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HomeMy WebLinkAboutDrainage McDONALD'S RESTAURANT RE- DEVELOPMENT PROJECT 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 1 McDONALD'S RESTAURANT RE-DEVELOPMENT PROJECT 750 E. Carmel Drive Carmel, Indiana DRAINAGE SYSTEM DESIGN TECHNICAL INFORMATION REPORT PROFESSIONAL CERTIFICATION ```��tiS11 N t,,Q,1114.5.6744,1i G\STEq a;P 1-. O• No.920256 ri o -o STATE OF Martin S. Mann, P.E. SEAL Indiana Registration No. 920256 The following report and accompanying computations have been developed by me or under my direct supervision. 11 DELORME XMap®6 � N ! _ _ i J. p � Y •i, r til, t, , ® -� I....Il t .„,•a, , I `` , a Lpt / 1_ j 0,4/N I /. _ •• ___ J ? _ — - _}7 a oN Ro” i 1 �r °liil �,l Y f '' '�; 6t . iI _• :y 1 i. 1 (1_ re'kr It 1 r. w ° I {moo• itl 1�!ou ;ul V'4 /r, ' 1,`N, ' 1: ,- ;T.11 `.( •5 1• •A •: i — • •• .;, r ,','.4 5 I ' r.j rI,1:— P. 4 04:__ ' 'R_ _Ytl `1 - • y 'M' STREET ''I V ..-78i _: ��-.._ y ,‘_ _ alb 7-1 Cr ' 1'. B6' - ,, • L I.,-1, •-f 1 i 1�;' l -41. '. .t� ,`Y� ,• .R D•,647l�, ?i 'i M^,• /. • I /"_ ',McDonald's Project Site. ( ' w• 4. - r' � f- j •t_T. G1,a 1. l `�,r '� • r: 'v r -. -3,:r1.11, • i..• .� , �i 71 / .6 1* . ," •ss. .:10. 0 . ....': .• : • ...I:4.' .„2111; , ) - - \" • •• .,..:: ::,,.• •........:I• ‘..,_ 2 r? 01 0 —_ ., .- •� •P 1 it !� - y-. �'' • X -.-1, -'o ,� \ jr .--._._may'' \\,\ -s_G. 1 t - •t--- +�ws -�'1. 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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 1 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. B- C) 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. 2 McDonald's Restaurant Re-Development Project Landwater Group, Inc. 750 E. Carmel 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. 3 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. 4 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: 1. 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. 5 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 6 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 (I1) 4 HYD2 Insert sub-basin hydrograph 11 (direct runoff to detention basin) to hydrograph 2 storage register. ADD HYD1 + HYD2 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) HYD4 Route post developed sub-basin runoff through the onsite detention system ADD HYD4 + HYD5 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 INLET GRATE 10YR/24HR 10YR/24HR SEWER GRATE SPECIFICATION PEAK Q INLET DEPTH INLET TYPE (CFS) (INCHES) 13 Sumped Curb Inlet EJ-5250 0.88 2.4 14 Sumped Curb Inlet EJ-7220 1.31 3.1 15 Sumped Curb Inlet EJ-7220 0.59 1.8 16 Sumped Curb Inlet EJ-7220 0.65 1.9 17 Sumped Pavement EJ-7220 0.51 1.9 8 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 Design Storm STORM INLET GRATE 100YR/24HR 100YR/24HR SEWER GRATE SPECIFICATION PEAK Q INLET DEPTH INLET TYPE (CFS) (INCHES) 13 Sumped Curb Inlet EJ-5250 1.52 3.4 14 Sumped Curb Inlet EJ-7220 2.24 4.0 15 Sumped Curb Inlet EJ-7220 1.10 2.8 16 Sumped Curb Inlet EJ-7220 1.16 2.9 17 Sumped 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 System Performance Table STORM PIPE PIPE PIPE-FULL 10YR/24HR 100YR/24HR SEWER DIAMETER CAPACITY VELOCITY PEAK Q PEAK Q SEGMENT (INCHES) (CFS) (FPS) (CFS) (CFS) I1DN-EX 15 10.08 8.43 2.18 2.52 13-M2 15 4.56 3.82 3.93 6.91 14-13 12 2.77 3.62 2.55 4.50 15-14 12 2.71 3.54 1.24 2.26 16-15 12 2.72 3.55 0.65 1.16 17-13 12 2.71 3.54 0.51 0.89 M2-I1 UP 15 10.35 8.66 3.93 6.91 Table 4-Proposed Condition Hydraulic Grade at Inlet Grates Table STORM LOWEST 10YR/24HR 100YR/24HR DOES INLET SEWER RIM HGL DETENTION SURCHARGE INLET ELEVATION ELEVATION* ELEVATION 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 17 828.40 826.05 826.88 NO M2 828.00 823.41 826.88 NO *HGL computations based on submerged tailwater elevation at detention basin at time-of-hydrograph-peak. 9 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. 10 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. 11 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: • 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 • 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 POST DEVELOPED TARGET POST* PEAK RUNOFF PEAK RUNOFF DEVELOPED PEAK (CFS) (CFS) 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 . . . I 0 1�vn�v�\V'� fVA�I I I I=� uvwvvw\ �v>>y I s�i��'II �� I i° a -- ����, 11 LLv I I i I I i I I psi J`�i I= Jy2 :°°I �� N � --- I°q ❑ ❑ � I 8� S°z . 1 1 i 1 iii 1 i 1 1 H-rfs;O; � I I !. 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