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Home My WebLink AboutSW6090701_Stormwater Report_20110829STORMWATER REPORT If -Till GRAYS CREEK MIDDLE SCHOOL CUMBERLAND COUNTY, NORTH CAROLINA PREPARED FOR MCBRIDE HESS DESIGN GROUP 1073 BULLARD CT RALEIGH, 27615 PREPARED BY . W.K. DICKSON & CO., INC. 3101 JOHN HUMPHRIES WYND RALEIGH, NC 27612 t-c• JULY 2009 DAVID J. KIKER, PE STORMWATER REPORT FOR GRAY'S CREEK MIDDLE SCHOOL Introduction The following report supports the design of the proposed stormwater wet pond at Gray's Creek Middle School located on Celebration Drive, Hope Mills, North Carolina. The school is located in the unincorporated areas of Cumberland County and is subject to local stormwater requirements for the County. The pond will bring the project into compliance for both detention and water quality. Because the proposed project does not impact any waters of the U.S. or any jurisdictional wetlands the only stormwater requirements that need to be adhered to are the local design requirements. Per discussions with Mike Caldwell at NCDENR Division of Water Quality, the project will be subject to the following design requirements: • Detain the 1-year, 24-hour duration storm so that the post project conditions flows are at or below those of the pre -project conditions. • Treat the runoff generated from the 1st inch of rainfall for TSS removal. The proposed pond accepts all runoff from the proposed impervious areas and will removed 90% of the TSS per Cumberland County design requirements. This report does not support the design of the local stormwater system that will collect runoff and direct it to the proposed wet pond. The on -site storm drainage pipe system was designed to convey a 10 year storm event based on calculations provided by McBride Hess Design Group, P.A. Proposed Site Conditions The proposed project involves the construction of a middle school with several parking areas, sidewalks, school buildings, grass fields and open spaces. The proposed project is composed of 11.32 acres of new impervious area and roughly has a 28% impervious coverage for the total site and 31.6% for that portion draining to the wet pond. Impervious calculations have been tabulated and are shown in Appendix A This triggers the need for stormwater detention and water quality treatment. Soil classifications are Norfolk Loamy Sand - NoA (B), Norfolk Loamy Sand - NoB (B), Autryville Loamy Sand — Aua (A), Wagram Loamy Sand — WaB (A) and Woodington Loamy Sand — Wo (B/D). The predominate SCS soil hydrologic group classification is B. Methodology Intelisolve's computer model Hydraflow Hydrographs 2007 (Hydraflow Hydrographs) was used to size the proposed pond and verify that the post -project conditions flows were less than or equal to the pre -project conditions flows. A copy of the Hydraflow Model output is provided in Appendix B. Natural Resources Conservation Service (NRCS) methodologies were used to simulate the rainfall -runoff process. An NRCS Type II storm with a 24-hour duration was input along with rainfall depths obtained for Johnston County. Hydrology The Hydraflow Hydrographs model offers a variety of methods for simulating rainfall -runoff response and hydrograph development. The SCS Type II storm was chosen along with precipitation data for the 1-, 5-, 10-, and 25-year frequency; 24-hour duration. The NRCS curve number approach was selected to calculate runoff volumes from the precipitation data. Sub - basin unit hydrographs for these flood volumes were developed using the NRCS lag times. WK Dickson & Co., Inc. Page 1 STORMWATER REPORT FOR GRAYS CREEK MIDDLE SCHOOL Drainage Areas Existing and proposed conditions drainage area maps have been provided separately from this report. Drainage areas were delineated using 2-foot contour interval topographic mapping generated from survey data based on the NGVD 1988 vertical datum. The following table summarizes all the input data determined in this report and directly input into the Hydraflow model. Table 1-Summa of Hydrologic Data Drainage Drainage Tc Tc Drainage Area Area RCN RCN (Existing) (Proposed) Basin Existing Proposed (Existing) (Proposed) (Min) (Min) (Acres) (Acres) #1 40.52 35.79 61 73 26.6 8.1 Rainfall Rainfall data was taken from the NOAA's online website for Johnston County as shown in Table 2. NRCS Curve Numbers Runoff curve numbers (RCN's), which are presented in this report, were generated using the hydrologic soil group and corresponding existing and proposed landuse. Runoff curve numbers were developed by weighting the area and corresponding curve numbers for each type of landuse. This methodology is based on the NRCS document entitled Urban Hydrology for Small Watersheds, dated June 1986 and commonly referred to as TR-55. Time of Concentration The time of concentration refers to the time required for water to flow from the most hydraulically remote point of the basin to the location being analyzed. The time of concentration values found in this report were determined based on the "segmental method" as described in TR55. As shown in Table 1 the time of concentration has reduced from 26.6 minutes to 8.1 minutes from the existing to the proposed conditions. Stormwater Detention Evaluation An in-house spreadsheet (Appendix D) and Hydraflow model were used to design the proposed wet pond. The following is a summary of the proposed pond that was designed to detain the 1- year storm event and provide water quality benefits to meet County requirements: • Percent Impervious to pond: 31.63% • Total drainage area to pond: 35.79 acres • Runoff volume needed for water quality treatment: 42,873 cubic feet • Average depth of the pond: 3.5 feet • Pond surface: 29,309 square feet • Orifice needed to achieve 2 to 5 day draw down time: 4" diameter WK Dickson & Co., Inc. Page 2 STORMWATER REPORT FOR GRAYS CREEK MIDDLE SCHOOL The stage storage relationship of the pond has been included in the in-house spreadsheet prepared by WK Dickson. The following table summarizes the results of the Hydraflow model used to evaluate the site and design the proposed wet pond. Because the peak flows in Basin #1 increase in the proposed conditions a wet pond is proposed to bring the project into conformance with County design requirements. As shown in Table 3, the peak flow for the site has decreased from the pre -project to the post -project conditions. A copy of the Hydraflow model results has been provided with this report as Appendix B. As shown in Table 4 the top of dam does not overtop in a 25-year flood. A 10-year flood does not activate the spillway therefore a 25-year flood was chosen for erosion control purposes. The proposed pond does not have a significant embankment. The pond is essentially "a hole in the ground". Therefore it is not vulnerable to overtopping and erosive forces taking out the downstream toe of the embankment causing a breach. Table'4- Spillway Ca acit Top of Dam Elevation Spillway Crest Elevation 10-Year Water 25-Year Water (ft NGVD) (ft NGVD) Surface Elevation Surface Elevation (ft NGVD (ft NGVD 161.00 159.75 159.72 160.63 Hydraulic Calculations Rip -Rap Pad Rip -rap was sized using the NYDOT method for both the outfall into the wet pond and also the outfall into the receiving open channel. A copy of the calculation is shown in Appendix D. The 10-year peak flow of 27 cfs from the barrel leaving the pond (Hydraflow output) was used to size the rip -rap pad. Anti -seep Collar Because the subject dam does not have an embankment there is no need for an anti -seep collar. Antifloation Calculation for Riser Appendix E includes the buoyancy calculation used to confirm that the proposed riser will not float. The resultant factor of safety for the riser against flotation is 1.55 which includes a base thickness of 30". Erosion Control Erosion control measures for the overall site development have been prepared by McBride Hess Design Group, P.A. This plan was reviewed and approved by NCDENR as shown in the attached approval letter. Additional erosion control calculations were prepared by WK Dickson to support the design of the rip -rap pad at the outfall of the 36" diameter barrel that leaves the pond. In addition, a channel liner calculation was prepared for the emergency spillway at the pond assuming a 25-year storm event. WK Dickson & Co., Inc. Page 3 STORMWATER REPORT FOR GRAYS CREEK MIDDLE SCHOOL Conclusions As shown in this report, the proposed stormwater wet pond brings the proposed school improvements into compliance Cumberland County stormwater design requirements. The proposed wet pond will reduce peak flows to at or below the pre -project flows for the 1-year frequency, 247hour duration storm. In addition, the wet pond also removes 90% of the TSS entering the pond. WK Dickson & Co., Inc. Page 4 Grays Creek Middle School Impervious Surface AREA DRIVES & PARKING SIDEWALKS ROOFS TOTAL 11 120,474 2,568 0.00 123,042 2c 28,536 10,085 20,138 58,759 2D 90093 2234 14540 106,867 2A 0 0 0 - 2B 10200 0 0 10,200 2B RE 38824 0 0 38,824 2E 47373 2902 906 51,181 3 1622 17313 18,935 3B 0 0 0 - 4 0 2998 31372 34,370 4B 0 0 0 - 5 0 3002 31364 34,366 5B 0 0 0 - 6 0 3585 13108 16,693 6B 0 0 0 - 7 0 0 0 81 0 0 0 - TOTALI 335.500 1 28,996 1 128,741 493,237 ACRES 7.70 0.67 2.96 11.32 ACRES 2.82 1.35 2.45 0.23 0.89 1.17 0.43 0.79 0.79 0.38 11.32 u Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations WEST DRAINAGE AREA Area 1 Surface Square Feet Acres C Drive/Parking 120,474 2.8 0.85 2.4 Sidewalks 2,568 0.1 0.82 0.0 Roof - 0.0 0.8 0.0 Lawn 113,305 2.6 0.45 1.2 Total 236,347 5.4 3.6 C = 0.66 From fig 8.03 6' elev. - Max length 550' = Tc 6 min From Fig 8.03e Intensity for a 10 - 25 year storm in "/Hr 8 Q = 28.6 CFS DRAINAGE INTO CATCH BASINS 513, 6, & 7 28.6 CFS TOP ELEVATION 164.23 INVERT OUT AT CB #7 160.00 175.00' OF 36"RCP A .3% SLOPE TO CB #8 INVERT IN AT CB # 8 = 159.46 Area 2C Surface Square Feet Acres C Drive/Parking 28,536 0.7 0.85 _ 0.6 Sidewalks 10,085 0.2 0.82 0.2 Roof 20,138 0.5 0.80 0.4 Lawn 53,202 1.2 0.45 0.5 Total 111,961 2.6 1.7 C = 0.65 From fig 8.03 1.0' elev. - Max length 275" = Tc 4.5 min From Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 13.3 CFS DRAINAGE INTO CATCH BASIN 8 28.6 CFS + 13.3 CFS = 42.9 CFS TOP ELEVATION 165.25 INVERT OUT AT CB #8 159.46 198.00' OF 42" RCP A .3% SLOPE TO CB #9 INVERT IN AT CB #9 = 158.87 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 2D Surface Square Feet Acres C Drive/Parking 9,093 0.2 0.85 0.2 Sidewalks 2,234 0.1 0.82 0.0 Roof 14,540 0.3 0.80 0.3 Lawn 7,504 0.2 0.45 0.1 Total 33,371 0.8 0.6 C = 0.74 From fig 8.03 1' elev. - Max length 100' = To 2. min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 4.5 CFS DRAINAGE INTO CATCH BASIN 9 28.6CFS +13.3CFS+4.5CFS=47.4CFS TOP ELEVATION 165.50 INVERT OUT AT CB #9 158.87 153.00' OF 48" RCP A .3% SLOPE TO CB #10 INVERT IN AT CB #10= 158.40 Area 2A Surface Square Feet Acres C Drive/Parking 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof 0.0 0.80 0.0 Lawn 47,968 1.1 0.45 0.5 Total 47,968 1.1 0.5 C = 0.45 From fig 8.03 3' elev. - Max length 300" = Tc 6. min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 4.0 CFS DRAINAGE INTO FES #1 4.0 CFS INVERT IN AT FES #1 = 162.5 30' OF 18" RCP A 2% SLOPE TO CB #12 INVERT IN AT CB #12 = 161.90 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 9%954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 2B PARTIAL Surface Square Feet Acres C Drive 10,200 0.2 0.85 0.2 Sidewalks 0.0 0.82 0.0 Roof 0.0 0.80 0.0 Lawn 0.0 0.45 0.0 Total 10,200 0.2 0.2 C = 0.85 From fig 8.03 V elev. - Max length 160" = Tc 2.5 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 1.6 CFS DRAINAGE INTO CATCH BASINS #12 + #13 4.0CFS+1.6CFS=5.6CFS CB #12 TOP ELEVATION 164.61 INVERT OUT AT CB #12 = 161.90 27' OF 18" RCP A 2% SLOPE TO CB #13 INVERT IN AT CB #13 = 161.38 #13 TOP ELEVATION 164.53 INVERT OUT AT CB #13 = 161.38 45' OF 18" RCP A 20% SLOPE TO FES #2 INVERT OUT AT FES#2 = 160.48 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954,8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 2B REMAINDER Surface Square Feet Acres C Drive 38,824 0.9 0.85 0.8 Sidewalks 0.0 0.82 0.0 Roof 0.0 0.80 0.0 Lawn 69,814 1.6 0.45 0.7 Total 108,638 2.5 1.5 C = 0.59 From fig 8.03 5' elev. - Max length 120" = Tc 1.5 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 11.8 CFS DRAINAGE INTO FES #3 4.0 CFS + 1.6 CFS +11.8 CFS = 17.4 CFS INVERT IN AT FES #3 = 160.50 200' OF 30" RCP A .3% SLOPE TO CB #10 INVERT IN AT CB #10 = 159.90 DRAINAGE INTO CATCH BASIN 10 28.6 CFS + 13.3 CFS + 4.5 CFS = 47.4 CFS 4.0 CFS + 1.6 CFS +11.8 CFS = 17.4 CFS 64.8 CFS TOP ELEVATION 165.50 INVERT IN FROM CB#9 = 158.40 147.00' OF 48" RCP A .34% SLOPE TO CB #11 INVERT OUT AT CB#10 = 158.40 INVERT IN AT CB #11 = 157.90 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 2E Surface Square Feet Acres C Drive/Parking 47,373 1.1 0.85 0.9 Sidewalks 2,902 0.1 0.82 0.1 Roof 906 0.0 0.80 0.0 Lawn 16,166 0.4 0.45 0.2 Total 67,347 1.5 1.2 C = 0.76 From fig 8.03 3' elev. - Max length 300' = Tc 3.5 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 9.3 CFS DRAINAGE INTO CATCH BASIN 11 28.6 CFS + 13.3 CFS + 4.5 CFS = 47.4 CFS 4.0 CFS + 1.6 CFS +11.8 CFS = 17.4 CFS 9.3 CFS 74.1 FS TOP ELEVATION 163.00 INVERT OUT AT CB #11 = 157.90 416.00' OF (2) 36" RCP A .3% SLOPE TO FES #4 & #5 INVERT OUT AT FES #4 & #5 = 156.65 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations EAST DRAINAGE AREA Area 3 Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 1,622 0.0 0.82 0.0 Roof 17,313 0.4 0.80 0.3 Lawn 14,659 0.3 0.45 0.2 Total 33,594 0.8 0.5 C = 0.65 From fig 8.03 V elev. - Max length 200" = Tc 4 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 4.0 CFS DRAINAGE AD #1 4.0 CFS TOP ELEVATION 166.00 INVERT OUT AT AD #1 = 163.50 165' OF 18"RCP A 1.2% SLOPE TO AD #2 INVERT IN AT AD #2 = 161.50 Area 3B Surface Square Feet Acres C Drive/Parking 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof - 0.0 0.80 0.0 Lawn 42,879 1.0 0.45 0.4 Total 42,879 1.0 0.4 From fig 8.03 Im 4.5' elev. - Max length 500" = Tc 12 min Frrom Fig 8.03e Intensity 10 - 25 year storm in "/Hr = Q = 2.2 CFS DRAINAGE AD #2 4.OCFS +2.2CFS =6.2CFS TOP ELEVATION 164.00 INVERT OUT AT AD #2 161.50 155" OF 24"RCP A .3% SLOPE TO AD#4 INVERT IN AT AD#4 161.03 INVERT OUT AT AD #4 160.32 0.45 5 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 4 Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 2,998 0.1 0.82 0.1 Roof 31,372 0.7 0.80 0.6 Lawn 26,613 0.6 0.45 0.3 Total 60,983 1.4 0.9 C = 0.65 From fig 8.03 V elev. - Max length 200" = Tc 4 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 7.3 CFS DRAINAGE AD #3 7.3 CFS TOP ELEVATION 165.75 INVERT OUT AT AD #3 163.00 205" OF 18" RCP A .75% SLOPE TO AD#4 INVERT IN AT AD #4 161.43 Area 46 Surface Square Feet Acres C Drive/Parking 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof - 0.0 0.80 0.0 Lawn 22,100 0.5 0.45 0.2 Total 22,100 0.5 0.2 C= From fig 8.03 2' elev. -Max length 2100" = Tc 4 min Frrom Fig 8.03e Intensity 10 - 25 year storm in 7Hr = Q = 1.8 CFS DRAINAGE AD #4 6.2 CFS + 7.3 CFS +1.8 CFS = 15.3 CFS TOP ELEVATION 163.85 INVERT IN FROM AD #3 161.43 INVERT IN FROM AD #2 161.03 190' OF 30" RCP A .3% SLOPE TO AD #6 INVERT OUT AT AD #4 159.32 INVERT IN AT AD #6 158.75 0.45 8 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 5 Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 3,002 0.1 0.82 0.1 Roof 31,364 0.7 0.80 0.6 Lawn 26,618 0.6 0.45 0.3 Total 60,984 1.4 0.9 C = 0.65 From fig 8.03 V elev. - Max length 200" = Tc 4 min From Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 7.3 CFS DRAINAGE AD #5 7.3 CFS TOP ELEVATION 165.75 INVERT OUT AT AD #5 163.00 205" OF 18"RCP A 1.2% SLOPE TO AD#6 INVERT IN AT AD #6 160.50 Area 56 Surface Square Feet Acres C Drive/Parking 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof - 0.0 0.80 0.0 Lawn 22,068 0.5 0.45 0.2 Total 22,068 0.5 0.2 C = 0.45 From fig 8.03 V elev. - Max length 200" = Tc 4 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 8 Q = 1.8 CFS DRAINAGE AD #6 6.2 CFS + 7.3 CFS +1.8 CFS = 15.3 CFS 7.3 CFS +1.8 CFS = 9.1 CFS TOTAL = 24.4 CFS TOP ELEVATION 163.25 INVERT IN FROM AD #5 160.50 INVERT IN FROM AD #4 158.75 200' OF 36" RCP A .3% SLOPE TO AD #8 INVERT OUT AT AD #6 158.65 INVERT IN AT AD #8 158.05 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 6 Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 3,585 0.1 0.82 0.1 Roof 13,108 0.3 0.80 0.2 Lawn 32,040 0.7 0.45 0.3 Total 48,733 1.1 0.6 C = 0.57 From fig 8.03 1' elev. - Max length 200" = Tc 4 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 5 Q = 3.2 CFS DRAINAGE AD #7 3.2 CFS TOP ELEVATION 165.75 INVERT OUT AT AD #7 163.00 205" OF 18"RCP A 1.95% SLOPE TO AD#8 INVERT IN AT AD #8 159.00 Area 6B Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof 0.0 0.80 0.0 Lawn 17,612 0.4 0.45 0.2 Total 17,612 0.4 0.2 C = 0.45 From fig 8.03 1' elev. - Max length 200" = Tc 4 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 5 Q = 0.9 CFS McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 7 Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof 0.0 0.80 0.0 Lawn 147.681 3.4 0.45 1.5 Total 147,681 3.4 1.5 C = 0.45 From fig 8.03 1.5' elev. - Max length 550" = Tc 8 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 6 Q = 9.2 CFS DRAINAGE AD #8 6.2 CFS + 7.3 CFS +1.8 CFS = 15.3 CFS 7.3 CFS +1.8 CFS = 9.1 CFS 3.2CFS +.9CFS+9.2CFS =13.3CFS TOTAL 37.7 CFS TOP ELEVATION 162.50 INVERT IN FROM AD #7 159.00 INVERT IN FROM AD #6 158.05 300' OF 42" RCP A .3% SLOPE TO FES #6 INVERT OUT AT AD #8 157.40 INVERT OUT AT FES #6 156.75 McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Grays Creek Middle School Cumberland County North Carolina Soil and Erosion Control Calulations Area 8 Surface Square Feet Acres C Drive/Parking - 0.0 0.85 0.0 Sidewalks 0.0 0.82 0.0 Roof 0.0 0.80 0.0 Lawn 324,086 7.4 0.45 3.3 Total 324,086 7.4 3.3 C = 0.45 From fig 8.03 3' elev. - Max length 620' = Tc 9.5 min Frrom Fig 8.03e Intensity 10 - 25 year storm in '/Hr = 7 Q = 23.4 US McBride Hess Design Group 1073 Bullard Court Raleigh, NC 27615 919.954.8200 Watershed Model Schematic 1 - POST DEVELOPMENT DRAINAGE 3 - Routing to Wet Pond W Hydraflow Hydrographs by Intelisolve v9.22 2-PRE DEVELOPMENT UD Project: Wet Pond without Perm Pool Included.gpw Thursday, Jul 2, 2009 2 Hydrograph Return Period Recap HydraFlow Hydrog raphs by Intelisolve v9.22 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 SCS Runoff ------- 51.06 ----- -- 122.51 161.36 218.71 ------ 318.44 POST DEVELOPMENT DRAINAG 2 SCS Runoff ------- 8.675 ------- ------- 41.36 62.43 95.19 156.03 PRE DEVELOPMENT DRAINAGE 3 Reservoir 1 1.278 ------ ------ 18.08 27.48 69.89 ------ 0.000 Routing to Wet Pond Proj. file: Wet Pond without Perm Pool Included.gpw Thursday, Jul 2, 2009 3 Hydrograph Summary Report Hydraflow Hydrographs by lntelisolve v9.22 Hyd. No. Hydrograph type (origin) Peak flow (cfs) Time interval (min) Time to peak (min) Hyd. volume (cuft) Inflow hyd(s) Maximum elevation (ft) Total strge used (cuft) Hydrograph description 1 SCS Runoff 51.06 1 719 110,160 --- ----- ----- POST DEVELOPMENT DRAINAG 2 SCS Runoff 8.675 1 733 55,243 --- ----- ------ PRE DEVELOPMENT DRAINAGE 3 Reservoir 1.278 1 1015 66,133 1 158.17 74,808 Routing to Wet Pond Wet Pond without Perm Pool Included.gpw Return Period: 1 Year Thursday, Jul 2, 2009 4 Hydrograph Report Hydraflow Hydrographs by Inlelisolve v9.22 Hyd. No. 2 PRE DEVELOPMENT DRAINAGE Hydrograph type = SCS Runoff Storm frequency = 1 yrs Time interval = 1 min Drainage area = 40.520 ac Basin Slope = 0.0 % Tc method = TR55 Total precip. = 3.02 in Storm duration = 24 hrs Q (cfs) 10.00 : 11 . ie 4.00 2.00 Thursday, Jul 2, 2009 Peak discharge = 8.675 cfs Time to peak = 733 min Hyd. volume = 55,243 cuft Curve number = 61 Hydraulic length = 0 ft Time of conc. (Tc) = 26.60 min Distribution = Type II Shape factor = 484 PRE DEVELOPMENT DRAINAGE Hyd. No. 2 -- 1 Year Q (cfs) 10.00 IR �1 4.00 2.00 0.00 0.00 0 120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560 Hyd No. 2 Time (min) — 5 Hydrograph Summary Report Hydraflow Hydrographs by lntelisolve v922 Hyd. No. Hydrograph type (origin) Peak flow (cfs) Time interval (min) Time to peak (min) Hyd. volume (cuft) Inflow hyd(s) Maximum elevation (ft) Total strge used (cuft) Hydrograph description 1 SCS Runoff 122.51 1 719 258,421 -- -- POST DEVELOPMENT DRAINAG 2 SCS Runoff 41.36 1 731 175,939 --- ---- ----- PRE DEVELOPMENT DRAINAGE 3 Reservoir 18.08 1 733 213,662 1 158.98 113,366 Routing to Wet Pond Wet Pond without Perm Pool Included.gp Return Period: 5 Year Thursday, Jul 2, 2009 Fz Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Hyd. No. 2 PRE DEVELOPMENT DRAINAGE Hydrograph type = SCS Runoff Storm frequency = 5 yrs Time interval = 1 min Drainage area = 40.520 ac Basin Slope = 0.0 % Tc method = TR55 Total precip. = 4.69 in Storm duration = 24 hrs Q (cfs) 50.00 40.00 30.00 20.00 10.00 Thursday, Jul 2, 2009 Peak discharge = 41.36 cfs Time to peak = 731 min Hyd. volume = 175,939 cuft Curve number = 61 Hydraulic length = 0 ft Time of conc. (Tc) = 26.60 min Distribution = Type II Shape factor = 484 PRE DEVELOPMENT DRAINAGE Hyd. No. 2 -- 5 Year 120 240 360 480 600 720 840 Hyd No. 2 Q (cfs) 50.00 40.00 30.00 20.00 10.00 ' 0.00 960 1080 1200 1320 1440 1560 Time (min) 7 Hydrograph Summary Report Hyd raFlow Hydrographs by Intelisolve v9.22 Hyd. No. Hydrograph type (origin) Peak Flow (cfs) Time interval (min) Time to peak (min) Hyd. volume (cuft) Inflow hyd(s) Maximum elevation (ft) Total strge used (cuft) Hydrograph description 1 SCS Runoff 161.36 1 718 341,381 --- ---- ----- POST DEVELOPMENT DRAINAG 2 SCS Runoff 62.43 1 730 250,743 ---- ------ ------ PRE DEVELOPMENT DRAINAGE 3 Reservoir 27.48 1 730 296,443 1 159.72 151,025 Routing to Wet Pond Wet Pond without Perm Pool Included.gp Return Period: 10 Year Thursday, Jul 2, 2009 n Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Thursday, Jul 2, 2009 Hyd. No. 2 PRE DEVELOPMENT DRAINAGE Hydrograph type = SCS Runoff Peak discharge = 62.43 cfs Storm frequency = 10 yrs Time to peak = 730 min Time interval = 1 min Hyd. volume = 250,743 cuft Drainage area = 40.520 ac Curve number = 61 Basin Slope = 0.0 % Hydraulic length = 0 ft Tc method = TR55 Time of conc. (Tc) = 26.60 min Total precip. = 5.52 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Q (cfs) 70.00 N-11411A 40.00 30.00 10.00 0.00 0 120 240 Hyd No. 2 40 PRE DEVELOPMENT DRAINAGE Hyd. No. 2 -- 10 Year Q (cfs) 70.00 50.00 40.00 30.00 20.00 10.00 0.00 480 600 720 840 960 1080 1200 1320 1440 1560 Time (min) 0 Hydrograph Summary Report Hydraflow Hydrographs by lntelisolve v9.22 Hyd. No. Hydrograph type (origin) Peak flow (cfs) Time interval (min) Time to peak (min) Hyd. volume (cuft) Inflow hyd(s) Maximum elevation (ft) Total strge used (cuft) Hydrograph description 1 SCS Runoff 218.71 1 718 464,803 -- -- POST DEVELOPMENT DRAINAG 2 SCS Runoff 95.19 1 730 367,709 ---- ------ ----- PRE DEVELOPMENT DRAINAGE 3 Reservoir 69.89 1 726 419,682 1 160.63 198,898 Routing to Wet Pond Wet Pond without Perm Pool Included.gp Return Period: 25 Year Thursday, Jul 2, 2009 10 Hydrograph Report HydraFlow Hydrographs by Intelisolve v9.22 Thursday, Jul 2, 2009 Hyd. No. 2 PRE DEVELOPMENT DRAINAGE Hydrograph type = SCS Runoff Peak discharge = 95.19 cfs Storm frequency = 25 yrs Time to peak = 730 min Time interval = 1 min Hyd. volume = 367,709 cuft Drainage area = 40.520 ac Curve number = 61 Basin Slope = 0.0 % Hydraulic length = Oft Tc method = TR55 Time of conc. (Tc) = 26.60 min Total precip. = 6.69 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Q (cfs) 100.00 90.00 80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 PRE DEVELOPMENT DRAINAGE Hyd. No. 2 -- 25 Year Q (cfs) 100.00 90.00 80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00 ' 0.00 0 120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560 Hyd No. 2 Time (min) WK DICKSON Engineering Planning Surveying 616 Colonnade Drive Charlotte, NC 28205 (704)334-5348 GREY'S CREEK ELEMENTARY SCHOOL - WET DETENTION POND #1 W.K. DICKSON DESIGN: TOTAL AREA DRAINING TO POND = 1,559,013 SF = 35.79 AC TOTAL PROPOSED IMPERVIOUS SURFACE AREA DRAINING TO POND: = 493,099 SF PERCENT IMPERVIOUS DRAINAGE AREA FOR POND: = (493,099 / 1,559,013 SF) x 100 = 31.63% REQUIRED STORAGE FOR 1-YR 24-HR STORM EVENT OF RUNOFF FOR POND: (use Simple Method) NORMAL POND DEPTH (FT) 4.0 Rv= 0.05 + 0.009 (IA) Rv= 0.05 + 0.009(31.63) = 0.33 in./in. V=3630*RD*Rv*A = 3630 * 1.0 * 0.33 * 35.79 = 42,873 CF (00000 CF PROVIDED) AVERAGE DEPTH SA/DA % POND SURFACE AREA (MIN.) (FT) (SF) 3.50 1.88 29,309 AVERAGE DEPTH = PERM. POOL VOLUME / PERM. SURFACE AREA IF POND DEPTH = 3.5 FT, MINIMUM SURFACE AREA = 29,309 SF USED 90% TSS CHART TO SIZE POND TOP OF PERM. POOL/BOTTOM AREA OF STORAGE ELEV 157.00 = 38,437 SF > 29,309 SF Revised: PROJECT: GRAY'S CREEK SUBJECT: POND CALCULATIONS - #1 JOB NO.: CLN00289.UN.CO PREPARED BY: BR DATE: 06.29.09 CHECKED BY: DJK DATE: 07.02.09 PAGE NO I OF 3 WK DICKSON Engineering 616 Colonnade Drive Planning Charlotte, NC 28205 Surveying (704)334-5348 REQUIRED STORAGE FOR FOREBAY: 20% OF TOTAL PERMANENT POOL VOLUME = (0.2)(38,437 SF)(3.5') = 26,906 CF (29,710 CF PROVIDED) Pond Volume Contour Incremental Cumulative Elevation Area Volume Volume (ft) (sf) (cf) (cf) Bottom Elevation a153i. '. 29338:� 0 0 :;l5,4 �31520 ; 30429 30429 32647 63076 156 :36081,. 34928 98004 Permanent Pool 157 ..,,, 38437,, 37259 135263 Forebay Volume Contour Incremental Cumulative Elevation Area Volume Volume Bottom Elevation ., 153- 6263 0 0 154.,. 72z86 6775 6775 155,,. --8378' ,' 7832 14607 - 156; ,, .9538;; 8958 23565 Permanent Pool .:, .157 ... J 1330: 10434 33999 Forebay Volume is of Pond Volume Revised: PROJECT: GRAY'S CREEK SUBJECT: POND CALCULATIONS - #1 JOB NO.: CLN00289.UN.CO PREPARED BY: BR DATE: 06.29.09 CHECKED BY: DJK DATE: 07.02.09 PAGE NO 2 OF 3 WK DICKSON Engineering Planning Surveying Permanent Pool 616 Colonnade Drive Charlotte, NC 28205 (704)334-5348 1" Storage & Release Time Contour Incremental Cumulative Outflow Release Elevation Area Volume Volume Rate Time (ft) (sf) (cf) (cf) (cfs) (hrs) .156.5 40054 40054 0 0.000 0.000 157 ,,, 441,84, 21060 21060 0.297 19.7 158',,h r 46738; 45461 66521 0.515 24.5 159x 493„ 48, 48043 114564 0.664 20.1 160 g52013 , ;, 50681 165244 0.786 17.9 161 _ . -54737 _ 104085 218649 0.891 32.4 TOTAL HOURS 114.7 DAYS 4.8 Orifice = 4.0 inches USED ORIFICE EQUATION TO DETERMINE OUTFLOW RATE Q = Cd`A'(2gh)Al/2 Revised: PROJECT: GRAY'S CREEK SUBJECT: POND CALCULATIONS - #1 JOB NO.: CLN00289.UN.CO PREPARED BY: BR DATE: 06.29.09 CHECKED BY: DJK DATE: 07.02.09 PAGE NO 3 OF 3 25 20 s- Appendices NEW YORK DOT DISSIPATOR METHOD FOR USE IN DEFINED CHANNELS (Source: `Bank and channel lining procedures", New York Department of Transportation, Division of Design and Construction, 1971.) NOTE: To use the following chart you must know: (1) Q full capacity (2) Qw (3) V full (4) Vm where Q = discharge in cfs and V = Velocity in FPS. ESTIMATION OF STONE SIZE AND DIMENSIONS FOR CULVERT APRONS Step 1) Compute flow velocity V at culvert or paved channel outlet. Step 2) For pipe culverts De is diameter. For pipe arch, arch and box culverts, and paved channel outlets. D. = A. where A. = cross -sectional area of flow at outlet. For multiple culverts, use D. =1.25 x D. of single culvert. Step 3) For apron grades of 10% or steeper, use recommendations For next higher zone. (Zones 1 through 6). YYY ®IaYYYYInmAYY®Y®YY ..YY,9.aY.mOW m... \Y.OnW annYn.Wn®Yp mma.mYYmmY®am va.unfYYaaaamuvvuamaW ca W. Y\....muvvvauuuu0Yai0nfY.. aron.aa®uu aYIY YnaY.mlm YYYYYYY�® Y Yaa WammaaWauaavvYauYaaaa asvY sanfWmaamaamuuavuaaas»a®a. amX aarYl ®aramYYumlYaaaaaaaaaa.n®n® immmW lmlamauammYimvamaaam Imna...anvYam. W.m..Yun®mnYY YYa..�----muYa.aaam.ammmmmYY na.>ou aaa uamuu lave aavvYY m as W Y .m\. 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YYap�YaaYYYY .Wl■an alaaYlla.�YYaaaWaaY.nY=YYWaaI my vl..Y.uluY.wuuaanam..YsvaaYaa mm ®aa.lnlmuaaamuuamuuvs�maa maY.mlJ.mallaa{a�LaYaaaa W tlalYaaaL1Y a CN �fK 0 $ Figure 8.08n i 10' 1$ a7' 25'. DIAMETER (FL) Rev. 12/93 8.06.5 W p N APRON. MATERIAL LENGTH OF APRON TO PROTECT CULVERT L1 TO PREVENT SCOUR HOLE USE L2 ALWAYS L2 t STONE FILLING (FINE) CL.A 3 X Do 4 x D. 2 STONE FILLING (LIGHT) CL B 3 X Da 6 % D, 3 STONE FILLING (MEDIU1r1) L t 4 X Da) 8 x 0, 4 STONE FILLING (HEAVY) CL t 4 X D. 8 x Dv 5 STONE FILLING (HEAVY) CI-2 5 X De lox D. 6 STONE FILLING (HEAVY) CL 2 6 X Do 10 x De 7 1 SPECIALSTUDY REQUIRED (ENERGY DISSIPATORS, STILLING BASIN OR LARGER SIZE STONE). I ' Figure 8.06d d Width —3 times pipe dia. (min.) DETERMINATION OF STONE SIZES FOR DUMPED STONE CHANNEL LININGS AND REVETMENTS Step 1. Use figure 8.06.b:3 to determine maximum stone size (eg: for 12 Fps = 20" of 550 lbs. Step 2. Use figure 8.06.b.4 to determine acceptable size range for stone (for 12 FPS it is 125-500lbs. for 75% of stone, and the maximum and minimum. range in weight should be 25-500 lbs.). NOTE: In determining channel velocities for stone linings and revetment, use the following coefficients of roughness: Diameter Manning's Min. thickness (inches) "n" of lining (inches) Fine 3 0.031 9 12 Light 6 0.035 12 18 Medium 13 0.040 IB 24 Heavy 23 0.044 30 36 (Channcls) (Dissapatm) 8.06.6 Rev. 12193 er Project: Grays Creek Middle School Engineer: DJK Date: 7-2-09 Mannings Equation, Q=�A�(1.49Rh66sn51 n J (DEHNR) Channel Location Design Flow(cis) Chan Bot Width Side Slope Side Slope Length Design Depth Chan Area Wetted Perim.,Pw Hydraulic Radius Mann. "n" Channel Slope Q Allow. Calc. Depth Calc. Velocity Shear Stress Temp. Liner Perm. Liner 42 50 A 1 3. 4.0 1,1.25 23 1 23 1 1.0 10.027,1 0.021 1 188 0.6 1 5.1 -1 0.7 Straww/net I Grass Shear Stress, T = yds T = shear stress in lb/sq. ft. y = unit weight of water, 62.4 lb/cu. ft. d = flow depth in ft. s = channel slope in ft./ft. TemporaryLiners Allow$heaSuess Material (lb/sqd) Tacked Mulch 0.35 Jute Net 0.45 Straww/Net 145 SythetiaMet 2.00 ChusA 1.25 C1assB 2.00 Classl 3.40 Max Peoussibb Velocities5r Unprotcted Soilsin Ex Channels Material Max Po utissibb Velocity(9/s) FineSand(noncollidl) 2.5 Saud Loam(noncollidl) 2.5 Silt Lonm(noncollidl) In OrdinaryFirm Losm 3.5 FineG.t,l 5.0 Stiff Clay(verycollidan 5.0 Gmded,Silt toCobbles 50 Notes: Side slope = horiz./vert. Needs input Depth and Velocity calculated using Dodson's Hydrocalc model For temporary liners with a short term of service (6 months or less), use 2-year design flow Max. Allow. Design V for Vegetative Channels CheonelSlope Soil Gmss Lining Pervssibb V (1/s) 0-5% Sands/Silt Bemuda 5.0 Tall Fescue 4.5 KY Bluegrass 4.5 Grass-legut toix 3.5 Clay Mims Berttuda 6.0 Tall Fescue 5.5 KY Bluegrass 5.5 Grass-legunetnx 4.5 5-10% Surds/Silt Bemuda 4.5 Tall Fescue 4.0 KYBluegmas 40 Grass-legurremx 3.0 Clay Mims Bermuda 5.5 Tell Fescue 5.0 KYBluegrass 50 Grass-legunemx 3.5 � i J Map Ihrit Map Unit Name percent of Symbol AOI AOI BaO Blaney loamy 9.8 3.5% sand, 8 to 15 Percent slopes CaB Candor sand, 1 to 86.2 30.9% 8 percent slopes FaA Facevdle loamy 09 0.3% sand, 0 to 2 Percent slopes FaB Facevdle loamy 1.6 0.6% sand, 2 to 6 percent slopes GOA Goldsboro loamy L7 0.6% sand, 0 to 2 percent slopes JI 101V14LDf1 loam 4.8 1.7%. NoA Norfolk loamy 12.6 4.5% sand, 0 to 2 percent slopes NOB c bato 18.5 6.6% 2 to 6 slopes Ra jpemcent sandy loam 5.3 1.9% WaB m loamy 137.6 49.3% O to 6 rt slopes Totals for Area of Interest 278.8 100.046 to AL Warning: Soil Map may not he valid at this scale. You have zoomed in beyond the scale at which the sod map for this area is intended to be used. Mapping of si is done at a particular scale. The sod surveys that comprise your AOI were mapped at 1:24,000. The design of map units and the level of detail shown in the resulting sod map are dependent on that map scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and arrurary of coil hoe olarement. The mans do not ahnw the small areas of rontrachnn srntc ttur rrrndd navp hw i I.; i WaB CaB uz� .. ..-.. Jt ili aB Wnli '`y z Coq y i N°A r`: y NoB B kL JAd Ai 6*11 �V q •_, `, ,�� 1 � ICA \ ✓ 'r WFam(@L,.o L.' JSv. 1O 0I T r-W`1 a�� NZI AW