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Home My WebLink AboutSW6100905_COMPLIANCE_20101206STORMWATER DIVISION CODING SHEET POST -CONSTRUCTION PERMITS PERMIT NO. SW_� DOC TYPE ❑ CURRENT PERMIT ❑ APPROVED PLANS ❑ HISTORICAL FILE COMPLIANCE EVALUATION INSPECTION DOC DATEj�j� YYYYMMDD N Permit Number SW6100905 Program Category State SW Permit Type State Stormwater Primary Reviewer cory.larsen Coastal SW Rule Permitted Flow Central Files: APS_ SWP_ 12/06/10 Permit Tracking Slip Status Project Type Active New Project Version Permit Classification 1.00 Individual Permit Contact Affiliation Facility Name Major/Minor Region Colonial Landing Minor Fayetteville Location Address County 212 Constitution Dr Hoke Raeford NC 28376 Facility Contact Affiliation Greg M. Caulder 5011 Cumberland Rd Fayetteville INC 28306 Owner Owner Name Owner Type G&G Land Development Inc Non -Government Owner Affiliation Greg M. Caulder 5011 Cumberland Rd Fayetteville NC 28306 Dates/Events Scheduled Ong Issue App Received Draft Initiated Issuance Public Notice Issue Effective Expiration 12/06/10 09/07/10 12/06/10 12/06/10 12/05/20 Regulated Activities Requested/Received Events State Stormwater - HD - Detention Pond Deed restriction requested Additional information requested 09/28/10 Additional Information received 10/25/10 Deed restriction received 11 /15/10 Additional information received 11/15/10 Additional information requested 11/22/10 Additional information received 11/29/10 Outfall NULL Waterbody Name Stream Index Number Current Class Subbasm Wet Detention Pond Colonial Landing ohn E. Prevette 111, I nc 4905 3 Goden and Associates 1745 Cypress Lakes Rd \\\�\ES 4 Hope Mills, NC 28348 = SEAL r 910-223-7766 = L-4905 jprevette@goodensurveying.com %�y'•l9,1, \(Ar4" Method Find permanent pool dimensions: o Iterate dimensions to achieve an acceptable surface area to drainage area ratio Find temporary pool dimensions: o Calculate the required treatment volume o Find the max stage needed to store the treatment volume Find temporary pool draw -down orifice dimensions o Iterate orifice dimensions to find an acceptable flow that will drain the treatment volume in 2-5 days o The orifice flow must not exceed the pre -development 1-yr 24-hr peak flow Overflow considerations: o Set the overflow riser at the temporary pool max stage o Route the 10-year 24-hour storm and find the max stage o Set the emergency overflow weir at the 10-year 24-hour max stage o Route the 100-year 24-hour storm and find the max stage o Set the top of the pond 1 foot above the 100-year 24-hour max stage NOTE: All formulas are taken from the Stormwater BMP Manual Given Drainage Area (DA) = 350,000 sf Proposed Impervious Percentage (f) = 25.5% Top Pond: 266.6' (Z=7.1) Temp Pool: 265.2' (Z=5.7) V Perm Pool: 264.0' (Z= Nov 10, 2010 Emgcy Overflow Weir 265.3' (Z=5.8). (Excavate 1' below Pond Bottom for Sediment Accumulation) Page 1 of 5 Project # 09219 Gooden & Associates, Inc. Permanent Pool Pollutant removal within a wet detention basin is dependent upon two integral components — average depth and permanent pool surface area. Average depth, is a function of the surface area, total depth, and side slopes, and must be recalculated if any of these dimensions change. Surface area is directly proportional to the total drainage area by a factor found in a table in the Manual. To find the factor in the table, the designer must know the percent of impervious area and the average depth of the permanent pool. Because surface area is dependent upon average depth, and average depth is dependent upon surface area, it becomes necessary to iterate pond dimensions to find an acceptable solution. This is an easy task with a spreadsheet. As shown in [Table 1] we have chosen a 60' by 80' Main Pond and a 25' by 45' forebay. The spreadsheet calculates incremental contour areas and prismoidal volumes as well as stage and storage values. Using the formula found on page 10-13 of the Manual: Permanent Pool Area (PP) = 5,925 sf Bottom of Shelf Area (BS) = 4,489 sf Bottom of Pond Area (BP) = 1,385 sf Depth from Bottom to Shelf (Depth)= 4.0 feet Average Depth da = [0.25 x (1 + PP)l + [lasZerl x (° s`hlJ = 3.1 feet We use Table 10-3 in the Manual to find the ratio of surface area to drainage area. Table 10-3 is fed into the spreadsheet and a ratio is found by double interpolation. Using 3.1 feet as the average depth and 25.5% as the impervious percentage, a SA/DA ratio of 1.69% is calculated. The drainage area is 350,000 so the surface area must be at least 5,919 sf. The surface area of 5,925 sf shown in [Table 1] is acceptable. Regulations also require that the forebay contain 20% of the permanent pool volume. The chosen dimensions yield a forebay storage of 2,675 cf and a total permanent pool storage of 13,731 cf. The forebay contains 19.5% of the total volume and is sized appropriately. Temporary Pool Regulations require treatment of the first one inch of rainfall because most common surface pollutants are carried away within this volume. Surface characteristics reduce the treatment depth from one inch of rainfall to some particular depth of runoff. This runoff depth — not the rainfall depth — is then applied to the Drainage Area to compute the runoff volume that must be treated. Using the Simple formula described in Chapter 3 of the Manual: Impervious Percentage (1)= 0.255 Drainage Area (DA) = 350,000 sf Treatment Depth (dT) = (0.05+0.9x I)x 1 inch rainfall = 0.44 in Treatment Volume (WQV) = (dT x DA)12 - 81152 cf [Table 1 ] shows that at stage 5.7, the temporary pool will hold the required treatment volume. Nov 10, 2010 Page 2 of 5 Project # 09219 Gooden & Associates, Inc. Top Pond Temp Pool Top Shelf Perm Pool Btm Shelf Btm Pond [Table 11- Pool Dimensions p dcGocC LJ 2cL0c LU >lU LJ >A LQm f0 . Nm(I OLL O> !0L 7.1 82.6 102.6 35.4 55.4 10436 10808 18d9 5.7 74.2 94.2 29.8 49.8 8474 4649977 5.0 70 90 27 47 7569 2767598417096365 4.5 60 80 25 45 5925 2066 528 13731 4 50 70 23 43 4489 8989 2147 11136 0 26 46 7 27 1385 0 0 0 Draw -Down Orifice In order to maximize filtration, the temporary pool volume should be completely drained no sooner than 2 days. However, maximum inundation should be limited to 5 days to reduce adverse affects on plant life and to free up space for the next storm. Regulations also require that the treatment volume be discharged at a rate equal to or less than the 1-yr 24-hr pre -development peak discharge rate. Using the Orifice Equation to determine draw -down time: Coefficient of Discharge (C) = 0.60 Orifice Diameter (D) = 1 inch (0.083 feet) - (2652-264.0) = 0.6 ft Average driving head (ha g) - 2 (rzD4) j 0.0832� Q=Cx 4 x 2x32.2xh(rz x a„9=0.bx 4 x 2x32.2x0.6=0.020ft3/sec WQV 1 8152 1 Drawdown Time =- x = x = 4.7 days Q 60x60x24 0.020 60x60x24 Using the Rational Method to determine the pre -development flow: Pre -development C = 0.20 Post -development C = 0.35 Drainage Area = 8.03 acres 1-yr 24-hr storm intensity = 0.13 in/hr Pre -development Q = C i A = 0.20 x 0.13 x 8.03 = 0.21 ft3/sec Post -development Q = C i A = 0.35 x 0.13 x 8.03 = 0.36 ft3/sec Flow through the orifice is 0.02 ft3/sec, which is less than the pre -development flow. The orifice will draw -down the temporary pool volume in 4.7 days, which is meets the 2-5 day restriction. Therefore, a 1 inch orifice is adequate for this pond. Nov 10, 2010 Page 3 of 5 Project # 09219 Gooden 8 Associates, Inc. Overflow Considerations To find appropriate overflow structures, storms with volumes larger than the treatment volume must be routed through the pond. To accomplish this, we utilize the SCS TR-20 Method with a Type III storm. 10-year 24-hour Storm 10-year 24-hour depth = 5.52 inches Drainage Area = 350,000 sf Tc = 15 minutes [Figure 2] shows the 10-year 24-hour hydrographs and [Figure 41 shows depths in the pond. The inflow hydrograph represents the unrouted storm. The outflow hydrograph is generated from two outflow structures. A 4'x4' brick riser is set at the top of the temporary pool elevation (5.7') and a 4" draw -down orifice is set at the top of the permanent pool elevation (4.5'). Routing the storm produces the following results: Max Inflow = 6.9 cfs Max Outflow = 1.3 cfs Max Stage = 5.8 ft 100-year 24-hour Storm 100-year 24-hour depth = 5.52 inches Drainage Area = 350,000 sf Tc = 15 minutes [Figure 3] shows the 100-year 24-hour hydrographs and [Figure 4] shows depths in the pond. The outflow hydrograph is generated from the riser, the draw -down orifice, and a weir with a length of 10'. The overflow weir is at the maximum stage of the 10-year 24-hour storm (5.8'). Routing the storm produces the following results: Max Inflow = 21.1 cfs Max Outflow = 19.7 cfs Max Stage = 6.1 ft With 1 foot freeboard, the top of the pond will be at stage 7.1 feet. Nov 10, 2010 Page 4 of 5 Project # 09219 Gooden & Associates, Inc. 8 7 6 v, 5 w u 3 4 LL 3 2 1 0 9 25 20 ; 15 u 3 LL° 10 5 0 9 6.5 6.0 x 5.5 a M N 5.0 4.5 4.0 9 Nov 10, 2010 [Figure 2] - 10yr 24hr Hydrographs 10 11 12 13 14 15 16 Time (hours) [Figure 3] - 100yr 24hr Hydrographs 10 11 12 13 14 15 16 Time (hours) [Figure 4] - Depths 10 11 12 13 14 15 16 Time (hours) —10y Inflow —10y outflow —100y Inflow —100y Outflow — 10y Depth — 100y Depth Page 5 of 5 Project # 09219 Gooden & Associates, Inc. CLIFDALE QUADRANGLE 0� UNITED STATES UNITED STATES N.AT. CeN. INe J DEPARTMENT OF THE INTERIOR DEPARTMENT O P TXE ARNT ?6 MINUTE SERIES 1OPOONS1N111 GEOIAGICAL SURVEY _ CORPS OF ENGINEERS q TIO w.euro.0 �� w.o..n 19� n aAm S I E 51