HomeMy WebLinkAboutDrainage•� ., � ' •; � � ,,n. 1 I F'� : �6 Jll, i .nom
750 E. Carmel Drive
Carmel, Indiana
DRAINAGE SYSTEM DESIGN
TECHNICAL INFORMATION REPORT
July 30, 2012
Prepared by:
Landwater Group, Inc.
250 Franklin Street
Columbus, Indiana 47201
(812) 342 -9703
INTRODUCTION
The existing McDonald's Restaurant facility located at 750 E. Carmel Drive in Carmel, Indiana,
is scheduled to be updated via re- development of the site. The existing building will be
replaced with a new building and the surrounding parking lots will be reconstructed. As part of
this infrastructure upgrade, the onsite stormwater management system will be improved to
provide both water quantity and water quality treatment of runoff in accordance with the City of
Carmel Stormwater Technical Standards Manual.
This report presents the stormwater computations as required by the City of Carmel drainage
regulations. The enclosed computations and conclusions support the request for a stormwater
permit from the City of Carmel.
FLOODPLAIN & WATERSHED INFORMATION
According to the FEMA Carmel Flood Insurance Map Panel 228 (February 19, 2003 ed.), the
project site is not located in a Special Flood Hazard Area (see map in appendix 2). The project
site will not be flooded by the base flood elevation of a floodplain area based on the FEMA
mapping and site location.
According to the Indiana Map website, the 14 -digit Watershed Hydrologic Unit Code for the
project site is 05120201090040. According to the Hamilton County GIS site, the project site is
located within the Moffit- Williamson watershed.
PROJECT DESCRIPTION
The 1.3 acre McDonald's restaurant re- development project site is located along the north side
of E. Carmel Drive. The site is bounded on the west by Keystone Way and on the east by
Keystone Avenue.
The proposed McDonald's re- development project will consist replacement of the existing
building with a new 114 -foot by 45 -foot restaurant building and reconstruction of the
surrounding paved parking lot and access drives. The existing onsite stormsewer system
consisting of four parking lot area inlets will be replaced with a new system consisting of one
parking lot area inlet and four parking lot curb inlets.
The project will result in a reduction in impervious surfaces at the site. Under existing
conditions, the 1.32 acre site contains 1.00 acres of impervious surfaces for a percent
impervious value of 75.8 %. Under proposed conditions, the site will contain 0.82 acres of
impervious surfaces for a percent impervious value of 62.1 %.
Under existing conditions, stormwater runoff is discharged from the project site's stormsewer
system directly to offsite stormsewer systems without detention or water quality treatment. The
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
proposed re- development of the site would decrease peak stormwater runoff rates from the
site even without detention due to the reduction in the amount of impervious surfaces.
In accordance with Carmel drainage regulations, additional runoff rate reductions will be
achieved with the construction of an onsite detention basin. A water quality BMP unit will
provide water quality treatment of runoff prior to water quantity treatment in the detention
basin.
PRE - DEVELOPMENT SITE DRAINAGE
The pre - project condition at the site consists of a McDonald's restaurant facility surrounded by
a paved parking lot and access drives. Based on a site inspection and review of available
mapping, there appears to be no offsite drainage areas contributing runoff into the project site.
The existing onsite drainage system consists of an onsite stormsewer system connecting four
parking lot area inlets. The stormsewer system drains to offsite inlet structures along the west
side of Keystone Avenue near the northeast corner of the project site.
Soils at the project site consist of the Crosby silt loam; Brookston silty clay loam and Miami silt
loam associations with NRCS Soil Group ratings of C, B & B, respectively ( Brookston
assumed as drained condition). It is noted that in accordance with Carmel drainage standards,
these soil ratings will be increased to the next more impervious value (ex. B4C) for the
proposed condition runoff computations.
PROPOSED SITE DRAINAGE
Stormwater runoff at the re- developed McDonald's project site will be collected via one onsite
pavement area inlet (structure 7) and four combination curb inlets (structures 3, 4, 5 & 6). The
pavement area inlet grate will consist of an East Jordan 5250 casting and the curb inlets will
be furnished with East Jordan 7220 castings. These inlet structures will connect to an onsite
stormsewer system that will consist of 12 -inch to 15 -inch high density polyethylene (HDPE)
stormsewer pipes.
The new onsite stormsewer system will connect to a new onsite water quality BMP system that
consists of an Aqua -Swirl AS -3 rate limited BMP structure (structure "BMP "). The first flush
design runoff rate of 1.2 cfs will be diverted from and returned to manhole structure 2, with a
weir wall inside structure 2 diverting the first flush treatment flow to the BMP.
The internal weir has been sized to force the first flush treatment flow rate into the BMP, but
allows larger flows to bypass the BMP diversion weir. BMP diversion and return flows will be
conveyed via 10 -inch diameter PVC pipes.
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McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Camel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
It is noted that the BMP diversion weir /pipe system has been designed to convey water quality
flow rates that significantly exceed the design value of 1.2 cfs computed in accordance with
the Carmel drainage regulations. This is explained as follows.
The diversion wall inside structure 2 has a crest elevation approximately equal to the top of the
10 -inch diversion pipe to the BMP. The flow capacity of the diversion pipe (assuming no BMP
head losses) therefore limits the maximum flow that can divert to the BMP before water begins
to overtop the internal structure 2 10 -inch high diversion weir wall, thereby bypassing the BMP.
Based on the Manning's Equation, the BMP diversion pipe has an open channel flow capacity
of 2.6 cfs. Therefore the proposed BMP structure should treat approximately twice as much
runoff than required. This additional water quality treatment improves water quality in runoff
discharged from the project site during larger storm events. It also provides additional
sediment removal upstream of the detention facility which should reduce accumulation of
sediment at the detention basin outlet.
The proposed detention facility will consist of an onsite dry detention basin that will excavated
in the grassy yard area near the northeast corner of the project site. The detention basin will
have a grass surface with a north /south total length of approximately 100 feet and an
east/west total width of approximately 50 feet. The detention basin bottom elevation is 824.0
and the top -of -bank elevation is 828.2. From the top -of -bank elevation, internal basin side
slopes are 4- horizontal -to -1- vertical (4:1) and external side slopes are 5:1 or flatter.
To achieve mandated pipe cover and reduce wetness - related maintenance problems at the
detention basin bottom, the outlet control structure for the detention basin will consist of a
orifice plate /surcharge structure (structure 1). The onsite stormsewer system and BMP outflow
runoff will drain into the upstream side of detention basin outlet control structure 1. A 6 -inch
orifice plate (minimum allowable size) will limit flow from the incoming 15 -inch stormsewer pipe
from exiting the downstream side of the structure.
Excess runoff will be stored in the upstream stormsewer system and for larger events be
forced to rise up into the detention basin through the control structure grate. For this type of
detention design, it is critical that the elevation -vs- discharge rating properly utilize the orifice
equation for pressure flow instead of the Manning's Equation to assure that adequate head is
developed to force the water into the detention basin. The model utilized for this study properly
applies the orifice equation for the submerged orifice.
As runoff from a storm event dissipates the stormwater in the detention basin will drain back
into structure 1 via its grate and flow out of the site through the 6 -inch orifice plate opening.
It is noted that the proposed detention basin will perform similarly to a traditional basin in which
the incoming stormsewer pipe is directed across the bottom of the basin to the orifice - affixed
outlet control structure. The primary benefits of the proposed detention system are flatter,
more maintainable side slopes and a dryer, more maintainable grass bottom because trickle
flow stormwater will pass underneath the grassy bottom instead of across it.
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
DESIGN REQUIREMENTS
The site has been designed to comply with the City of Carmel stormwater standards as
follows:
Hydrology
❑ NRCS hydrograph method used via TR -20 type computer program and NRCS Type 2, 24-
hour duration design storms using rainfall amounts contained in the City of Carmel
Stormwater Technical Standards manual.
❑ Proposed condition runoff computations shall utilize NRCS soil groups that are one group
more impervious than the actual soil group existing at the site as mandated by the City of
Carmel Stormwater Technical Standards Manual.
Inlets
❑ The inlet grates will convey the peak discharge from the post development 10 year
frequency, 24 hour duration, NRCS Type 2 rainfall /runoff event (Q10) with 50% blockage
of the grate with a maximum depth of water over the grate no greater than 6 inches. The
inlet grates will also be checked for the 100 year storm.
Stormsewer Pipes
❑ The onsite stormsewer system will covey the peak discharge from the post development
10 year frequency, 24 hour duration, NRCS Type 2 rainfall /runoff event (Q10) in an open
channel flow conditions with a full pipe flow velocity of at least 2.5 feet per second and a
maximum pipe -full flow velocity less than 10 feet per second. The stormsewer system will
also be checked for the 100 year storm.
❑ The Q10 hydraulic grade elevation will not surcharge the stormsewer system (HGL below
all stormsewer inlet rim elevations).
❑ All stormsewer system inlet rim elevations will be set at or above the 100 year detention
basin water surface elevation.
Water Quality BMP
❑ The rated limited BMP will be sized to remove 80% of total suspended solids from the 1-
inch, 24 hour duration, NRCS Type 2 rainfall /runoff event (Qwq) using specialized water
quality NRCS Curve Numbers as mandated by the City of Carmel Stormwater Technical
Standards Manual.
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McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
Detention
❑ The detention basin peak runoff attenuation standards require that the post developed 10
year and 100 year storm peak discharges rates be reduced to values of 0.1 cfs per acre
and 0.3 cfs per acre, respectively. Based on the City of Carmel Stormwater Technical
Standards Manual, the minimum orifice size for the detention basin outlet control is 6-
inches, even if the 6 -inch diameter orifice results in discharge that exceeds the
predetermined maximum authorized peak flow releases as determined using
methodologies in section 302.02 (allowable release rates).
❑ A minimum of 90% of the detention basin storage capacity will be restored within 48 hours
of the start of the 100 year design storm.
❑ The detention basin shall be designed with an additional 10 percent available capacity to
allow for sediment accumulation.
❑ The lowest adjacent grade for all buildings will be set at least 2.0 feet above the computed
100 year storm maximum detention basin water surface elevation or 2.0 feet above the
crest of the emergency overflow weir at the detention basin, whichever is higher.
❑ The detention basin emergency overflow facility (weir in this case) shall be designed to
convey 1.25 times the peak inflow discharge for the 100 -year storm without overtopping
the grass portion of the containment berm and without reliance on flow through the
principal outlet control orifice or basin storage. Emergency spillway runoff must be directed
to an adequate receiving system.
METHODOLOGY FOR STORM ROUTING SIMULATIONS
The pre - developed and post developed stormwater drainage conditions were analyzed by
studying the available information for the project as follows:
Available information was collected or processed including the site survey, grading and
drainage plans, the NRCS Soil Survey for Hamilton County, Hamilton County GIS 1 -foot
contour mapping, the City of Carmel Stormwater Design and Construction Specifications
Manual rainfall data and observations obtained during a site inspection by Landwater
Group, Inc.
2. The pre - developed and post developed condition onsite drainage areas were delineated
and measured on the site topographic survey and grading plans, respectively.
3. The NRCS Hamilton County Soil Survey layer was used to determine the NRCS Soil Class
percentages within each pre - developed and post developed drainage sub - basin. As
required by the ordinance, the next more impervious soil group was assigned to each soil
classification for the post developed drainage sub - basins.
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
4. For the pre - developed drainage sub - basin, the assumed surface cover consisted of short
grass in good condition.
5. For the post developed drainage sub - basins, the total area of impervious surfaces within
each sub -basin was measured using the site plans. The impervious surfaces included
road surfaces, parking lot surfaces, roof -tops, driveways and sidewalks.
6. For the post developed drainage sub - basins, the pervious area of each sub -basin was
computed by subtracting the combined impervious area from the total drainage area.
7. The TR55 computer simulation model was used to determine the NRCS Curve Number for
each sub -basin based on values for impervious surfaces and for pervious surfaces. The
pervious surfaces Curve Number was based on short grass based on the typical
subdivision lawn. Although the NRCS Soil Survey indicated NRCS Hydrologic Soil Group
"B" and "C" soils at the site, a Group "D" Curve Number was computed for each proposed
sub -basin based on the City of Carmel Stormwater Technical Standards Manual.
8. The TR55 computer simulation model was used to determine the time of concentration for
each sub -basin based on the longest flow path within each sub - basin. For parking lot time
of concentration computations, only the paved part of the flow path was used to assure
that a conservatively low Tc value was computed.
9. The proposed stormsewer and detention system geometry and hydrologic data were
inserted into the stormwater runoff and hydraulics computer model WATERWORKS that
develops and routes runoff hydrographs through the drainage system similar to the NRCS
TR -20 computer model. This model is the basis of all design storm simulations for the
project.
10. The WATERWORKS model was executed for the 2 -year, 10 -year and 100 -year, 24 -hour
duration NRCS Type 2 storms to evaluate the performance of the proposed
inlet/stormsewer /detention system. All rainfall amounts for the simulated design storms
are based on City of Carmel rainfall data contained in the Carmel Stormwater Technical
Standards Manual.
11. The TR -20 model was executed for the 1 -inch, 24 -hour NRCS Type 2 water quality first
flush storm to compute the water quality flow rate for the proposed rated limited water
quality BMP. The NRCS Curve Numbers for the water quality computations were revised
based on the methodology for computing water quality Curve Numbers based on percent
imperviousness as described in the City of Carmel Stormwater Technical Standards
Manual.
12. The hydraulic capacity /performance of each inlet grate was evaluated using the
Flowmaster computer program that is based on the standard HEC 22 methodology. Each
inlet;grate was assumed to be 50% clogged as required by the City of Carmel Stormwater
Technical Standards Manual and the actual grate dimensions were modified in the model
0
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
input as needed to match the net flow area for each grate reported in the Neenah product
catalog.
13. The Flowmaster computer program was also used to compute the Manning's equation
capacity and depth for the BMP diversion and return flows in the 12 -inch diameter PVC
pipes to assure that the first flush flow will not bypass the BMP structure.
OUTPUT DESCRIPTION
The WATERWORKS summary output data area arranged as follows:
BASIN SUMMARY -These summary tables provide hydrologic data for each drainage area
flowing to an inlet structure. The input data include total rainfall amount, storm duration,
rainfall distribution, peaking factor, contributing drainage area, time of concentration, NRCS
Curve Number, and hydrograph time increment. The output data include peak discharge and
total runoff volume.
HYDROGRPAH SUMMARY TABLE -This table provides summary peak flow and volume data
for the hydrographs used in the WATERWORKS program.
REACH SUMMARY -These summary tables provide stormsewer pipe reach hydraulic data.
Some of the input data include design flow, total contributing watershed area, pipe diameter,
pipe length, pipe slope, Mannings N value, and upstream /downstream structure numbers. The
output data include the percent of pipe capacity used, flow depth, full -flow velocity and actual
velocity.
STRUCTURE REPORT- These summary tables provide stormsewer inlet and manhole
hydraulic data. Some of the input data include the structure number, northing and easting
coordinates (referenced to those on the plans), rim elevation and the invert elevations, pipe
diameters, Manning N values and flow direction for each connecting pipes.
STAGE STORAGE TABLE -These summary tables provide elevation versus storage volume
ratings for the detention pond
STAGE DISCHARGE TABLE -These summary tables provide elevation versus discharge
ratings for the detention pond based on the application of the inlet control culvert equations
within the WATERWORKS program.
LEVEL POOL ROUTING TABLE -These summary tables provide the detention basin hydraulic
data. Input data include inflow and outflow hydrograph numbers, inflow peak and match Q (for
pre and post analyses) values. Output data include the peak pool elevation, peak outflow rate,
peak volume and time step at the peak.
7
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
WATERWORKS PROGRAM HYDROGRAPH INDEX
The WATERWORKS program uses 20 hydrograph storage registers to add and route the
various sub -basin hydrographs through a site's stormsewer and detention system. For this
project a separate file was established for each of the three design storm events described
above. Since each file is separate, the hydrograph storage registers were re -used for the
same parameters in each design run.
Following is a description of how the sub -basin data were entered into and manipulated with
the hydrograph storage registers.
ROUTE HYD1 Thru Stormsewer Network (MAIN) 4 HYD1
Route post developed sub -basin runoff through the onsite stormsewer system
MOVE Sub -Basin Hydrograph (11) 4 HYD2
Insert sub -basin hydrograph 11 (direct runoff to detention basin) to hydrograph 2 storage register.
ADD HYD1 + HYD2 4 HYD3
Combined total stormsewer system routed hydrograph 1 to direct hydrograph 2 for combined total inflow
hydrograph to detention basin.
ROUTE HYD3 Thru Detention Basin System (DETBASIN) 4 HYD4
Route post developed sub -basin runoff through the onsite detention system
ADD HYD4 + HYD5 4 HYD5
Create a copy of the detention basin outflow hydrograph for subsequent routing through the detention basin outlet
pipe while preserving the detention basin outflow hydrograph.
ROUTE HYD5 Thru Detention Basin outlet pipe (OUTLET) 4 HYD5
Route post developed detention basin outflow hydrograph through the outlet pipe to the offsite system.
STUDY RESULTS -INLET CAPACITY
The FLOWMASTER computer program was used to compute the 10 year and 100 year design
storm inlet capacity. The following tables illustrate key data from these computations.
Table 1- Proposed Condition Stormsewer Inlet Grate Capacity -10 Year Design Storm
STORM
SEWER
INLET
INLET
GRATE
TYPE
GRATE
SPECIFICATION
10YR /24HR
PEAK Q
(CFS)
10YR /24HR
INLET DEPTH
(INCHES
13
Sumped Curb Inlet
EJ -5250
0.88
2.4
14
Sum ed Curb Inlet
EJ -7220
1.31
3.1
15
Sum ed Curb Inlet
EJ -7220
0.59
1.8
16
Sumped Curb Inlet
EJ -7220
0.65
1.9
17
Sum ed Pavement
EJ -7220
0.51
1.9
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
Table 2- Proposed Condition Stormsewer Inlet Grate Capacity -100 Year Desiqn Storm
STORM
SEWER
INLET
INLET
GRATE
TYPE
GRATE
SPECIFICATION
100YR /24HR
PEAK Q
(CFS)
100YR /24HR
INLET DEPTH
(INCHES)
13
Sum ed Curb Inlet
EJ -5250
1.52
3.4
14
Sum ed Curb Inlet
EJ -7220
2.24
4.0
15
Sumped Curb Inlet
EJ -7220
1.10
2.8
16
Sum ed Curb Inlet
EJ -7220
1.16
2.9
17
Surn ed Pavement
EJ -7220
0.89
2.8
The inlet capacity computations indicate that the maximum Q10 inlet depth will be less than 6
inches with the assumption that 50% of the inlet area is blocked. The Q100 check indicates
that even during the 100 year storm event, the water depth at the inlets will maintain all runoff
within the project site curbing.
STUDY RESULTS- STORMSEWER ROUTING
The following tables illustrate key data from the stormsewer system routings.
Table 3-Proposed Condition Stormsewer S stem Performance Table
STORM
SEWER
SEGMENT
PIPE
DIAMETER
(INCHES
PIPE
CAPACITY'
(CFS '
PIPE -FULL
VELOCITY
FPS
10YR/24HIR
PEAK Q
;CFS'`
100YR /24HR
PEAK Q
CFS
11 DN -EX
15
' 10.0,8
8.43
2-,18
2.52
13 -M2
15
4.56
3.82
3,'3
6.91
14 -13
12
2:77
3.62
2.55
4.50
15 -14
12
2.71
3.54
4:24
2.26
16 -15
12
212;
3.55
6-65'
1.16
17 -13
12
2.71
3.54
0,.51
0.89
M2 -11 UP
15
10.35_ .,
8.66
3'.93
6.91
Table 4- Proposed Condition Hydraulic Grade at Inlet Grates Table
STORM
SEWER
INLET
LOWEST
RIM
ELEVATION
10YR/24HR
HGL
ELEVATION*
100YR /24HR
DETENTION
ELEVATION
DOES INLET
SURCHARGE
13
827.50
823.54
826.88
NO
14
827.40
824.54
826.88
NO
15
826.90
824.71
826.88
NO
16
827.75
824.75
826.88
NO
17
828.40
1 826.05
826.88
NO
M2
828.00
1 823.41
826.88
NO
"HGL computations based on submerged tailwater elevation at detention basin at time -of- hydrograph -peak.
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McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
The output data from the 10 year storm routing of the stormsewer system indicate that all pipe
segments will have open channel flow capacity based on the application of the Manning's
equation within the WATERWORKS storm routing computer program. The 100 year storm
check indicates that some of the pipe segments will also have Q100 capacity.
The summary data indicate that the proposed stormsewer system will comply with the design
standards inasmuch as it will convey the peak Q10 flow in open channel conditions and will
have a minimum pipe -full flow velocity of at least 2.5 feet per second and a maximum pipe -full
flow velocity less than 10 feet per second
The WATEWORKS storm routing program utilizes a backwater algorithm to compute the
hydraulic grade within the stormsewer system. The starting tailwater for the HGL modeling is
based on the water surface at the detention basin when the peak of the incoming hydrograph
arrives, since this is the point of time of maximum HGL impacts.
The hydraulic grade computations within the WATERWORKS simulations indicate that the
hydraulic grade line for the 10 year design storms will remain below the inlet rim elevations.
Also in accordance with the City of Carmel design standards, the rim of each inlet remains
below the peak water surface. elevation in the detention basin for the 100 year storm.
STUDY RESULTS -WATER QUALITY DESIGN
The proposed water quality BMP for the project site is Aqua -Swirl AS -3 rate limited BMP
structure (structure "BMP "). Under proposed conditions, the runoff from all impervious
surfaces at the project site will drain to the water quality BMP structure that will be installed
immediately upstream of the grass detention basin. Runoff draining directly to the detention
basin without passing through the BMP will be entirely pervious (grass) surfaces.
The BMP has been sized in accordance with design procedures set forth in the City of Carmel
Stormwater Technical Standards Manual. The drainage area contributing runoff to the BMP is
1.01 acres, containing 0.82 acres of impervious surfaces. Therefore the percent impervious is
0.82/1.01 = 81.2 %.
Based on Exhibit 701 -1 Curve Number Calculation for Water Quality Storm Event contained in
the City of Carmel Stormwater Technical Standards Manual, the corresponding Water Quality
Curve Number for an 81.2% imperviousness ratio is 98.
A TR -55 model was developed in accordance with the design standards for the proposed
project site based on the WQ CN value of 98, the 1.01 BMP drainage area, the actual time of
concentration for the contributing site area, the type 2 rainfall distribution provided in the City
of Carmel Stormwater Technical Standards Manual and 1 inch of rainfall.
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McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel; Indiana July 30, 2012
Drainage Design Study
The computed peak discharge from the TR -55 program is 1.18 cfs. This is the water quality
design flow rate for the proposed AS -3 rate limited BMP structure.
According to the current Aqua -Swirl Sizing Chart published by the manufacturer, the AS -3 rate
limited BMP has a maximum water quality treatment flow rate of 1.8 cfs. Therefore the AS -3
structure satisfies the water quality standards contained in the City of Carmel Stormwater
Technical Standards Manual.
STUDY RESULTS - DETENTION BASIN PERFORMANCE
The following tables illustrate the results from the detention basin system flood routing
computations.
Table 5- Proposed Condition Detention Basin Performance
DESIGN
EXISTING
PROPOSED
DETENTION
DETENTION*
DETENTION
STORM
CONDITION
CONDITION
BASIN PEAK
BASIN PEAK
BASIN PEAK
EVENT
PROJECT
PEAK
OUTFLOW
WATER
WATER
SITE PEAK
INFLOW
DISCHARGE
SURFACE
VOLUME
RUNOFF
DISCHARGE
ELEVATION
CFS )
(CFS)
(CFS)
(FT
CUBIC FT
2 YR -24 HR
3.03
2.76
1.92
824.11
276
10 YR -24 HR
4.81
4.29
2.18
825.21
1272
100 YR -24 HR
8.78
7.79
2.52
826.88
4425
"Detention basin emergency overflow weir elevation = 827.5 feet
The storm routing modeling confirms that since the re- development project will reduce the
percent impervious at the project site, peak runoff rates will be reduced even without the
detention facility. The detention basin will further reduce peak runoff rates from the project site.
The peak 100 year design storm water surface elevation at the detention basin is 826.88 feet.
This water surface elevation will back up into the stormsewer system but remain below all rim
elevations as required by the design standards.
The 0.62 feet of headroom between the peak 100 year detention basin water surface elevation
and the emergency spillway weir crest will provide the additional storage required for sediment
accumulation in the basin.
During the 100 year design storm event, the modeling data indicates that the detention basin
will completely drain within 24.5 hours of the beginning of the rainfall event. This drawdown
time is well within the 48 -hour standard contained in the City of Carmel design standards.
The proposed finish floor /adjacent grade elevation for the proposed restaurant building at the
project site is 829.50 feet. This elevation is 2.62 feet above the 100 year detention basin water
surface elevation and 2.00 feet above the emergency overflow weir crest elevation. The
building grade is compliant with the design standards.
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McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
STUDY RESULTS - EMERGENCY SPILLWAY /OVERFLOW FACILITY
Pursuant to the City of Carmel design standards, the detention basin must have an emergency
overflow facility that will:
O Convey 1.25% of the peak 100 year frequency inflow without overtopping the
unprotected grass portion of the detention basin containment berm.
Following are the design computations for the emergency spillway design at the proposed
detention basin.
The emergency spillway consists of a 6 -foot wide rip- rapped weir. Using the flow equation for
a broad crested weir.... Q = CL(H)3i2
Where: Q is flow in cubic feet per second.
C is discharge coefficient = 3.0
W is weir length = 6 ft
H is water depth in feet
Max Q100(in) for Detention Basin = 7.79 cfs
Emergency Spillway Design Flow = 1.25 X 7.79 = 9.74 cfs
Applying Weir Equation ... ............................... 9.74 = 3.0(6)(H)312
H = 0.66 FEET
6 Crest of emergency spillway is at elevation 827.50 feet.
® Spillway design overflow elevation = 827.50 + 0.66 = 828.16 feet.
® Surrounding grass berm crest elevation = 828.20 feet.
• Flow does not overtop grass berm.
STUDY RESULTS -PRE- DEVELOPED & POST DEVELOPED DISCHARGES
The detention basin peak runoff attenuation standards require that the post developed 10 year
and 100 year storm peak discharges rates be reduced to values of 0.1 cfs per acre and 0.3 cfs
per acre, respectively. Based on the City of Carmel Stormwater Technical Standards Manual,
the minimum orifice size for the detention basin outlet control is 6- inches, even if the 6 -inch
diameter orifice results in discharge that exceeds the predetermined maximum authorized
peak flow releases as determined using methodologies in section 302.02 (allowable release
rates).
12
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study
The following table illustrates the total site pre - developed, post developed and target
stormwater release rates for the McDonald's re- development project.
Table 6 -Total Site Pre- Developed, Post Developed & Target Peak Flows
DESIGN STORM
PRE - DEVELOPED
PEAK RUNOFF
CFS )
POST DEVELOPED
PEAK RUNOFF
(CFS)
TARGET POST*
DEVELOPED PEAK
RUNOFF
2 YEAR, 24 HOUR
3.03
1.92
N/A
10 YEAR, 24 HOUR
4.81
2.18
0.13
100 YEAR, 24 HOUR
8.78
2.52
0.40
*Based on 0.1 cfs per acre for Q10 and 0.3 cfs per acre for Q100. Site area= 1.32 acres.
Performance data from table 6 illustrates the difficulty in achieving the target runoff rate
reductions contained in the design standards when dealing with relatively small project sites.
Although the 6 -inch minimum sized orifice cannot achieve the target reduction rates, the
detention basin does significantly reduce peak discharges from the project site, even lower
than the existing discharge values.
Since the runoff from the site will be directed to an adequate drainage outlet (offsite
stormsewer system), the proposed detention system should be considered acceptable for the
project site.
CONCLUSION
The proposed re- development project has been designed to comply with the City of Carmel
drainage standards and standard engineering practice. This report presents detailed
information in support of the request for drainage approval for the project site plans.
The proposed stormwater management system design will collect, attenuate and safely
convey onsite stormwater runoff from the project site to an appropriate drainage outlet and will
provide required storm water quality treatment for the proposed updated McDonald's site.
13
McDonald's Restaurant Re- Development Project Landwater Group, Inc.
750 E. Carmel Drive, Carmel, Indiana July 30, 2012
Drainage Design Study