HomeMy WebLinkAboutFile Documents.1230 Snowbunny Ln.0116.2019 (11).ARBK
DRAINAGE REPORT FOR
VALENTINE RESIDENCE
1230 SNOWBUNNY LANE, ASPEN, CO
I hereby affirm that this report and the accompanying drawings for the analysis of 1230
Snowbunny Lane, 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. 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 #: 0166.2018.ARBK
September 9, 2019
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
09/16/2019 4:58:47 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.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design i
DRAINAGE REPORT FOR
VALENTINE RESIDENCE
1230 SNOWBUNNY LANE, ASPEN, CO
REVIEWED BY
RICK BARTH
SGM Project # 2018-458.001
I:\projects\2018\2018-458-SnowbunnyLn\001\E-Reports\SGM\Drainage Report\Valentine-Drainage-
Report.docx
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
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 1,2,3, & 4) 5
Sub-basin 1 5
Sub-basin 2 5
Sub-basin 3 6
Sub-basin 4 6
3.2 Basin B (sub-basins 5 & 6) 6
Sub-basin 5 7
Sub-basin 6 7
4.0 Peak Flow Methodology 7
5.0 Water Quality Methodology 8
6.0 Detention Methodology 9
7.0 Maintenance 10
7.1 Grass Buffer Maintenance 10
7.2 Pervious Pavers 12
8.0 Conclusion 12
LIST OF APPENDICES
Appendix A – Proposed Drainage Routing Schematic
Appendix B – Water Quality Credit Map
Appendix C – Water Quality Capture Volume Calculations
Appendix D – Peak Flow Calculations – Proposed Conditions
Appendix E – Detention Storage Calculations – Rational Volume Method
Appendix F – Detention Outfall Pipe Sizing
Appendix G – Soil Survey
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 3
1.0 Existing Site
1.1 Description of Existing Site
The physical address of the project is 1230 Snowbunny Lane, Aspen, Colorado and it is
located on parcel no. 2735-122-07-004. This parcel is approximately 0.38 acres and is
owned by 1230 Snowbunny Lane LLC. Current zoning of the property is medium-density
residential (R-6). The existing structure on the parcel is a single-family residential home
built in 1965. The project focuses on renovating the house while preserving the majority of
the existing landscaping. The existing house is 3816 square feet and has two stories with a
two-car garage.
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. Proposed routing patterns will continue to match existing
routing, which follows historical runoff patterns to the best of our knowledge. The age of
vegetation indicates this likely has been the general drainage pattern since the original
home.
On the west and north sides of the site, runoff sheet flows over grass towards the adjacent
property to the north, with no indication of concentrated flow or channelization. Runoff on the
east and south side of the building currently drains to the backyard which drains northeast to
an existing ditch. The ditch outfalls to the tail end of Castle Creek right before joining into the
Roaring Fork River. The only existing BMP on site is a drywell that collects runoff near the
front of the house by the garage. This drywell will be protected during construction and
remain in place for future use. Runoff will be routed to the existing drywell by means of a
new trench drain at the bottom of the driveway. There are no obvious indications of
drainage, erosion, or offsite flow issues.
A Web Soil Survey through the NRCS website indicates that the hydrologic soil group for the
site is Type B. The Web Soil Survey is attached to this report in Appendix G. There is no
existing offsite runoff that enters the property except the paved driveway that is outside the
property limits but within the City of Aspen ROW. Historic and Existing peak flow
calculations have been calculated for the site and runoff will maintain historic flow patterns.
There is no City of Aspen (COA) storm sewer system that the runoff from the site will
discharge to.
2.0 Proposed Project
The primary purpose of this project is to remodel the interior of the residence while adding
patio areas and permeable pavement locations, including a portion of the driveway. The
proposed remodel 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.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 4
The proposed residential development will have a footprint of 4520 square feet, up from the
existing footprint of 3526 square feet. Generally, runoff from the proposed building will not
change from existing to proposed conditions unless otherwise noted for water quality
treatment.
Storm water quality improvements consist of two rain gardens, 1548 square feet of pervious
pavers, and utilization of existing grass yard as grass buffers. The grass buffer will be
primarily located on the east portions of the site. The pervious pavers will be located on the
western portion of the house around the front entry, the driveway, and the eastern portion of
the house. Runoff from the south and east side of the lot will sheet flow north-easterly to a
proposed rain garden and detention area before infiltrating into the ground. The proposed
stormwater BMPs and routing can be seen in Appendix B.
Above ground detention is being proposed for this site because it will have a lesser impact
and easier maintenance than drywells.
3.0 Proposed Basins
The proposed site has 6 sub-basins, generally divided by rooflines, as shown in Appendix A.
To clearly describe these sub-basins, they have been grouped into 2 drainage basins based
on shared discharge locations, water quality treatment, and detention. 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 and detention through a combination of grading and gutters with downspouts. All
proposed storm water runoff will discharge from the site as sheet flow to the neighboring
property to the north or to the existing ditch to the east, which is the current drainage
pattern. 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, and
proposed pervious paver area.
Table 1 – Basin Summary
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 5
3.1 Basin A (sub-basins 1,2,3, & 4)
Basin A encompasses the south and east side of the site with a portion that touches the
north-east side of the house, generally between the residence and the ditch to the east.
Sub-basins 1, 2, 3 and 4 comprise Basin A. This basin is 9674 square feet of which 3032
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. Grass buffers, pervious pavers, and a
rain garden will treat the remaining WQCV. See Appendix B for the proposed location of
BMPs in this basin.
Table 2
Basin A (Sub-basins 1,2,3, & 4)
Basin Area (sf) 9674
% Impervious 31%
Required WQCV (cf) 53.8
Credits
Pervious Paver Area (sf) 744
Grass Buffer Area (sf) 2667
Tree Canopy Credit Area (sf) 205
Rain Garden Volume (cf) 4.5
Results
Effective Impervious Area (sf) -1328
Effective % Imperviousness -13.7%
Available WQCV (cf) 87.2
Surplus WQCV (cf) 33.3
Sub-basin 1
Sub-basin 1 is located on the south side of the property and has a total and impervious area
of 2616 and 869 square feet, respectively. This corresponds to 33% impervious area for the
sub-basin. This area is a grassed area containing trees with the south portion of the roof
included. The existing site will be impacted by the addition of a living space over the garage,
a second story deck, a covered patio, and flagstone pavers that lead to a hot tub. Storm
water will be routed from the roof to grass buffer via roof drainage and gutter downspouts
then east and ultimately north by existing vegetated topography to the proposed detention
along the east side of the site. Grass buffer area is at or above a 1:1 ratio to impervious
area, after applying the tree canopy credit as shown in Appendix C.
Sub-basin 2
Sub-basin 2 is located at the south-east corner of the property and has a total and
impervious area of 2717 and 537 square feet, respectively. This corresponds to 20%
impervious area for the sub-basin. This area is mostly a grassed area with the south-eastern
portion of the roof included. The existing site will be impacted by the expansion of the
kitchen and the addition of a covered patio and a portion of pervious pavers. Storm water
will be routed from the roof to grass buffer via gutter and sheet flow. Runoff will then route
east via existing grading and a proposed berm, ending up at a proposed detention along the
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 6
east side of the site. Grass buffer area is at or above a 1:1 ratio to impervious area, after
applying the tree canopy credit.
Sub-basin 3
Sub-basin 3 is located on the east side of the property and has a total and impervious area
of 2789 and 1297, respectively. This corresponds to 46% impervious area for the sub-basin.
This area is partly grassed with the eastern portion of the roof included. The existing site will
be impacted by the addition of pervious pavers to the uncovered, backyard patio. Storm
water will be routed from the roof to the grass buffer via gutters, down spouts, and splash
blocks. Additional stormwater will fall onto the pervious pavers. The runoff will then route
east via existing grading and a proposed berm, ending up at a proposed detention along the
east side of the site.
Sub-basin 4
Sub-basin 4 is located on the north-east side of the property and has a total and impervious
area of 1552 and 329, respectively. This corresponds to 21% impervious area for the sub-
basin. This area is partly grassed area with a portion of trees and the north-east portion of
the roof included. Storm water will be collected from the roof and routed around the corner
to the east and ultimately south to the proposed rain garden and detention via gutter and
sheet flow. The runoff will utilize the rain garden for the remaining water quality treatment in
the detention at the north-east corner of the site. Grass buffer area is at or above a 1:1 ratio
to impervious area, after applying the tree canopy and rain garden credit.
3.2 Basin B (sub-basins 5 & 6)
Basin B is located on the western boundary of the site, with a portion that reaches the
northern boundary of the site. Sub-basins 5 and 6 comprise Basin B. Storm water runoff will
flow from roof areas toward the west property boundary over pervious pavers and then over
the front lawn. This basin is 4693 square feet of which 1740 square feet is impervious.
Pervious pavers, tree canopy credit, and a rain garden will be used to treat the WQCV. The
proposed pervious pavers are 804 square feet. See Appendix B for the proposed location of
BMPs in this basin.
Table 3
Basin B
Basin Area (sf) 4693
% Impervious 37%
Required WQCV (cf) 29.7
Credits
Pervious Paver Area (sf) 804
Grass Buffer Area (sf) 0.0
Tree Canopy Credit Area (sf) 168
Rain Garden Volume (cf) 8.0
Results
Effective Impervious Area (sf) -35
Effective % Imperviousness -0.8%
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 7
Available WQCV (cf) 40.1
Surplus WQCV (cf) 10.4
Sub-basin 5
Sub-basin 5 is located at the north-western portion of the property and has a total and
impervious area of 1601 and 427 square feet, respectively. This corresponds to 27%
impervious area for the sub-basin. The existing sub-basin will remain unaffected in terms of
added impervious area between existing and proposed conditions. Most of the impacts to
this sub-basin include interior work, proposed berms and a rain garden. All storm water in
sub-basin 5 will be routed by grading, downspouts and gutters to a detention area at the
most north-west portion of the property. Remaining WQCV is accounted for by tree canopy
credit and the rain garden.
Sub-basin 6
Sub-basin 6 is located on the western portion of the property, along Snowbunny Lane and
has a total and impervious area of 3092 and 1313 square feet, respectively. This
corresponds to 42% impervious area for the sub-basin. A portion of the driveway within COA
right of way and outside property limits, will contribute runoff down the driveway into Sub-
basin 6. This excess runoff will get treated for water quality by excess pervious pavers on
the remaining part of the driveway. The existing site will be impacted by modifications to the
front entryway as well as the addition of a roof overhang and an uncovered pervious paver
patio. Additionally, the driveway within this section will be removed and replaced with
pervious pavers. Excess runoff that does not get infiltrated by the pervious pavers will be
collected by a trench drain at the base of the driveway and garage and routed by 4” PVC
storm drain to an existing dry-well west of the garage. All other storm water in sub-basin 6
will be routed by grading, downspouts and gutters to the rain garden in sub-basin 5. This
sub-basin contains a surplus of 371 square feet of pervious area that can accommodate
untreated runoff from the driveway that is impervious and exists in COA right of way (see
Section 3.2.3).
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 for the entire site.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 8
Table 4 Watershed Characteristics
Imperviousness
Overland
Flow Time
(min)
Channel
Flow Time
(min)
Time of
Concentration
(min)
Max
Watershed
Length (ft)
Watershed
Slope (ft/ft) C5
33.7% 14.34 N/A 14.34 165 0.025 0.263
Table 5 Proposed Conditions for 10-Year Storm
C10 – Hydro Soil
Group B I10 (in/hr) A - (acres)
Contributing Area Q10 (cfs)
0.330 2.79 0.33 0.304
Table 6 Proposed Conditions for 100-Year Storm
C100 – Hydro Soil
Group B I100 (in/hr) A - (acres)
Contributing Area Q100 (cfs)
0.584 4.46 0.33 0.859
For the calculation of proposed peak flows, the property was modeled as two catchments.
This approach was done based on flow patterns and discharge locations. A soils map was
researched for the site using a web soil survey from NRCS. The results found the soil to be
Type B. The intensity for the 10-year and 100-year storm were determined using Equation
2-1 of the URMP. A modified rational method calculation was used to determine the storage
volume required for the site. Work on the site fell into the major classification and required
storage of all developed portions of the site. The modified rational method was used to
determine the storage for the developed runoff with a historical release rate.
5.0 Water Quality Methodology
The Required WQCV (cf) for each basin on the property was calculated from Figure 8.13 of
the URMP based on the basin imperviousness and basin area. Then WQCV credits were
calculated based on the proposed areas for grass buffers (Section 8.5.1.3), pervious pavers
(8.5.1.4), and preserved tree canopy (Section 8.4.1, 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. A negative effective percent imperviousness was calculated. The absolute value of
this was used with Figure 8.13 to find the amount of WQCV each basin could treat in
excess. Next, Effective WQCV (cf) was calculated from the WQCV depth and the basin
area. This Effective WQCV was combined with the Rain Garden Volume to find the Net
WQCV. The negative value indicates a surplus in WQCV that can treat additional runoff,
should it be required in the future.
The Net WQCV is defined in this report to be the difference between the Required WQCV
(cf) and the Available 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. For this site, the Available WQCV far exceeds the Required
WQCV indicating a surplus in treatment capacity for each basin and the entire site.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 9
The Tree Canopy Credit was calculated following the methodology 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
and pervious pavers.
Rain gardens were installed in both basins to treat the remaining required WQCV. Locations
and WQCV calculations for the rain gardens can be found in Appendix B and C,
respectively.
6.0 Detention Methodology
The required detention volume was calculated using the Rational Volume Method for
Detention Volume (Watershed <90acres) as outlined in Chapter 5 the URMP. Calculations
of the required detention volume can be seen in Appendix E. After consideration of the site
and the importance of minimizing disturbance to the existing lawn, localized detention is
sized and located based on existing drainage paths. The most effective approach that
remains aesthetically pleasing is to build berms around the downstream points of each
basin. The detention areas will rely on berms and the existing grading to hold the developed
runoff from the site. The west detention area will incorporate an outlet pipe that will release
runoff at, or below historic rates, in a historic manner. Released runoff will be dispersed over
rip rap at the outlet to spread the flow and avoid point discharge. The east detention will be
created by raising the grade at the northeast corner roughly 6” and lowering down the
middle of the yard roughly 6”. The east detention will rely on infiltration to release the storm
runoff.
The berms were designed to overtop should an event greater than the 100-year storm event
occur. The overflow will spill into the ditch to the east and maintain historical drainage
patterns. Weir calculations showing the overflow is less than historic can be seen in the
appendix.
Using positive drainage away from the house and preserving as much healthy vegetation as
possible, the detention areas only needed berms built up on two sides to store runoff.
Utilizing quantities calculated from the Rational Volume Method for Detention Volume, the
berms would only need to be built up 6” from the existing ground. This approach will satisfy
not only the detention requirements but also be minimally invasive to the existing
landscaping.
Outfall discharge of the detention area was calculated using Equation 5-10 of the URMP
and can be seen in Appendix F. The outlet pipe was sized to drain the detention area at or
below the historic 100-year flow rate, while storing the 100-year storm. This approach was
used to avoid multiple stage discharge, thus eliminating a larger impact to the site.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 10
7.0 Maintenance
7.1 Grass Buffer Maintenance
The following maintenance recommendations were found in the Urban Drainage Flood Control
District USDCM: Volume 3 Stormwater Quality (page 6-5 to 6-7) and will be used as a guide for the
maintenance of the proposed grass buffer. Grass buffers should be inspected at least twice a year
for cover and vehicle impacts. Trash and debris should be removed from the buffer at least twice a
year. The buffer should be aerated once a year with holes of 2 inches in depth and no more than
four inches apart. When aerating the buffer, sprinkler heads and shallow utilities should be marked
to ensure they are not damaged. It is recommended to mow only when weeds need to be removed
in the first 3 years. After the first 3 years the grass should be maintained at 6 inches or more in
height. Additional information on mowing the grass buffer is provided in Figure 1. Irrigation is
recommended to maintain a healthy vegetation cover. In the early summer and fall, irrigation is
typically needed less, but in July and August more irrigation is needed. If native grasses are used,
irrigation should not be needed after establishment but could be useful during long dry periods.
Although irrigation is necessary for the establishment of the grass buffer, overwatering could cause
uneven growth and should be avoided. Every year the irrigation system should be drained and
blown out before the first winter freeze. The irrigation system should be inspected before
reactivation in the spring. Fertilizers and herbicides should be used only on an as-needed basis and
it is recommended to use biodegradable nontoxic fertilizers and herbicides.
The following maintenance recommendations were found in the City of Aspen URMP.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 11
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.
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design 12
7.2 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.
8.0 Conclusion
The proposed BMPs (grass buffer, pervious pavers, rain gardens, and detention) are
expected to treat 100% of the required WQCV and maintain historic discharge rates at the
1230 Snowbunny Lane project. With proper maintenance and installation, these BMPs will
meet the criteria outlined in the City of Aspen’s current Urban Runoff Management Plan.
Localized detention was sized to store the historic undeveloped rate and release at or below
the historic rate for the 10-year and 100-year storm to reduce the impacts of peak runoff on
downstream conveyances and properties.
HISTGraphic ScaleIn Feet: 1" = 6'03612Historic DrainageGreen Line Architects
1230 Snowbunny LanePE:QC:2018-458JK09.09.2019RBVR-DrainageAE118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004 www.sgm-inc.com APP APreliminary Not For ConstructionProject Milestone:5 (B)
1 (A)2 (A)3 (A)4 (A)5 (B)6 (B)Graphic ScaleIn Feet: 1" = 6'03612WQCV CreditsGreen Line Architects
1230 Snowbunny LanePE:QC:2018-458JK09.09.2019RBVR-DrainageAE118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004 www.sgm-inc.com APP BPreliminary Not For ConstructionProject Milestone:5 (B)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Appendix C
Water Quality Capture Volume Calculations
Basin A
𝑷𝒄𝒍𝒄𝒄𝒍𝒍 𝑰𝒍𝒍𝒄𝒍𝒍𝒉𝒍𝒍𝒍𝒍𝒄𝒍𝒍=𝑰𝒍𝒍𝒄𝒍𝒍𝒉𝒍𝒍𝒍 𝑨𝒍𝒄𝒂
𝑷𝒍𝒍𝒂𝒍 𝑨𝒍𝒄𝒂× 𝟎𝟎𝟎=𝟎𝟎𝟎𝟎 𝒄𝒍𝟎
𝟎𝟎𝟎𝟎 𝒄𝒍𝟎× 𝟎𝟎𝟎=𝟎𝟎%
Then lookup Required WQCV from Figure 8.13, which is 𝟎.𝟎𝟎𝟎 𝒉𝒍 𝒄𝒄𝒍𝒍𝒉 and…
𝑷𝒄𝒍𝒍𝒉𝒍𝒄𝒄 𝑽𝑷𝑨𝑽 (𝒍𝒍𝒍.)=𝟎.𝟎𝟎𝟎 𝒉𝒍 (𝟎 𝒄𝒍
𝟎𝟎 𝒉𝒍)× 𝑨𝒂𝒍𝒉𝒍 𝑨𝒍𝒄𝒂
=𝟎.𝟎𝟎𝟎 𝒉𝒍 (𝟎 𝒄𝒍
𝟎𝟎 𝒉𝒍)(𝟎𝟎𝟎𝟎 𝒄𝒍𝟎)
=𝟎𝟎.𝟎 𝒄𝒍𝟎
Now, find the effective impervious area (𝑬𝑰𝑨)…
𝐴𝐼𝐴 = 𝐼𝑙𝑙𝑐𝑟𝑟.𝐴𝑟𝑐𝑎−𝑃𝑟𝑐𝑐 𝐴𝑎𝑙𝑙𝑙𝑦 𝐴𝑟𝑐𝑐�ℎ𝑟−𝐺𝑟𝑎𝑟𝑟 𝐴𝑟𝑐𝑐𝑐𝑟 𝐴𝑟𝑐𝑎−2(𝑃𝑐𝑟𝑟.𝑃𝑎𝑟𝑐𝑟 𝐴𝑟𝑐𝑎)
where…
𝐼𝑙𝑙𝑐𝑟𝑟�ℎ𝑙𝑟𝑟 𝐴𝑟𝑐𝑎=3032 𝑐𝑟2
𝐺𝑟𝑎𝑟𝑟 𝐴𝑟𝑐𝑐𝑐𝑟 𝐴𝑟𝑐𝑎=2667 𝑐𝑟2
𝑃𝑐𝑟𝑟�ℎ𝑙𝑟𝑟 𝑃𝑎𝑟𝑐𝑟 𝐴𝑟𝑐𝑎=744 𝑐𝑟2
and…
𝑃𝑟𝑐𝑐 𝐴𝑎𝑙𝑙𝑙𝑦 𝐴𝑟𝑐𝑐�ℎ𝑟=0.15(𝐴𝑐𝑐�ℎ𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎)+0.3(𝐴𝑙𝑙𝑐�ℎ𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎)
where…
𝐴𝑐𝑐�ℎ𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎=1173 𝑐𝑟2
𝐴𝑙𝑙�ℎ𝑐𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎=96 𝑐𝑟2
so…
𝐴𝐼𝐴=3032 𝑐𝑟2 −0.15(1173 𝑐𝑟2)−0.3(96 𝑐𝑟2)−2667 𝑐𝑟2 −2(744 𝑐𝑟2)
𝐴𝐼𝐴= −1328 𝑐𝑟2
and…
𝑬𝒄𝒄.𝑷𝒄𝒍𝒄𝒄𝒍𝒍 𝑰𝒍𝒍𝒄𝒍𝒍𝒉𝒍𝒍𝒍𝒍𝒄𝒍𝒍=−𝟎𝟎.𝟎%
(Negative indicates credits being greater than impervious area)
Use Eff. Imperviousness to find Effective WQCV from Figure 8.13, which is −𝟎.𝟎𝟎𝟎𝟎 𝒉𝒍 𝒄𝒄𝒍𝒍𝒉
and…
𝐴𝑐𝑐𝑐𝑐𝑟�ℎ𝑟𝑐 𝑉𝑃𝐴𝑉 (𝑐𝑟3 )=−0.0358 �ℎ𝑙(1 𝑐𝑟
12 �ℎ𝑙)× 𝐴𝑎𝑟�ℎ𝑙 𝐴𝑟𝑐𝑎
= −0.0358 �ℎ𝑙 (1 𝑐𝑟
12 �ℎ𝑙)(9674 𝑐𝑟2)
=−28.86 𝑐𝑟3
𝑃𝑎�ℎ𝑙 𝐺𝑎𝑟𝑐𝑐𝑙 𝑉𝑙𝑙𝑟𝑙𝑐=4.5 𝑐𝑟3
Finally…
𝑵𝒄𝒍 𝑽𝑷𝑨𝑽=(𝐴𝑐𝑐𝑐𝑐𝑟�ℎ𝑟𝑐 𝑉𝑃𝐴𝑉)−𝑃𝑎�ℎ𝑙 𝐺𝑎𝑟𝑐𝑐𝑙 𝑉𝑙𝑙.
=−28.86 𝑐𝑟3 −4.5 𝑐𝑟3
𝑵𝒄𝒍 𝑽𝑷𝑨𝑽=−𝟎𝟎.𝟎 𝒄𝒍𝟎 (Negative indicates surplus WQCV)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Basin B
𝑷𝒄𝒍𝒄𝒄𝒍𝒍 𝑰𝒍𝒍𝒄𝒍𝒍𝒉𝒍𝒍𝒍𝒍𝒄𝒍𝒍=𝑰𝒍𝒍𝒄𝒍𝒍𝒉𝒍𝒍𝒍 𝑨𝒍𝒄𝒂
𝑷𝒍𝒍𝒂𝒍 𝑨𝒍𝒄𝒂× 𝟎𝟎𝟎=𝟎𝟎𝟎𝟎 𝒄𝒍𝟎
𝟎𝟎𝟎𝟎 𝒄𝒍𝟎× 𝟎𝟎𝟎=𝟎𝟎%
Then lookup Required WQCV from Figure 8.13, which is 𝟎.𝟎𝟎𝟎 𝒉𝒍 𝒄𝒄𝒍𝒍𝒉 and…
𝑷𝒄𝒍𝒍𝒉𝒍𝒄𝒄 𝑽𝑷𝑨𝑽 (𝒍𝒍𝒍.)=𝟎.𝟎𝟎𝟎 𝒉𝒍 (𝟎 𝒄𝒍
𝟎𝟎 𝒉𝒍)× 𝑨𝒂𝒍𝒉𝒍 𝑨𝒍𝒄𝒂
=𝟎.𝟎𝟎𝟎 𝒉𝒍 (𝟎 𝒄𝒍
𝟎𝟎 𝒉𝒍)(𝟎𝟎𝟎𝟎 𝒄𝒍𝟎)
=𝟎𝟎.𝟎 𝒄𝒍𝟎
Now, find the effective impervious area (𝑬𝑰𝑨)…
𝐴𝐼𝐴 = 𝐼𝑙𝑙𝑐𝑟𝑟.𝐴𝑟𝑐𝑎−𝑃𝑟𝑐𝑐 𝐴𝑎𝑙𝑙𝑙𝑦 𝐴𝑟𝑐𝑐�ℎ𝑟−𝐺𝑟𝑎𝑟𝑟 𝐴𝑟𝑐𝑐𝑐𝑟 𝐴𝑟𝑐𝑎−2(𝑃𝑐𝑟𝑟.𝑃𝑎𝑟𝑐𝑟 𝐴𝑟𝑐𝑎)
where…
𝐼𝑙𝑙𝑐𝑟𝑟�ℎ𝑙𝑟𝑟 𝐴𝑟𝑐𝑎=1740 𝑐𝑟2
𝐺𝑟𝑎𝑟𝑟 𝐴𝑟𝑐𝑐𝑐𝑟 𝐴𝑟𝑐𝑎=0 𝑐𝑟2
𝑃𝑐𝑟𝑟�ℎ𝑙𝑟𝑟 𝑃𝑎𝑟𝑐𝑟 𝐴𝑟𝑐𝑎=804 𝑐𝑟2
and…
𝑃𝑟𝑐𝑐 𝐴𝑎𝑙𝑙𝑙𝑦 𝐴𝑟𝑐𝑐�ℎ𝑟=0.15(𝐴𝑐𝑐�ℎ𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎)+0.3(𝐴𝑙𝑙𝑐�ℎ𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎)
where…
𝐴𝑐𝑐�ℎ𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎=654 𝑐𝑟2
𝐴𝑙𝑙�ℎ𝑐𝑐𝑟𝑙𝑟𝑟 𝐴𝑟𝑐𝑎=234 𝑐𝑟2
so…
𝐴𝐼𝐴=1740 𝑐𝑟2 −0.15(654 𝑐𝑟2)−0.3(234 𝑐𝑟2)−0 𝑐𝑟2 − 2(804) 𝑐𝑟2
𝐴𝐼𝐴= −35 𝑐𝑟2
and…
𝑬𝒄𝒄.𝑷𝒄𝒍𝒄𝒄𝒍𝒍 𝑰𝒍𝒍𝒄𝒍𝒍𝒉𝒍𝒍𝒍𝒍𝒄𝒍𝒍=−𝟎.𝟎%
(Negative indicates credits being greater than impervious area)
Then use it to find Effective WQCV from Figure 8.13, which is −𝟎.𝟎𝟎𝟎𝟎𝟎 𝒉𝒍 𝒄𝒄𝒍𝒍𝒉 and…
𝑬𝒄𝒄𝒄𝒄𝒍𝒉𝒍𝒄 𝑽𝑷𝑨𝑽 (𝒄𝒍𝟎)=−0.00618 �ℎ𝑙(1 𝑐𝑟
12 �ℎ𝑙)× 𝐴𝑎𝑟�ℎ𝑙 𝐴𝑟𝑐𝑎
= −0.00618 �ℎ𝑙 (1 𝑐𝑟
12 �ℎ𝑙)(4693 𝑐𝑟2)
=−2.42 𝑐𝑟3
𝑃𝑎�ℎ𝑙 𝐺𝑎𝑟𝑐𝑐𝑙 𝑉𝑙𝑙𝑟𝑙𝑐=8.0 𝑐𝑟3
Finally…
𝑵𝒄𝒍 𝑽𝑷𝑨𝑽=(𝐴𝑐𝑐𝑐𝑐𝑟�ℎ𝑟𝑐 𝑉𝑃𝐴𝑉)−𝑃𝑎�ℎ𝑙 𝐺𝑎𝑟𝑐𝑐𝑙 𝑉𝑙𝑙.
=−2.42𝑐𝑟3 −8.0 𝑐𝑟3
𝑵𝒄𝒍 𝑽𝑷𝑨𝑽=−𝟎𝟎.𝟎 𝒄𝒍𝟎 (Negative indicates surplus WQCV)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Appendix D
Peak Flow Calculations for Project Site – Proposed Conditions Basin A
• Watershed Area, 𝐴
𝑨=𝟎𝟎𝟎𝟎 𝒄𝒍𝟎=𝟎.𝟎𝟎 𝒂𝒄𝒍𝒄𝒍
• Percent Imperviousness, �ℎ
�ℎ=𝐴𝑖𝑙𝑜
𝐴𝑡𝑜𝑡𝑎𝑙
× 100
�ℎ=3032 𝑐𝑟2
9674 𝑐𝑟2 × 100
𝒉=𝟎𝟎.𝟎𝟎 %
• Maximum Watershed Length, 𝐿0
𝑳𝟎=𝟎𝟎𝟎 𝒄𝒍,routing on property from SW around building corner to NE corner
• Watershed Slope, 𝑃0
𝑃0 =𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑟𝑙𝑟𝑟� 𝑐𝑙𝑐−𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑙𝑙𝑟𝑟� 𝑐𝑙𝑐
𝐿0
𝑃0 =7857.96 𝑐𝑟−7854.17 𝑐𝑟
165 𝑐𝑟
𝑷𝟎=𝟎.𝟎× 𝟎𝟎−𝟎
• Runoff Coefficients, 𝐴5, 𝐴10, & 𝐴100
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). Soil Type B was used in
determining the runoff coefficients.
𝐴5 =0.2439
𝐴10 =0.3113
𝐴100 =0.5736
• Computed Time of Concentration, 𝑃𝑐
𝑃𝑐=𝑃𝑜+𝑃𝑓
where…
𝑃𝑜=0.395(1.1−𝐶5√𝐾0)
𝑅00.33 (Overland flow time, Equation 3-4 in COA URMP)
𝑃𝑓=𝐾𝑓
60𝐾√𝑅𝑓
(Channelized flow time, Equation 3-5 in COA URMP)
And since there’s no channelized flow, 𝑃𝑓=0 so…
𝑃𝐶=𝑃𝑜=0.395(1.1 −0.2439)√165 𝑐𝑟
(2.3 × 10−2)0.33
𝑷𝑨=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
• Design Time of Concentration, 𝑃𝑐
The design time of concentration, 𝑃𝑐, is the lesser of the computed time of
concentration, 𝑃𝑐, and the regional time of concentration, 𝑃𝑅, which is given by
𝑃𝑅=10 +𝐿0/180
=10 +(165 𝑐𝑟/180)
=10.92 𝑙�ℎ𝑙𝑟𝑟𝑐𝑟
so…
𝑷𝒄=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Peak Runoff for 10-year Storm Return Period, 𝑃10
𝑃10 =𝐴10𝐼10𝐴
where 𝐴10 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 +10.92)1.052
𝑰𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 10-year peak runoff is given by..
𝑃10 =(0.3113)(2.73 �ℎ𝑙
�𝑟)(0.222 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
• Peak Runoff for 100-year Storm Return Period, 𝑃100
𝑃100 =𝐴100𝐼100𝐴
where 𝐴10 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…
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
𝐼100 =88.8(1.23)
(10 +10.92)1.052
𝑰𝟎𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 100-year peak runoff is given by
𝑃100 =(0.5736)(4.35 �ℎ𝑙
�𝑟)(0.222 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
Peak Flow Calculations for Project Site – Proposed Conditions Basin B
• Watershed Area, 𝐴
𝑨=𝟎𝟎𝟎𝟎 𝒄𝒍𝟎=𝟎.𝟎𝟎𝟎 𝒂𝒄𝒍𝒄𝒍
• Percent Imperviousness, �ℎ
�ℎ=𝐴𝑖𝑙𝑜
𝐴𝑡𝑜𝑡𝑎𝑙
× 100
�ℎ=1740 𝑐𝑟2
4693 𝑐𝑟2 × 100
𝒉=𝟎𝟎.𝟎𝟎 %
• Maximum Watershed Length, 𝐿0
𝑳𝟎=𝟎𝟎𝟎 𝒄𝒍,routing on property from SW around building corner to NE corner
• Watershed Slope, 𝑃0
𝑃0 =𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑟𝑙𝑟𝑟� 𝑐𝑙𝑐−𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑙𝑙𝑟𝑟� 𝑐𝑙𝑐
𝐿0
𝑃0 =7858.40 𝑐𝑟−7855.56 𝑐𝑟
105 𝑐𝑟
𝑷𝟎=𝟎.𝟎× 𝟎𝟎−𝟎
• Runoff Coefficients, 𝐴5, 𝐴10, & 𝐴100
Table 3.3 of the URMP was used to determine the runoff coefficients. Soil Type B
was used in determining the runoff coefficients.
𝐴5 =0.2924
𝐴10 =0.3575
𝐴100 =0.6004
• Computed Time of Concentration, 𝑃𝑐
𝑃𝑐=𝑃𝑜+𝑃𝑓
where…
𝑃𝑜=0.395(1.1−𝐶5√𝐾0)
𝑅00.33 (Overland flow time, Equation 3-4 in COA URMP)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
𝑃𝑓=𝐾𝑓
60𝐾√𝑅𝑓
(Channelized flow time, Equation 3-5 in COA URMP)
And since there’s no channelized flow, 𝑃𝑓=0 so…
𝑃𝐶=𝑃𝑜=0.395(1.1 −0.2924)√105 𝑐𝑟
(2.7 × 10−2)0.33
𝑷𝑨=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Design Time of Concentration, 𝑃𝑐
The design time of concentration, 𝑃𝑐, is the lesser of the computed time of
concentration, 𝑃𝑐, and the regional time of concentration, 𝑃𝑅, which is given by
𝑃𝑅=10 +𝐿0/180
=10 +(105 𝑐𝑟/180)
=10.58 𝑙�ℎ𝑙𝑟𝑟𝑐𝑟
so…
𝑷𝒄=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Peak Runoff for 10-year Storm Return Period, 𝑃10
𝑃10 =𝐴10𝐼10𝐴
where 𝐴10 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 +10.58)1.052
𝑰𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 10-year peak runoff is given by..
𝑃10 =(0.3575)(2.84 �ℎ𝑙
�𝑟)(0.108 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
• Peak Runoff for 100-year Storm Return Period, 𝑃100
𝑃100 =𝐴100𝐼100𝐴
where 𝐴100 is the 100-year runoff coefficient, 𝐼100 is the 100-year storm rainfall
intensity (units of �ℎ𝑙/�𝑟), and 𝐴 is the watershed area in units of 𝑎𝑐𝑟𝑐𝑟
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
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…
𝐼100 =88.8(1.23)
(10 +10.58)1.052
𝑰𝟎𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 100-year peak runoff is given by
𝑃100 =(0.6004)(4.53 �ℎ𝑙
�𝑟)(0.108 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Peak Flow Calculations for Project Site – Historic Conditions Basin A
• Watershed Area, 𝐴
𝑨=𝟎𝟎𝟎𝟎 𝒄𝒍𝟎=𝟎.𝟎𝟎 𝒂𝒄𝒍𝒄𝒍
• Percent Imperviousness, �ℎ
�ℎ=𝐴𝑖𝑙𝑜
𝐴𝑡𝑜𝑡𝑎𝑙
× 100
�ℎ=193 𝑐𝑟2
9674 𝑐𝑟2 × 100
𝒉=𝟎.𝟎 %
• Maximum Watershed Length, 𝐿0
𝑳𝟎=𝟎𝟎𝟎 𝒄𝒍,routing on property from SW around building corner to NE corner
• Watershed Slope, 𝑃0
𝑃0 =𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑟𝑙𝑟𝑟� 𝑐𝑙𝑐−𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑙𝑙𝑟𝑟� 𝑐𝑙𝑐
𝐿0
𝑃0 =7857.96 𝑐𝑟−7854.17 𝑐𝑟
165 𝑐𝑟
𝑷𝟎=𝟎.𝟎× 𝟎𝟎−𝟎
• Runoff Coefficients, 𝐴5, 𝐴10, & 𝐴100
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). Soil Type B was used in
determining the runoff coefficients.
𝐴5 =0.0122
𝐴10 =0.0732
𝐴100 =0.4354
• Computed Time of Concentration, 𝑃𝑐
𝑃𝑐=𝑃𝑜+𝑃𝑓
where…
𝑃𝑜=0.395(1.1−𝐶5√𝐾0)
𝑅00.33 (Overland flow time, Equation 3-4 in COA URMP)
𝑃𝑓=𝐾𝑓
60𝐾√𝑅𝑓
(Channelized flow time, Equation 3-5 in COA URMP)
And since there’s no channelized flow, 𝑃𝑓=0 so…
𝑃𝐶=𝑃𝑜=0.395(1.1 −0.0122)√165 𝑐𝑟
(2.3 × 10−2)0.33
𝑷𝑨=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Design Time of Concentration, 𝑃𝑐
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
The design time of concentration, 𝑃𝑐, is the lesser of the computed time of
concentration, 𝑃𝑐, and the regional time of concentration, 𝑃𝑅, which is given by
𝑃𝑅=10 +𝐿0/180
=10 +(165 𝑐𝑟/180)
=10.92 𝑙�ℎ𝑙𝑟𝑟𝑐𝑟
so…
𝑷𝒄=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Peak Runoff for 10-year Storm Return Period, 𝑃10
𝑃10 =𝐴10𝐼10𝐴
where 𝐴10 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 +10.92)1.052
𝑰𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 10-year peak runoff is given by..
𝑃10 =(0.0732)(2.73 �ℎ𝑙
�𝑟)(0.222 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
• Peak Runoff for 100-year Storm Return Period, 𝑃100
𝑃100 =𝐴100𝐼100𝐴
where 𝐴10 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…
𝐼100 =88.8(1.23)
(10 +10.92)1.052
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
𝑰𝟎𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 100-year peak runoff is given by
𝑃100 =(0.4354)(4.53 �ℎ𝑙
�𝑟)(0.222 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
Peak Flow Calculations for Project Site – Historic Conditions Basin B
• Watershed Area, 𝐴
𝑨=𝟎𝟎𝟎𝟎 𝒄𝒍𝟎=𝟎.𝟎𝟎𝟎 𝒂𝒄𝒍𝒄𝒍
• Percent Imperviousness, �ℎ
�ℎ=𝐴𝑖𝑙𝑜
𝐴𝑡𝑜𝑡𝑎𝑙
× 100
�ℎ=94 𝑐𝑟2
4693 𝑐𝑟2 × 100
𝒉=𝟎.𝟎 %
• Maximum Watershed Length, 𝐿0
𝑳𝟎=𝟎𝟎𝟎 𝒄𝒍,routing on property from SW around building corner to NE corner
• Watershed Slope, 𝑃0
𝑃0 =𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑟𝑙𝑟𝑟� 𝑐𝑙𝑐−𝐴𝑙𝑐𝑟𝑎𝑟�ℎ𝑙𝑙 𝑎𝑟 𝑙𝑙𝑟𝑟� 𝑐𝑙𝑐
𝐿0
𝑃0 =7858.40 𝑐𝑟−7855.56 𝑐𝑟
105 𝑐𝑟
𝑷𝟎=𝟎.𝟎× 𝟎𝟎−𝟎
• Runoff Coefficients, 𝐴5, 𝐴10, & 𝐴100
Table 3.3 of the URMP was used to determine the runoff coefficients. Soil Type B
was used in determining the runoff coefficients.
𝐴5 =0.0122
𝐴10 =0.0732
𝐴100 =0.4354
• Computed Time of Concentration, 𝑃𝑐
𝑃𝑐=𝑃𝑜+𝑃𝑓
where…
𝑃𝑜=0.395(1.1−𝐶5√𝐾0)
𝑅00.33 (Overland flow time, Equation 3-4 in COA URMP)
𝑃𝑓=𝐾𝑓
60𝐾√𝑅𝑓
(Channelized flow time, Equation 3-5 in COA URMP)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
And since there’s no channelized flow, 𝑃𝑓=0 so…
𝑃𝐶=𝑃𝑜=0.395(1.1 −0.2924)√105 𝑐𝑟
(2.7 × 10−2)0.33
𝑷𝑨=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Design Time of Concentration, 𝑃𝑐
The design time of concentration, 𝑃𝑐, is the lesser of the computed time of
concentration, 𝑃𝑐, and the regional time of concentration, 𝑃𝑅, which is given by
𝑃𝑅=10 +𝐿0/180
=10 +(105 𝑐𝑟/180)
=10.58 𝑙�ℎ𝑙𝑟𝑟𝑐𝑟
so…
𝑷𝒄=𝟎𝟎.𝟎𝟎 𝒍𝒉𝒍𝒍𝒍𝒄𝒍
• Peak Runoff for 10-year Storm Return Period, 𝑃10
𝑃10 =𝐴10𝐼10𝐴
where 𝐴10 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 +10.58)1.052
𝑰𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 10-year peak runoff is given by..
𝑃10 =(0.0732)(2.84 �ℎ𝑙
�𝑟)(0.108 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
• Peak Runoff for 100-year Storm Return Period, 𝑃100
𝑃100 =𝐴100𝐼100𝐴
where 𝐴100 is the 100-year runoff coefficient, 𝐼100 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)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
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…
𝐼100 =88.8(1.23)
(10 +10.58)1.052
𝑰𝟎𝟎𝟎=𝟎.𝟎𝟎𝒉𝒍
𝒉𝒍
then the 100-year peak runoff is given by
𝑃100 =(0.4354)(4.53 �ℎ𝑙
�𝑟)(0.108 𝑎𝑐𝑟𝑐𝑟)
𝑷𝟎𝟎𝟎=𝟎.𝟎𝟎𝟎 𝒄𝒄𝒍
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Appendix E
Detention Storage Calculations – Rational Volume Method
InputCalculationSub BasinBasin Name Total Area Imp. Area Imperv-iousnessReq'd WQCV (depth) Req'd WQCV (vol.) Decid. Area Conif. AreaPerv. Paver AreaGrass Buffer AreaRain Garden Vol.Perv. Paver RatioGrass Buffer RatioCanopy CreditPerv. Paver CreditGrass Buffer CreditEff. Imp. AreaEff. Imperv-iousnessEff. WQCV (depth) Net WQCV (#)(sq. ft.) (sq. ft.) (%) (in) (cu. ft.) (sq. ft.) (sq. ft.) (sq. ft.) (sq. ft.) (cu. ft.) (sq. ft.) (sq. ft.) (sq. ft.) (sq. ft.) (%) (in) (cu. ft.)1 2616 86933% 0.070 15.2645.8 0.00 0.0 900 0 2 196.9 0 900 -128 -4.9% 0.00 0.002 2717 53720% 0.047 10.70.0 0.00 251 1004 0 2 10.0 502 1004 -969 -35.7% 0.00 0.003 2789 129746% 0.092 21.399.1 23.00 493 393 4.5 2 121.8 986 393 -104 -4% 0.00 -4.504 1552 32921% 0.050 6.4427.7 73.43 0.0 37002 186.2 0 370 -127 -8% 0.00 0.005 1601 42727% 0.059 7.9140.3 234.00 0.0 08.02 191.2 0 0 336 21% 0.05 -1.426 3092 131342% 0.085 21.8513.9 0.00 804 0 0 2 177.1 1607 0 -371 -12% 0.00 0.00Total 14367 4772 33% 0.4 83 1827 330 1548 2667 12.5 373 3095.0 2667.0 -1363 -7% 0.05 -5.9Basin Name Total Area Imp. Area % Imperv-iousnessReq'd WQCV (depth)Req. WQCV (vol.) Decid. Area Conif. AreaPerv. Paver AreaGrass Buffer AreaRain Garden Vol.Perv. Paver RatioGrass Buffer RatioCanopy Area CreditPerv. Paver Area CreditGras Buffer Area CreditEff. Imperv. AreaEff. Imperv-iousnessNet WQCV Depth (in)Net WQCV (cu. ft.)1,2,3,4A9674 3032 31% 0.067 53.8 1173 96 744 2667 4.52 1205 1488 2667 -1328 -13.7% -0.03575 -33.35,6B4693 1740 37% 0.076 29.7 654 234 804 0 8.02 1168 1607 0 -35 -0.8% -0.00618 -10.414367 4772 33% 0.1 84 1827 330 1548 2667 12.5 373 3095 2667 -1363 -7% -0.04 -43.7Basin WQCV Calculations Valentine Legend
Major Drainage Report Calculations3Jordan Kehoe47Basin A- 100 YearBasin B - 100 YearDuration (min.)Rainfall Intensity (in/hr)Inflow Volume (ft3)Outflow Volume (ft3)Storage Volume (ft3)Duration (min.)Rainfall Intensity (in/hr)Inflow Volume (ft3)Outflow Volume (ft3)Storage Volume (ft3)ConditionDeveloped Historic Developed Historic0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Area / ft^29674 9674 4693 46931.00 8.55 65.91 150.36 -84.45 1.00 8.55 33.46 72.10 -38.64Aimp / ft^23032 193 1740 942.00 7.80 120.28 162.98 -42.69 2.00 7.80 61.07 78.32 -17.26Soil TypeB B B B3.00 7.17 165.86 175.60 -9.74 3.00 7.17 84.20 84.55 -0.34Lo / ft165 165 105 1054.00 6.64 204.56 188.21 16.34 4.00 6.64 103.85 90.77 13.08So / ft/ft0.023 0.023 0.027 0.0275.00 6.17 237.79 200.83 36.96 5.00 6.17 120.73 97.00 23.73Lf / ft0 0 0 06.00 5.77 266.62 213.45 53.17 6.00 5.77 135.36 103.22 32.14Sf / ft/ft0 0 0 07.00 5.41 291.84 226.07 65.77 7.00 5.41 148.17 109.45 38.72K7 7 7 78.00 5.09 314.07 238.69 75.38 8.00 5.09 159.45 115.67 43.78L / ft0 0 0 09.00 4.81 333.79 251.30 82.49 9.00 4.81 169.46 121.90 47.5710.00 4.56 351.40 263.92 87.47 10.00 4.56 178.40 128.12 50.28% Imp31.40% 2.00% 37.10% 2.00%11.00 4.33 367.20 276.54 90.66 11.00 4.33 186.42 134.34 52.08C50.2439 0.0122 0.2924 0.012212.00 4.12 381.45 289.16 92.29 12.00 4.12 193.66 140.57 53.09C100.3113 0.0732 0.3575 0.073213.00 3.94 394.35 301.77 92.58 13.00 3.94 200.21 146.79 53.42C1000.5736 0.4354 0.6004 0.435414.00 3.76 406.09 314.39 91.70 14.00 3.76 206.17 153.02 53.15To /min15.08 19.17 10.77 14.5015.00 3.61 416.81 327.01 89.80 15.00 3.61 211.61 159.24 52.37Vf /ft/sec0.00 0.00 0.00 0.0016.00 3.46 426.63 339.63 87.00 16.00 3.46 216.60 165.47 51.13Tf /min0.000 0.000 0.000 0.00017.00 3.33 435.65 352.25 83.40 17.00 3.33 221.18 171.69 49.49Tc,i /min15.08 19.17 10.77 14.5018.00 3.20 443.96 364.86 79.09 18.00 3.20 225.40 177.92 47.48TR / min10.92 10.92 10.58 10.5819.00 3.08 451.64 377.48 74.16 19.00 3.08 229.30 184.14 45.16Td / min10.92 10.92 10.58 10.5820.00 2.98 458.75 390.10 68.65 20.00 2.98 232.91 190.36 42.54I10 / in/hr2.73 2.73 2.78 2.7821.00 2.88 465.36 402.72 62.64 21.00 2.88 236.26 196.59 39.67I100 / in/hr4.35 4.35 4.42 4.4222.00 2.78 471.50 415.33 56.17 22.00 2.78 239.38 202.81 36.57Q10 / cfs0.189 0.044 0.107 0.02223.00 2.69 477.23 427.95 49.28 23.00 2.69 242.29 209.04 33.25Q100 / cfs0.554 0.421 0.286 0.20724.00 2.61 482.59 440.57 42.02 24.00 2.61 245.01 215.26 29.7525.00 2.53 487.60 453.19 34.41 25.00 2.53 247.55 221.49 26.0626.00 2.46 492.29 465.81 26.49 26.00 2.46 249.94 227.71 22.2227.00 2.39 496.70 478.42 18.28 27.00 2.39 252.17 233.94 18.2428.00 2.32 500.85 491.04 9.81 28.00 2.32 254.28 240.16 14.12Return PeriodP1 / in/hr29.00 2.26 256.26 246.38 9.882-yr0.35430.00 2.20 258.13 252.61 5.525-yr0.6231.00 2.14 259.90 258.83 1.0610-r0.75332.00 2.09 261.56 265.06 -3.5025-yr0.93150-yr1.07100-yr1.2Friday, September 6, 2019IDF for Aspen, COBasin A Basin BFrom Table 2.2 of COA URMP (page 2-2), Rainfall Intensity-Duration-Frequency (IDF) in Aspen, CO0.00100.00200.00300.00400.00500.00600.000.00 5.00 10.00 15.00 20.00 25.00 30.00Basin A 100 YearBasin A- 100 Year Inflow Volume (ft3)Basin A- 100 Year Outflow Volume (ft3)Basin A- 100 Year Storage Volume (ft3)-50.000.0050.00100.00150.00200.00250.00300.000.00 10.00 20.00 30.00 40.00Basin B 100 YearBasin B - 100 Year Inflow Volume (ft3)Basin B - 100 Year Outflow Volume (ft3)Basin B - 100 Year Storage Volume (ft3)
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Appendix F
Detention Outfall Pipe Sizing
C₀gH₀A₀ - BD₀ - BI.D.Q - B-ft/sec²ftft²in.in.cfs0.632.20.250.0884.03.10.124Historic 100 year Flow Rate (Q)Discharge CoefficientHeadwater DepthGravitational ConstantOrifice AreaCalculated DiameterActual OutflowQ - Basin Bcfs0.213PVC Diameter*** Use Schedule 40 3.0" PVC (3.068" I.D.) for Basin B ***Outlet pipe sized to release less than 100-year historic flow rateHeadwater depth calculated from depth at 100-year ponding depth
CwLwHwQ - B-ft/sec²ftcfs2.3840.100.301Actual OutflowQ - Basin Acfs0.421Historic 100 year Flow Rate (Q)Discharge CoefficientHorizontal Weir LengthHeadwater Depth
1230 Snowbunny Lane, Aspen, Colorado September 9, 2019
Drainage Report for Major Design Appendix
Appendix G
Soil Survey
Hydrologic Soil Group—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
(1230 Snowbunny Lane HSG Map)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/22/2019
Page 1 of 4434082543408314340837434084343408494340855434086143408674340873434082543408314340837434084343408494340855434086143408674340873341419341425341431341437341443341449341455
341419 341425 341431 341437 341443 341449 341455
39° 12' 9'' N 106° 50' 11'' W39° 12' 9'' N106° 50' 9'' W39° 12' 7'' N
106° 50' 11'' W39° 12' 7'' N
106° 50' 9'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 10 20 40 60
Feet
0 3 7 14 21
Meters
Map Scale: 1:257 if printed on A portrait (8.5" x 11") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Aspen-Gypsum Area, Colorado, Parts of
Eagle, Garfield, and Pitkin Counties
Survey Area Data: Version 9, Sep 10, 2018
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Data not available.
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Hydrologic Soil Group—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
(1230 Snowbunny Lane HSG Map)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/22/2019
Page 2 of 4
Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
107 Uracca, moist-Mergel
complex, 1 to 6
percent slopes,
extremely s
B 0.2 100.0%
Totals for Area of Interest 0.2 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Hydrologic Soil Group—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin
Counties
1230 Snowbunny Lane HSG Map
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/22/2019
Page 3 of 4
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Hydrologic Soil Group—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin
Counties
1230 Snowbunny Lane HSG Map
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/22/2019
Page 4 of 4