :
Graphic Scale
In Feet: 1" = 10'
0 5 10 20
12/04/201810/16/2019
533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
Appendix C
Water Quality Capture Volume Calculations
Basin A
%
Then lookup Required WQCV from Figure 8.13, which is . ! " # and…
$ % " &'() * .+ . ! ,
- .
. ! ,
-/ 0
. 1
Now, find the effective impervious area (2 )… 345 46789:.598; < =988 >;?@7A >98BCD < E9;FF GHII89 598; < 2*K89:.K;:89 598;+
where… 46789:C@HF 598; 488 ID
N E9;FF GHII89 598; 150 ID
N K89:C@HF K;:89 598; 0 ID
N
and… =988 >;?@7A >98BCD 0.15
*R8SCBH@HF 598;+T 0.3
*>@?IC89@HF 598;+
where… R8SCBH@HF 598; 121 ID
N >@?CI89@HF 598; 295 ID
N
so… 3II.467.598; 480 ID
N < 0.15
*276 ID N +< 0.3
*295 ID N +< 150 ID
N 3II.467.598; 230.93 ID
N
and… 3II.K89S8?D 46789:C@HF?8FF 17.0%
Then lookup Required WQCV from Figure 8.13, which is . " # and…
Y8D Z[>\ R87Dℎ 0.042 C?,
1 ID
12 C?- G;FC? 598;
0.042 C? ,1 ID
12 C?-*1356 ID N +
4.8 ID ^
Y@4?IC_D9;DC@? K89:.K;:89 \@_H68 598; B87Dℎ K89S8?D \@CB `7;S8 *0 ID N +*aC?.0.5 ID B87Dℎ+*33%+ 0 ID ^ b;C? E;9B8? \@_H68 6.50 ID
^
Finally… =98;D8B Z[>\ *Y8D Z[>\+<*Y@ < 4?IC_D9;DC@? K89:.K;:89 \@_.+< b;C? E;9B8? \@_. 4.8 ID ^< 0 ID
^< 6.5 ID
^ " &'() < .!
Basin B
12/04/201810/16/2019
533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
1
! ! %
Then lookup Required WQCV from Figure 8.13, which is . ! " # and…
$ % " &'() * .+ . ! ,
- .
. ! ,
-/ 0
.
Now, find the effective impervious area (2 )… 345 46789:.598; < =988 >;?@7A >98BCD < E9;FF GHII89 598; < 2*K89:.K;:89 598;+
where… 46789:C@HF 598; 1921 ID
N E9;FF GHII89 598; 0 ID
N K89:C@HF K;:89 598; 0 ID
N
and… =988 >;?@7A >98BCD 0.15
*R8SCBH@HF 598;+T 0.3
*>@?IC89@HF 598;+
where… R8SCBH@HF 598; 28 ID
N >@?CI89@HF 598; 0 ID
N
so… 3II.467.598; 1921 ID
N < 0.15
*28 ID N +< 0.3
*0 ID N +< 0 ID
N 3II.467.598; 1917 ID
N
and… 3II.K89S8?D 46789:C@HF?8FF 76%
Then lookup Required WQCV from Figure 8.13, which is . ! " # and…
Y8D Z[>\ R87Dℎ 0.157 C?,
1 ID
12 C?- G;FC? 598;
0.157 C? ,1 ID
12 C?-*2537 ID N +
33.2 ID ^
Y@4?IC_D9;DC@? K89:.K;:89 \@_H68 598; B87Dℎ K89S8?D \@CB `7;S8 *315 ID N +*0.5 ID B87Dℎ+*33%+ 52.0 ID ^ b;C? E;9B8? \@_H68 0 ID
^
Finally… =98;D8B Z[>\ *Y8D Z[>\+<*Y@ < 4?IC_D9;DC@? K89:.K;:89 \@_.+< b;C? E;9B8? \@_. 33.2 ID ^< 52.0 ID
^< 0 ID
^ " &'() < 1.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
Basin C
1
! %
Then lookup Required WQCV from Figure 8.13, which is . 1 " # and…
$ % " &'() * .+ . 1 ,
- .
. 1 ,
-/! 0
.
Now, find the effective impervious area (2 )… 345 46789:.598; < =988 >;?@7A >98BCD < E9;FF GHII89 598; < 2*K89:.K;:89 598;+
where… 46789:C@HF 598; 259 ID
N E9;FF GHII89 598; 0 ID
N K89:C@HF K;:89 598; 0 ID
N
and… =988 >;?@7A >98BCD 0.15
*R8SCBH@HF 598;+T 0.3
*>@?IC89@HF 598;+
where… R8SCBH@HF 598; 23 ID
N >@?CI89@HF 598; 257 ID
N
so… 3II.467.598; 756 ID
N < 0.15*23 ID N +< 0.3*257 ID N +< 0 ID
N 3II.467.598; 178 ID
N
and… 3II.K89S8?D 46789:C@HF?8FF 22%
Then lookup Required WQCV from Figure 8.13, which is . 1 " # and…
Y8D Z[>\ R87Dℎ 0.069 C?,
1 ID
12 C?- G;FC? 598;
0.069 C? ,1 ID
12 C?-*756 ID N +
3.4ID ^
Y@4?IC_D9;DC@? K89:.K;:89 \@_H68 598; B87Dℎ K89S8?D \@CB `7;S8 135 ID N *0.5 ID B87Dℎ+ *33%+ 22.57 ID ^ b;C? E;9B8? \@_H68 0 ID
^
Finally… =98;D8B Z[>\
*Y8D Z[>\+<*Y@ < 4?IC_D9;DC@? K89:.K;:89 \@_.
+< b;C? E;9B8? \@_. 3.4 ID ^< 22.57 ID
^< 0 ID
^ " &'() < 1.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
Basin D
!
%
Then lookup Required WQCV from Figure 8.13, which is . ! " # and…
$ % " &'() * .+ . ! ,
- .
. ! ,
-/ 0
.!
Now, find the effective impervious area (2 )… 345 46789:.598; < =988 >;?@7A >98BCD < E9;FF GHII89 598; < 2*K89:.K;:89 598;+
where… 46789:C@HF 598; 720 ID
N E9;FF GHII89 598; 0 ID
N K89:C@HF K;:89 598; 0 ID
N
and… =988 >;?@7A >98BCD 0.15
*R8SCBH@HF 598;+T 0.3
*>@?IC89@HF 598;+
where… R8SCBH@HF 598; 693 ID
N >@?CI89@HF 598; 0 ID
N
so… 3II.467.598; 720 ID
N < 0.15*0 ID N +< 0.3*693 ID N +< 0 ID
N 3II.467.598; 616 ID
N
and… 3II.K89S8?D 46789:C@HF?8FF 47%
Then lookup Required WQCV from Figure 8.13, which is . 1 " # and…
Y8D Z[>\ R87Dℎ 0.0925 C?,
1 ID
12 C?- G;FC? 598;
0.0925 C? ,1 ID
12 C?-*756 ID N +
10.1 ID ^
Y@4?IC_D9;DC@? K89:.K;:89 \@_H68 598; B87Dℎ K89S8?D \@CB `7;S8 80.8 ID N *0.5 ID B87Dℎ+ *33%+ 13.47 ID ^ b;C? E;9B8? \@_H68 0 ID
^
Finally… =98;D8B Z[>\
*Y8D Z[>\+<*Y@ < 4?IC_D9;DC@? K89:.K;:89 \@_.
+< b;C? E;9B8? \@_. 10.1 ID ^< 13.47 ID
^< 0 ID
^ " &'() < .
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
Appendix D
Peak Flow Calculations for Project Site – Proposed Conditions
· Watershed Area, 5 .
· Percent Imperviousness, C
C
5cde5fgfhi 100
C
488 ID N T 1921 ID
N T 259 ID
N T 720 ID
N
6000 ID N 100
. %
· Maximum Watershed Length, jk l ,extent of property from South to North
· Watershed Slope, `k
`k 3_8:;DC@? ;D F@HDℎ 8?B < 3_8:;DC@? ;D ?@9Dℎ 8?B
jk
`k 7915.14 ID < 7912.54 ID
100 ID
| . }
· Runoff Coefficients, >~, >•k, & >•kk
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)
>~ 0.86C k.^^ 0.4621 >•k 0.81C T 0.057 0.5147 >•kk 0.47C T 0.426 0.6916
· Computed Time of Concentration, =€ =€ =g T =•
where…
=g k.^‚~/•.•}ƒ„…†‡0
ˆ‡‡.‰‰ (Overland flow time, Equation 3-4 in COA URMP)
=• †Š
‹kŒ…ˆŠ (Channelized flow time, Equation 3-5 in COA URMP)
And since there’s no channelized flow, =• 0 so…
=ƒ =g 0.395*1.1 < 0.4621+…100 ID
*2.6 10
}N +k.^^
( .
· Design Time of Concentration, =•
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Use Figure 3.3 of the URMP as opposed
to Urban Drainage Tables for runoff
coefficients.
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
The design time of concentration, =•, is the lesser of the computed time of
concentration, =€, and the regional time of concentration, =Ž , which is given by
=Ž 10 T jk /180 10 T *100 ID/180+ 10.6 6C?HD8F
so… " .
· Peak Runoff for 10-year Storm Return Period, [•k
[•k >•k 4•k 5
where >•k is the 10-year runoff coefficient, 4•k is the 10-year storm rainfall intensity
(units of C?/ℎ9), and 5 is the watershed area in units of ;S98F
4•k ••.•‘’*•k“”•+’.‡„– (Equation 2-1 in COA URMP)
and K• is the 1-hr precipitation intensity for a 10-year storm
K• 0.77 C?/ℎ9 (Table 2.2 of the COA URMP)
so…
4•k 88.8*0.77+
*10 T 8.4+•.k~N
. 1
#
then the 10-year peak runoff is given by..
[•k *0.5147+,3.19 C?
ℎ9-*0.14 ;S98F+
' .
· Peak Runoff for 100-year Storm Return Period, [•kk
[•kk >•kk4•k 05
where >•k is the 100-year runoff coefficient, 4•k is the 100-year storm rainfall
intensity (units of C?/ℎ9 ), and 5 is the watershed area in units of ;S98F
4•kk ••.•‘’*•k“”•+’.‡„– (Equation 2-1 in COA URMP)
and K• is the 1-hr precipitation intensity for a 100-year storm
K• 1.23 C?/ℎ9 (Table 2.2 of the COA URMP)
so…
4•kk 88.8*1.23+
*10 T 8.4
+•.k~N
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533 W. Hallam Street, Aspen, Colorado November 28, 2018
Drainage Report for Major Design Appendix
.
#
then the 100-year peak runoff is given by
[•k *0.6916+,5.10 C?
ℎ9-*0.14 ;S98F+
' .
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