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HomeMy WebLinkAboutStromwater Detention Design Report 042721STORMWATER DETENTION DESIGN ------------- * ------------- 7520 North Chicot Road Little Rock, Arkansas April 27, 2021 ETC Engineers & Architects, Inc. Little Rock, AR Table of Contents 1. Project Description...................................................................................................... 1 2. Watershed Condition .................................................................................................. 1 3. Design Methods and Parameters ................................................................................. 1 5. Design of Detention Pond and outlet structure ........................................................... 1 List of Table Table 1. Watershed parameters ........................................................................................... 2 Table 2. Estimated Pre- and Post-Project Peak outflow ..................................................... 2 Table 3. Designed dimensions of the detention ponds ....................................................... 2 List of Figure Map 1 (Pre-Project Watershed) .......................................................................................... 3 Map 2 (Post-Project Watersheds) ....................................................................................... 4 APPENDIX A: ‘Hydraflow’ Report on the Pre and Post Development Model 1 1. Project Description The Project is located in the City of Little Rock in Pulaski County Arkansas. The Project area comprises of about 0.73 acres of land. The elevations within the project site range from 270 ft NGVD 29 to 278 ft NGVD 29. General drainage pattern within the development site is from southeast to northwest to existing natural drainage way. A small area at the northwest corner is located in the FEMA regulatory floodplain with a Base Flood Elevation of 270 ft. Stormwater from about 77% of the area is proposed to be routed through a detention pond located at the northwest corner of the property. 2. Watershed Condition Watershed Map-1 shows the Pre-project topography with 1 ft contours. Watershed Map-2 shows the Post-project watershed. The project area is divided into two watersheds Area-1 to the north and Area-2 to the south. Area-1 is proposed to be routed through a detention pond. 3. Design Methods and Parameters The detention ponds were designed under the guidelines set forth in the Drainage Design Manual of the City of Little Rock. The storm runoff was computed by using the Rational method with a return period of 25- yr, and 100-yr return period. A computer model, Hydraflow Hydrograph of InteliSolve was used to design the detention ponds. 5. Design of Detention Pond and outlet structure The detention ponds were designed through an iterative process by comparing the inflow (pre-development) and outflow (post-development) hydrographs of the ponds. Table-1 Shows the watershed parameters used in the hydraulic model. Table 2&3 compare the peak outflows of the pond with the Pre-project peak outflows. Table 4 shows the design parameters of the pond and their outlet structure. Detailed model results are shown in Appendix A (Pre-development) and (Post-development). 2 Table 1. Watershed parameters Items Units Pre- Project Watershed Post Project Watershed Area-1 Area-2 Impervious Area Acres 0.09 0.35 0.034 Pervious Area Acres 0.64 0.21 0.136 Total Watershed Area Acres 0.73 0.56 0.17 Flow Length Ft 229 218 284 Overall Land Slope % 3.71 2.66 2.99 Composite Roughness Coeff. ‘C’ - 0.46 0.71 0.5 Composite Roughness Coeff. ‘Kirby n” - 0.4 0.30 0.35 Time of Concentration by Kirby Min 13 13 15 Table 2. Estimated Pre and Post Project Peak Outflow Return Period Pre-Project (cfs) Combine Post-Project (cfs) Water Depth in Pond A (ft) 25-yr 2.14 1.91 1.16 100-yr 2.58 2.35 1.24 Table 3. Designed dimensions of the detention ponds and outlet structure Pond No. Bottom Elevation (ft) Side Slope Top Elevation (ft) Outlet Structure 1 270 3:1 273 50 LF of 12” ADS Culvert w/ FES @ 0.1% Slope 3 Map 1 (Pre-Project Watershed) 4 Map 2 (Post-Project Watersheds) 5 APPENDIX A: ‘Hydraflow’ Report on the Pre and Post Development Model Model Layout 1. Pre-Project Watershed 2. Post-Project Watershed Area-1 3. Post-Project Watershed Area-2 4. Detention Pond Routing 5. Combine Outflow Table of Contents Detention.gpw Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:44 PM Hydrograph Return Period Recap ............................................................................ 1 25 - Year Summary Report ....................................................................................................................... 2 Hydrograph Reports ................................................................................................................. 3 Hydrograph No. 1, Rational, Pre-Project ................................................................................ 3 Hydrograph No. 2, Rational, Post-Project Area 1 ................................................................... 4 Hydrograph No. 3, Rational, Post Project-Area2 .................................................................... 5 Hydrograph No. 4, Reservoir, Detention Routing ................................................................... 6 Pond Report ....................................................................................................................... 7 Hydrograph No. 5, Combine, <no description> ...................................................................... 8 100 - Year Summary Report ....................................................................................................................... 9 Hydrograph Reports ............................................................................................................... 10 Hydrograph No. 1, Rational, Pre-Project .............................................................................. 10 Hydrograph No. 2, Rational, Post-Project Area 1 ................................................................. 11 Hydrograph No. 3, Rational, Post Project-Area2 .................................................................. 12 Hydrograph No. 4, Reservoir, Detention Routing ................................................................. 13 Pond Report ..................................................................................................................... 14 Hydrograph No. 5, Combine, <no description> .................................................................... 15 Hydrograph Return Period Recap 1 Hyd. Hydrograph Inflow Peak Outflow (cfs) Hydrograph No. type Hyd(s)description (origin) 1-Yr 2-Yr 3-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 1 Rational ------- ------- ------- ------- ------- ------- 2.14 ------- 2.58 Pre-Project 2 Rational ------- ------- ------- ------- ------- ------- 1.99 ------- 2.41 Post-Project Area 1 3 Rational ------- ------- ------- ------- ------- ------- 0.51 ------- 0.62 Post Project-Area2 4 Reservoir 2 ------- ------- ------- ------- ------- 1.44 ------- 1.77 Detention Routing 5 Combine 3, 4 ------- ------- ------- ------- ------- 1.91 ------- 2.35 <no description> Proj. file: Detention.gpw Tuesday, Apr 27 2021, 4:42 PM Hydraflow Hydrographs by Intelisolve Hydrograph Summary Report 2 Hyd. Hydrograph Peak Time Time to Volume Inflow Maximum Maximum Hydrograph No. type flow interval peak hyd(s) elevation storage description (origin) (cfs) (min) (min) (cuft) (ft) (cuft) 1 Rational 2.14 1 13 1,666 ---- ------ ------ Pre-Project 2 Rational 1.99 1 13 1,556 ---- ------ ------ Post-Project Area 1 3 Rational 0.51 1 15 461 ---- ------ ------ Post Project-Area2 4 Reservoir 1.44 1 17 1,525 2 271.16 827 Detention Routing 5 Combine 1.91 1 16 1,986 3, 4 ------ ------ <no description> Detention.gpw Return Period: 25 Year Tuesday, Apr 27 2021, 4:42 PM Hydraflow Hydrographs by Intelisolve Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:42 PM Hyd. No. 1 Pre-Project Hydrograph type = Rational Peak discharge = 2.14 cfs Storm frequency = 25 yrs Time interval = 1 min Drainage area = 0.730 ac Runoff coeff. = 0.46 Intensity = 6.359 in/hr Tc by User = 13.00 min IDF Curve = SampleFHA.idf Asc/Rec limb fact = 1/1 Hydrograph Volume = 1,666 cuft 0 5 10 15 20 25 30 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (min) Pre-Project Hyd. No. 1 -- 25 Yr Hyd No. 1 3 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Hyd. No. 2 Post-Project Area 1 Hydrograph type = Rational Peak discharge = 1.99 cfs Storm frequency = 25 yrs Time interval = 1 min Drainage area = 0.560 ac Runoff coeff. = 0.56 Intensity = 6.359 in/hr Tc by User = 13.00 min IDF Curve = SampleFHA.idf Asc/Rec limb fact = 1/1 Hydrograph Volume = 1,556 cuft 0 5 10 15 20 25 30 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 Q (cfs) Time (min) Post-Project Area 1 Hyd. No. 2 -- 25 Yr Hyd No. 2 4 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Hyd. No. 3 Post Project-Area2 Hydrograph type = Rational Peak discharge = 0.51 cfs Storm frequency = 25 yrs Time interval = 1 min Drainage area = 0.170 ac Runoff coeff. = 0.5 Intensity = 6.026 in/hr Tc by User = 15.00 min IDF Curve = SampleFHA.idf Asc/Rec limb fact = 1/1 Hydrograph Volume = 461 cuft 0 5 10 15 20 25 30 Q (cfs) 0.00 0.00 0.10 0.10 0.20 0.20 0.30 0.30 0.40 0.40 0.50 0.50 0.60 0.60 0.70 0.70 0.80 0.80 0.90 0.90 1.00 1.00 Q (cfs) Time (min) Post Project-Area2 Hyd. No. 3 -- 25 Yr Hyd No. 3 5 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Hyd. No. 4 Detention Routing Hydrograph type = Reservoir Peak discharge = 1.44 cfs Storm frequency = 25 yrs Time interval = 1 min Inflow hyd. No. = 2 Max. Elevation = 271.16 ft Reservoir name = Detention Pond Max. Storage = 827 cuft Storage Indication method used.Hydrograph Volume = 1,525 cuft 0.0 0.3 0.7 1.0 1.3 1.7 2.0 2.3 2.7 3.0 3.3 3.7 4.0 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 Q (cfs) Time (hrs) Detention Routing Hyd. No. 4 -- 25 Yr Hyd No. 4 Hyd No. 2 6 Pond Report 7 Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Pond No. 1 - Detention Pond Pond Data Pond storage is based on known contour areas. Average end area method used. Stage / Storage Table Stage (ft) Elevation (ft) Contour area (sqft) Incr. Storage (cuft) Total storage (cuft) 0.00 270.00 516 0 0 1.00 271.00 815 666 666 2.00 272.00 1,171 993 1,659 3.00 273.00 1,583 1,377 3,036 Culvert / Orifice Structures Weir Structures [A] [B] [C] [D] [A] [B] [C] [D] Rise (in)= 12.00 0.00 0.00 0.00 Span (in)= 12.00 0.00 0.00 0.00 No. Barrels = 1 0 0 0 Invert El. (ft)= 270.00 0.00 0.00 0.00 Length (ft)= 50.00 0.00 0.00 0.00 Slope (%)= 0.00 0.00 0.00 0.00 N-Value = .013 .000 .000 .000 Orif. Coeff.= 0.60 0.00 0.00 0.00 Multi-Stage = n/a No No No Crest Len (ft)= 0.00 0.00 0.00 0.00 Crest El. (ft)= 0.00 0.00 0.00 0.00 Weir Coeff.= 0.00 0.00 0.00 0.00 Weir Type = --- --- --- --- Multi-Stage = No No No No Exfiltration = 0.000 in/hr (Contour) Tailwater Elev. = 0.00 ft Note: Culvert/Orifice outflows have been analyzed under inlet and outlet control. 0.00 1.00 2.00 3.00 4.00 5.00 6.00 Stage (ft) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Stage (ft) Discharge (cfs) Stage / Discharge Total Q Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Hyd. No. 5 <no description> Hydrograph type = Combine Peak discharge = 1.91 cfs Storm frequency = 25 yrs Time interval = 1 min Inflow hyds. = 3, 4 Hydrograph Volume = 1,986 cuft 0.0 0.3 0.7 1.0 1.3 1.7 2.0 2.3 2.7 3.0 3.3 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 Q (cfs) Time (hrs) <no description> Hyd. No. 5 -- 25 Yr Hyd No. 5 Hyd No. 3 Hyd No. 4 8 Hydrograph Summary Report 9 Hyd. Hydrograph Peak Time Time to Volume Inflow Maximum Maximum Hydrograph No. type flow interval peak hyd(s) elevation storage description (origin) (cfs) (min) (min) (cuft) (ft) (cuft) 1 Rational 2.58 1 13 2,013 ---- ------ ------ Pre-Project 2 Rational 2.41 1 13 1,880 ---- ------ ------ Post-Project Area 1 3 Rational 0.62 1 15 559 ---- ------ ------ Post Project-Area2 4 Reservoir 1.77 1 16 1,850 2 271.24 906 Detention Routing 5 Combine 2.35 1 16 2,408 3, 4 ------ ------ <no description> Detention.gpw Return Period: 100 Year Tuesday, Apr 27 2021, 4:43 PM Hydraflow Hydrographs by Intelisolve Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Hyd. No. 1 Pre-Project Hydrograph type = Rational Peak discharge = 2.58 cfs Storm frequency = 100 yrs Time interval = 1 min Drainage area = 0.730 ac Runoff coeff. = 0.46 Intensity = 7.687 in/hr Tc by User = 13.00 min IDF Curve = SampleFHA.idf Asc/Rec limb fact = 1/1 Hydrograph Volume = 2,013 cuft 0 5 10 15 20 25 30 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (min) Pre-Project Hyd. No. 1 -- 100 Yr Hyd No. 1 10 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:43 PM Hyd. No. 2 Post-Project Area 1 Hydrograph type = Rational Peak discharge = 2.41 cfs Storm frequency = 100 yrs Time interval = 1 min Drainage area = 0.560 ac Runoff coeff. = 0.56 Intensity = 7.687 in/hr Tc by User = 13.00 min IDF Curve = SampleFHA.idf Asc/Rec limb fact = 1/1 Hydrograph Volume = 1,880 cuft 0 5 10 15 20 25 30 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (min) Post-Project Area 1 Hyd. No. 2 -- 100 Yr Hyd No. 2 11 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:44 PM Hyd. No. 3 Post Project-Area2 Hydrograph type = Rational Peak discharge = 0.62 cfs Storm frequency = 100 yrs Time interval = 1 min Drainage area = 0.170 ac Runoff coeff. = 0.5 Intensity = 7.301 in/hr Tc by User = 15.00 min IDF Curve = SampleFHA.idf Asc/Rec limb fact = 1/1 Hydrograph Volume = 559 cuft 0 5 10 15 20 25 30 Q (cfs) 0.00 0.00 0.10 0.10 0.20 0.20 0.30 0.30 0.40 0.40 0.50 0.50 0.60 0.60 0.70 0.70 0.80 0.80 0.90 0.90 1.00 1.00 Q (cfs) Time (min) Post Project-Area2 Hyd. No. 3 -- 100 Yr Hyd No. 3 12 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:44 PM Hyd. No. 4 Detention Routing Hydrograph type = Reservoir Peak discharge = 1.77 cfs Storm frequency = 100 yrs Time interval = 1 min Inflow hyd. No. = 2 Max. Elevation = 271.24 ft Reservoir name = Detention Pond Max. Storage = 906 cuft Storage Indication method used.Hydrograph Volume = 1,850 cuft 0.0 0.3 0.7 1.0 1.3 1.7 2.0 2.3 2.7 3.0 3.3 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (hrs) Detention Routing Hyd. No. 4 -- 100 Yr Hyd No. 4 Hyd No. 2 13 Pond Report 14 Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:44 PM Pond No. 1 - Detention Pond Pond Data Pond storage is based on known contour areas. Average end area method used. Stage / Storage Table Stage (ft) Elevation (ft) Contour area (sqft) Incr. Storage (cuft) Total storage (cuft) 0.00 270.00 516 0 0 1.00 271.00 815 666 666 2.00 272.00 1,171 993 1,659 3.00 273.00 1,583 1,377 3,036 Culvert / Orifice Structures Weir Structures [A] [B] [C] [D] [A] [B] [C] [D] Rise (in)= 12.00 0.00 0.00 0.00 Span (in)= 12.00 0.00 0.00 0.00 No. Barrels = 1 0 0 0 Invert El. (ft)= 270.00 0.00 0.00 0.00 Length (ft)= 50.00 0.00 0.00 0.00 Slope (%)= 0.00 0.00 0.00 0.00 N-Value = .013 .000 .000 .000 Orif. Coeff.= 0.60 0.00 0.00 0.00 Multi-Stage = n/a No No No Crest Len (ft)= 0.00 0.00 0.00 0.00 Crest El. (ft)= 0.00 0.00 0.00 0.00 Weir Coeff.= 0.00 0.00 0.00 0.00 Weir Type = --- --- --- --- Multi-Stage = No No No No Exfiltration = 0.000 in/hr (Contour) Tailwater Elev. = 0.00 ft Note: Culvert/Orifice outflows have been analyzed under inlet and outlet control. 0.00 1.00 2.00 3.00 4.00 5.00 6.00 Stage (ft) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Stage (ft) Discharge (cfs) Stage / Discharge Total Q Hydrograph Plot Hydraflow Hydrographs by Intelisolve Tuesday, Apr 27 2021, 4:44 PM Hyd. No. 5 <no description> Hydrograph type = Combine Peak discharge = 2.35 cfs Storm frequency = 100 yrs Time interval = 1 min Inflow hyds. = 3, 4 Hydrograph Volume = 2,408 cuft 0.0 0.3 0.7 1.0 1.3 1.7 2.0 2.3 2.7 3.0 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (hrs) <no description> Hyd. No. 5 -- 100 Yr Hyd No. 5 Hyd No. 3 Hyd No. 4 15