HomeMy WebLinkAboutFile Documents.533 E Main St.0004.2017 (14).ACBK
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REPORT
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970.94
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Prepared by
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Table of Contents
1.0 General ............................................................................................................................................... 1
1.1 Description of Existing Site.............................................................................................................. 1
1.2 Description of Existing Drainage ..................................................................................................... 1
1.3 Description of Proposed Project ...................................................................................................... 1
2.0 Drainage Basins ................................................................................................................................. 2
2.1 Discussion of Drainage Basins .................................................................................................... 2
2.2 Drainage Basins .......................................................................................................................... 2
2.3 Peak Discharge Calculations ....................................................................................................... 3
2.4 Analysis of Peak Discharge into Inlet 2 ....................................................................................... 4
3.0 Low Impact Site Design ...................................................................................................................... 5
3.1 Principles of LID .......................................................................................................................... 5
4.0 Water Quality Capture Volume ........................................................................................................... 6
4.1 WQCV Storage ............................................................................................................................ 6
5.0 Summary ............................................................................................................................................ 7
Basin Delineation Maps ........................................................................................................... Appendix A
Drainage Exhibit ...................................................................................................................... Appendix B
Equations ................................................................................................................................. Appendix C
1
1.0 General
1.1 Description of Existing Site
St. Mary Catholic Church is located in Township 10 South, Range 85 West of the 6th Principal Meridian, in Pitkin
County. The parcel on which the church sits faces the south side of Main St (Highway 82). The church address is
533 E Main St, which is in the Aspen Mountain Drainage Basin. The parcel is bordered by public right of way‐
sidewalk, curb and gutter‐ on all four sides. All building entrances are currently one foot or more above the
flowline of the adjacent gutter. Several large deciduous and coniferous trees are on the parcel. Grass, concrete,
and asphalt shingle roofs cover the large majority of the surface area. The site is gently‐sloped with general
drainage patterns from south to north. All utility lines lie outside of the parcel boundary. The receiving system
for onsite runoff is the City of Aspen (COA) storm system.
1.2 Description of Existing Drainage
The selected study area for this project is 0.995 acre. The study area is bounded on the north by Main St, on the
west by Galena St, on the east by Hunter St, and on the south by Hyman St. Topographic mapping obtained from
COA GIS was used to subdivide the study area into 3 basins. The basin delineation maps are presented in
Appendix A. The delineation maps show historic drainage conditions and developed drainage conditions.
Historic conditions for the site were defined to be the existing conditions as the church building dates back to
1882. Under historic conditions, surface runoff from the east and west sides of the parcel drains into five inlets
that surround the parcel. These inlets are all located in the street gutter in the public right of way. Surface runoff
from the middle of the property drains into an existing drywell located in that basin.
For additional information about the existing storm sewer system and its capacity, refer to the Aspen Mountain
Surface Drainage Master Plan that was completed for the City of Aspen in 2001 and updated in 2014. As shown
in the Aspen Mountain Mudflow Zone Map, the property is well outside of any zone for risk of a mudflow.
1.3 Description of Proposed Project
This project is classified as “Major” as the proposed interior work will demolish more than 75% of the existing
interior. The purpose of this report is to demonstrate that the project is in compliance with the requirements of
the Urban Runoff Management Plan (URMP). Throughout the design process, the Low Impact Design Principles
were used as a guide for mitigating stormwater runoff. As the project is located within the Aspen Mountain
Drainage Basin, only the Water Quality Capture Volume (WQCV) is required to be detained on the east side
where the impervious area is being increased when the sidewalk widens in that area.
The proposed project will remove and replace the sidewalks along Main St and Hunter St. The sidewalk along
Hunter St will be widened to the 6 feet as required by the City. The sidewalk along Main St will remain 5 feet in
width west of the church entrance, and the sidewalk will remain 6 feet in width on the east side of the church
entrance. The existing curb cut for the driveway along Main St will be removed and replaced with a new 16 foot
curb cut, meeting Colorado Department of Transportation (CDOT) requirements. 165 square feet of concrete
will be removed within the driveway area and replaced with 165 square feet of pervious pavers. Pervious pavers
will also replace 122 square feet of concrete in front of the church building entrance. A handicap ramp will be
added from Hunter St to access the sidewalk.
The proposed pervious area on the east side of the church will not sufficiently meet the 1:1 ratio with the
impervious roof area that drains onto the grass. Therefore, as requested by the city of Aspen, a grassed swale
with a subsurface detention area will be established on the east side of the church to adequately treat the
WQCV. The grassed swale will be approximately 88 feet long and 2 feet wide. The top 3” of the swale will be
wqcv is required to be
detained for the entire
remodel area.
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2.0 Draina
2.1 Discus
Drainage basi
delineation m
Appendix B sh
The Hydraulic
2.2 Draina
Basins 1 const
mostly grass,
between Main
the total surfa
being done th
the sidewalk a
runoff from th
Basin 2 consti
surface areas,
the sidewalk a
runoff from th
surface runoff
by a grassed s
swale, and the
Basin 3 is the
t consists mo
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age Basin
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hat will chang
and green spa
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and green spa
his basin drai
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swale and dra
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area betwee
ostly of grass,
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rainage Ba
ermined and d
ound in Appe
ins and lists t
HSG) was clas
Figure 3.1
outhwest port
ncrete walkwa
kins Ave drain
r Basin 1. This
ge the historic
ace in the pub
ns into Inlet 1
rtheast portio
6% imperviou
ace in the pub
ns into Inlet 2
of of the chur
ain pipe. The W
ill sheet flow
n Basins 1 an
pervious pav
sand. And las
rock section w
The screened
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t box of Inlet
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delineated us
ndix A. The si
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ssified as HSG
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tion of the pa
ays, and has 3
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s basin has th
c drainage pat
blic right of w
1 near the cor
on of the parc
us area. Unde
blic right of w
2 near the cor
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WQCV will be
across the sid
nd 2‐generally
vers, roof, and
tly, under the
will be wrapp
rock section
ock section. T
2 on Hunter
sing topograp
ite was divide
us area, the ru
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arcel. This bas
38.5% imperv
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cel. This basin
er historic con
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rner of Hunte
rass on the si
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dewalk and in
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d concrete, an
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ped in a Class
will be draine
This drain pipe
St (see Sheet
phic data deve
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unoff coefficie
to Figure 3.1
URMP: Soil M
sin is 11,121.2
vious area. Ru
te contributio
ric and devel
basin. Surfac
the gutter tha
na St and Mai
n is 4,766.3 SF
nditions, surfa
the gutter tha
er St and Mai
ide of the chu
a subsurface
nto Inlet 2.
portion of the
nd has 56.4%
e 6” of ¾‐inch
3 geotextile
ed by a 4”‐dia
e will have a 1
t CRD‐01 for d
eloped from C
ins. The Drain
ent, and peak
1 of the City o
Map for Asp
2 square feet
unoff from m
on was accoun
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at runs along
n St.
F. It consists o
ace runoff fro
at runs along
n St. Under d
urch will be c
e detention ar
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% impervious a
S
h screened roc
separator fab
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1% minimum
details).
COA GIS. The
nage Exhibit in
k flow of each
of Aspen URM
en
t (SF). It consi
much of the al
nted for and
ons as no wo
m Basin 1 flow
Galena St. U
of grass and r
om Basin 2 flo
Hunter St. U
developed con
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MP.
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e grassed
78.3 SF.
ped
urch
Include more
information about all
pervious paver
sections that are
proposed.
Basin 3 includes the pathway that is
proposed for removal and the new granite
fines path. Include description of how the
pathway changes impact drainage.
3
drainage conditions for Basins 3 will follow historic drainage patterns; however, a slight decrease in surface
runoff will result from the replacement of 165 SF of concrete driveway with pervious pavers. Surface runoff for
Basin 3 drains into a drywell that exists inside the basin.
2.3 Peak Discharge Calculations
Peak flows were calculated for the surface runoff of each basin for the 10‐Yr and 100‐Yr storm events using the
Rational Method since the site is less than 90 acres. The runoff coefficients (C) were determined using the
percentage of impervious area in each basin and Figure 3.3‐Runoff Coefficients for NRCS HSG B of the City of
Aspen URMP. Even though the actual Time of Duration (Td) for each basin was considerably less than 5 minutes,
the City of Aspen URMP requires a minimum Td of 5 minutes be used. Rainfall intensity was calculated using a Td
of 5 minutes and 1‐hour rainfall depth (P1) of 0.77 inch/hour for the 10‐Yr event and 1.23 inch/hour for the 100‐
Yr event. See Equation 1 in Appendix C. Lastly, the peak flow (Q) was determined for each basin using Equation 2
for the 10‐Yr and 100‐Yr storm events. For each basin the runoff coefficient (C), the rainfall intensity (I) for the
storm event, and the area (A) were multiplied together to determine the peak flow (see Appendix C). The tables
below show the peak flows for historic and developed conditions for each basin.
Table 1
1‐Hr P1:0.77
Return Period: 10
Basin ID Total Area Impervious Area Imperviousness C Value Duration (Td)Intensity Q10
See Basin Exhibit (SF) (SF) (%) From Table 3.3 (min)I=88.8*P1/(10+Td)1.052 (cfs)
1 11121.2 4276.7 38.5 0.36 5 3.96 0.36
2 3882.8 2267.2 58.4 0.45 5 3.96 0.16
3 13178.3 7591.3 57.6 0.44 5 3.96 0.53
On Site 10‐Yr Peak Discharge ‐ Historic Conditions
Table 2
1‐Hr P1:0.77
Return Period: 10
Basin ID Total Area Impervious Area Imperviousness C Value Duration (Td)Intensity Q10
See Basin Exhibit (SF) (SF) (%) From Table 3.3 (min)I=88.8*P1/(10+Td)1.052 (cfs)
1 11121.2 4276.7 38.5 0.36 5 3.96 0.36
2 4766.3 2267.2 47.6 0.39 5 3.96 0.17
Off Site Basin 2 13046.0 11990.8 91.9 0.78 5 3.96 0.92
3 13178.3 7426.3 56.4 0.43 5 3.96 0.52
On Site 10‐Yr Peak Discharge ‐ Developed Conditions
Table 3
1‐Hr P1:1.23
Return Period: 100
Basin ID Total Area Impervious Area Imperviousness C Value Duration (Td)Intensity Q10
See Basin Exhibit (SF) (SF) (%) From Table 3.3 (min)I=88.8*P1/(10+Td)1.052 (cfs)
1 11121.2 4276.7 38.5 0.49 5 6.33 0.79
2 3882.8 2267.2 58.4 0.56 5 6.33 0.32
3 13178.3 7591.3 57.6 0.55 5 6.33 1.05
On Site 100‐Yr Peak Discharge ‐ Historic Conditions
Table 4
1‐Hr P1:1.23
Return Period: 100
Basin ID Total Area Impervious Area Imperviousness C Value Duration (Td)Intensity Q10
See Basin Exhibit (SF) (SF) (%) From Table 3.3 (min)I=88.8*P1/(10+Td)1.052 (cfs)
1 11121.2 4276.7 38.5 0.49 5 6.33 0.79
2 4766.3 2267.2 47.6 0.52 5 6.33 0.36
Off Site Basin 2 13046.0 11990.8 91.9 0.82 5 6.33 1.55
3 13178.3 7426.3 56.4 0.54 5 6.33 1.03
On Site 100‐Yr Peak Discharge ‐ Developed Conditions
The western portion of
the church roof is in
basin 3 and needs to
be treated for WQCV
4
2.4 Analysis of Peak Discharge into Inlet 2
Under historic conditions, surface runoff from Basin 2 and Off Site Basin 2 drains into the city storm sewer via
Inlet 2. Runoff from Basin 2 flows into the gutter along Hunter St and then into Inlet 2. Runoff from Off Site Basin
2 flows into an inlet near the SE corner of the site, through an 18” reinforced concrete pipe, and into the inlet
box of Inlet 2. Inlet 2 outlets into an existing 10” PVC pipe that then carries the stormwater under Hunter St.
Under developed conditions, surface runoff from Basin 2 will collect in a 4” perforated drain pipe that will run
under a grass swale on the east side of the church. The 4” drain pipe will convey the runoff and daylight into the
inlet box of Inlet 2. No runoff increase will result for the city storm sewer system. In fact, with the grassed swale
and the subsurface detention area, a significant decrease in runoff for smaller storms is expected due to much of
the runoff being infiltrated into the subsoil.
Table 5 below shows the required diameter of the Inlet 2 outlet pipe that is needed to convey the runoff that
drains into Inlet 2 under developed conditions. Table 6 below shows the adequacy of the existing 10” PVC outlet
pipe to convey the 100‐Yr storm event at 80% full (refer to Equation 3 in Appendix C).
An analysis of the capacity of Inlet 2 was performed to determine if Inlet 2 had adequate capacity to intercept
the 100‐Yr storm event for Basin 2. To be conservative, it was assumed that all runoff from Basin 2 would drain
into the inlet box through the grate and curb‐opening. Table 7 below shows that Inlet 2 is adequate to intercept
runoff from Basin 2 (refer to Equations 4‐26 and 4‐27 from the URMP shown in Appendix C).
Table 5
K=0.462
Basin 2 Discharge Off Site Basin 2 Discharge Total Discharge Slope of Outlet Pipe Mannings n Depth Required Diameter
Q100 (cfs) Q100 (cfs) (cfs) SO (ft/ft) Value d=(nQ/(K*SO
0.5)3/8 (Inch)
0.36 1.55 1.91 0.02 0.010 0.62 7.39
Required Pipe Size to Convey Runoff from Basin 2 and Off Site Basin 2
Table 6
Total Discharge Dia. of Ex. Outlet Pipe Slope of Outlet Pipe Required Diameter Design Dia. @ 80% Capacity Adequate Capacity
(cfs) (Inch) SO (ft/ft) (Inch) (Inch) Yes/No
1.91 10 0.02 7.39 8.94 Yes
Existing Storm Sewer Capacity
Table 7 Orifice‐Flow Capacity Co=0.65 Effective Opening Area Cg=0.12 Return Period = 100
Inlet 2 Capacity Qo=CoAe(2g(YS‐YC))0.5 g=32.2 ft/sec2 Ae=(1‐Cg)HL H=.5'
YS=0.43', YC=0.18' L=3'
Inlet Type Inlet Width Inlet Length Effective Opening Area Orifice‐Flow Capacity Q100 Adequate Capacity
In‐Sump Wo (ft) LO (ft)Eq. 4‐27 (ft2)Eq. 4‐26 (cfs) (cfs) Yes/No
Curb Opening 2 3 1.32 3.44 1.19 Yes
5
3.0 Low Impact Site Design
Low Impact Development (LID) attempts to reduce runoff and increase infiltration. The St Mary Catholic Church
site was classified as “Institutional/Campus”. Specific design recommendations applied to that classification.
These recommendations and other LID principles were employed to increase the water quality of the runoff and
increase public safety.
3.1 Principles of LID
Principle 1: Consider stormwater quality needs early in the design process.
Opportunities to integrate stormwater quality facilities were limited for this project as the buildings already
existed and sidewalk replacement aimed to largely replace sidewalk in existing locations. However, the site had
favorable historic conditions. All runoff travels through green infrastructure before entering the city storm sewer
system‐with the exception of the areas where the concrete sidewalk extends all the way to the back of the curb.
To treat the runoff from the roof on the east side of the church, a subsurface detention area under a grass swale
will be installed to treat the WQCV.
Principle 2: Use the entire site when planning for storm water quality treatment.
The existing conditions of the site will be largely unchanged. The site currently employs good storm water
quality treatment as all impervious areas are bordered by a grass buffer. All runoff passes through this grass
buffer before entering the city storm system.
Principle 3: Avoid unnecessary impervious area.
Sidewalk replacement will largely replace sidewalk to historic widths and lengths. In the areas where the
sidewalk is to be widened, a grass swale or pervious pavers will be installed to increase infiltration of that area
and reduce runoff.
Principle 4: Reduce runoff rates and volumes to more closely match natural conditions.
This principle does not entirely apply, because historic drainage patterns will not be changed. The runoff on the
east side of the church where the grassed swale will be installed will reduce runoff rates as the WQCV will be
detained.
Principle 5: Integrate storm water quality management and flood control.
Runoff from Basin 3 drains into an existing drywell, which increases flood control. Runoff from large storm
events will pass through a grass buffer or grassed swale before entering the city storm system. This increases
both water quality and flood control as much of the runoff will infiltrate through the grass and soil.
Principle 6: Develop storm water quality facilities that enhance the site, the community and the environment.
Much of the contaminants will be removed as the runoff passes over the landscaped areas before it leaves the
site. All runoff from impervious areas will pass through a grass buffer before entering the city storm system.
Additionally, trees and shrubbery cover much of the site.
6
Principle 7: Use the treatment train approach.
For the majority of the site, runoff will travel through green infrastructure before entering the city storm sewer
system. Two feet of gravel will be placed along the east side of the church to capture runoff before it travels into
the grass swale. Also, all downspouts discharge onto pervious surfaces, increasing filtration and infiltration.
Principle 8: Design sustainable facilities that can be safely maintained.
The drain pipe and inlet on the east side of the church will be well outside of the travel area of the new concrete
sidewalk. The side slopes of the grass swale will drop at one inch per foot to the flowline (~8.3%). New sidewalks
will be cross‐sloped at 2% away from the church. All new handicap ramps will be per Americans with Disabilities
Act and City of Aspen Standards.
Principle 9: Design and maintain facilities with public safety in mind.
Proper grading and drainage of new sidewalks will alleviate flooded or icy walking surfaces.
4.0 Water Quality Capture Volume
4.1 Water Quality Capture Volume Storage
As the project site is located in the Aspen Mountain Drainage Basin, only the WQCV is required to be detained.
Minimizing Directly Connected Impervious Areas (MDCIA) helps to control flooding and improve water quality.
To this end, a “green space” between the concrete sidewalks and the back of the curb was left to act as a buffer
for the runoff before it entered the gutter. This green space allows water to sheet flow over its surface,
promoting filtration, infiltration, and settling to remove pollutants. The site qualifies as MDCIA Level 1 as runoff
is treated before entering the city storm sewer system.
Table 8 below shows the calculated WQCV storage required for Basin 2‐the area on the east side of the church.
Using Figure 8.14 in the City of Aspen URMP, the total imperviousness of the basin was reduced to a lower
effective imperviousness because MDCIA and other LID practices were used. The WQCV (in watershed‐inches)
was determined using the effective imperviousness and Figure 8.13 of the URMP. Finally, Equation 4 was used to
determine the WQCV storage based on the watershed‐inches and the area of the basin (see Appendix C).
Table 9 below shows the storage volume of the subsurface detention area. This illustrates that the WQCV
required storage is adequately met by the subsurface detention area.
Table 8
Basin ID Total Area Impervious Area Imperviousness MDCIA Effective Imperviousness WQCV WQCV Storage
See Basin Exhibit (SF) (SF) (%) Level from Figure 8.14 Value from Figure 8.13 WQCV‐In*Area/12
2 3674.8 2297.3 0.63 1 0.61 0.12 36.7 ft3
Water Quality Capture Volume Storage required for developed runoff of Basin 2
Table 9
Void Space Volume
Width Length Depth Volume Sand = 30%
(ft) (ft) (Inch) W*L*D/12*0.30 3/4" Screened Rock = 30%
287.710 43.9 ft3
Storage volume available for developed runoff of Basin 2 in the Subsurface Detention Area
Dimension of the Detention Area
Total area in Table 8 does
not match the total area in
Appendix A.
Basin 3 must be
included in WQCV
calculations.
Basin 3 must be
included in the
detention calculations.
7
5.0 Summary
In this report, it was demonstrated that the proposed project is in compliance with the City of Aspen URMP.
With the effective use of LID principles such as pervious pavers, green spaces between the sidewalks and the
back of curbs, a gravel filtration area, a grass swale, and a subsurface detention area to provide adequate
WQCV, the water quality of surface runoff can be increased. Also, by utilizing these principles, runoff can be
reduced by infiltration and percolation before it can enter the city storm sewer system. In addition to
demonstrating compliance with the URMP, this report also shows that the implementation of these best
management practices will not produce an increase of runoff to the city storm sewer system. Lastly, this report
shows that the existing inlets and culverts are adequate for handling the runoff that leaves the site.
Appendix A
Appendix A
Historic Site Basin Delineation Map
Developed Site Basin Delineation Map
Graphic Scale
In Feet: 1" = 40'
0 20 40 80
Job No.
Drawn by:
Date:
Of
File:
PE:QC:
Revision#Date By
Historic Site
Basin Delineation
2016-195
AWA
11/17/2016
DCS
StMarysBasinDelineationHist
DCS
1
2
St. Mary Catholic Church
Aspen, COFor Submittal 118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004 www.sgm-inc.com
Graphic Scale
In Feet: 1" = 40'
0 20 40 80
Job No.
Drawn by:
Date:
Of
File:
PE:QC:
Revision#Date By
Developed Site
Basin Delineation
2016-195
AWA
11/17/2016
DCS
StMarysBasinDelineationDev
DCS
2
2
St. Mary Catholic Church
Aspen, COFor Submittal 118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004 www.sgm-inc.com
Appendix B
Appendix B
Drainage Exhibit
Graphic Scale
In Feet: 1" = 40'
0 20 40 80
Job No.
Drawn by:
Date:
Of
File:
PE:QC:
Revision#Date By
Drainage Exhibit
2016-195
AWA
11/17/2016
DCS
StMarysBasinDrainExhibit
DCS
1
1
St. Mary Catholic Church
Aspen, COFor Submittal 118 West Sixth Street, Suite 200
Glenwood Springs, CO 81601
970.945.1004 www.sgm-inc.com
Appendix C
Appendix C
Equations
Appendix C
Equation 1 – Rainfall Intensity (Eq. 2-1 URMP)
I = (88.8)(𝑃1)
(10+𝑆𝑑)1.052 I = Rainfall Intensity (inch/hour)
P1 = 1-hour rainfall depth (inches)
Td = Time of Duration (minutes)
Equation 2 – Peak Flow (Eq. 3-1 URMP)
Q = (𝐶)(𝐻)(𝐴) C = Runoff Coefficient
(from Figure 3.3 in URMP)
I = Rainfall Intensity (inch/hour)
A = Area (acres)
Equation 3 – Required Depth
d = ((𝑛)(𝑃)
(𝐾)(𝑆𝑂).5)3 8⁄
N = Manning’s value
Q = Peak Flow (cfs)
K = 0.462
SO = Slope of pipe (ft/ft)
Equation 4 – WQCV Storage Volume
Volume (ft3) = (𝑉𝑄𝐶𝑉 �ℎ𝑛 𝑉𝑎𝑟𝑐𝑟𝑟�𝑐𝑐 �ℎ𝑛𝑐�𝑐𝑟)(1 𝑒𝑡
12 𝑖𝑛)(𝐴) WQCV-Inches from Figure 8.13 in URMP
I = Rainfall Intensity (inch/hour)
A = Area (square feet)
Equation 5 – In-Sump Curb-Opening Inlet Capacity (Eq. 4-26 URMP)
Qo = 𝐶𝑛𝐴𝑒√2𝑖(𝑌𝑆−𝑌𝐶) Qo=Inlet Capacity (cfs)
Co = Orifice Coeffecient
(0.65 recommended in URMP)
Ae = Effective Opening Area (sq. feet)
g = gravitational acceleration
(32.2 ft/sec2)
YS = Water Depth (feet)
YC = Center of opening above flow line
(feet)
Equation 6 – Effective Opening Area (Eq. 4-27 URMP)
Ae = (1 −𝐶𝑒)𝐻𝐿 Cg = Clogging Factor (0.12 from URMP)
H = Height of opening area (feet)
L = Width of opening area (feet)