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Home My WebLink AboutSW8050429_COMPLIANCE_20050601STORMWATER DIVISION CODING SHEET POST -CONSTRUCTION PERMITS PERMIT NO. SW DOC TYPE ❑ CURRENT PERMIT ❑ APPROVED PLANS ❑ HISTORICAL FILE COMPLIANCE EVALUATION INSPECTION DOC DATE �DIQO( YYYYMMDD DETENTION POND ANALYSIS FILE NAME: G:\DATA\WPDATA\WQS\POND1050429.WK1 PROJECT#: SW8 050429 REVIEWER: P.Bartlett PROJECT NAME: Advance Auto L DATE: 01-Jun-05 0A Receiving Stream: 816 e Creek Class: SC; NSW0A Drainage Basin: White Oak Index No. 19-8 Site Area 2.39 acres Drainage Area square feet Area = 1.34 acres IMPERVIOUS AREAS Rational C bldgs 6791.00 square feet 0.95 parking/driveways 25967.00 square feet 0.95 sidewalks 539.00 square feet 0.95 square feet square feet square feet TOTAL jjjjj!jjjjjj33297�1; square feet Rational Cc= 0.63 SURFACE AREA CALCULATION % IMPERVIOUS 56.92% SA/DA Ratio 4.99% Des. Depth 4 Req. SA 2920 sf TSS: 90 Prov. SA VOLUME CALCULATION place a "1" in the box if Rational is used Rational ?* FOREBAY Des. Storm inches Perm. Pool Volume= 4430 Rv= 0.56 Req. Forebay Volume= 886 Bottom 22 msl Provided Volume= 942.00 Perm. Pool msl Percent= Bala Design Pool s 26.89 msl Design SA 3746 sf ✓ Req. Volume 41: cf SA @ Prov. Volume =fl cf Elevation= ORIFICE CALCULATION Avg. Head = 0.43 ft Q2 Area = 0.72 sq. in. Flow Q2, cfs 0.016 cfs Q5 Area = 0.29 sq. in. Flow Q5, cfs 0,006 cfs Orifice Area 0.44 sq. in. No. of Orifices Q= 0.010 cfs Diameter, inches weir H x W in x in Drawdown = 3�� days COMMENTS Sur -face Area , Volume and Orifice are within Design Guideline: RECEIVED MAY 2 S 2005 BY: 1. PROJECT DESCRIPTION Design Calculations Stormwater Treatment Facilities for Advance Auto Pond located in Onslow County (r&VtsEa 5/W/6 } The Project consists of proposed business on a 2.39 acre tract and associated sitework. 2. COMPUTE POND AREA REQUIRED a. Compute Percent Impervious: Impervious Areas: On -Site Buildings On -Site Streets On -Site Parking On -Site Sidewalks Other on -site Off -site Sub -Total FUTURE FUTURE(reserved) Total Total Area Drainage to Pond: Percent Impervious = b. Compute Pond Surface Area 0 0 0 SF 0 33,297 33,297 SF 0.569 or 56,9 % ... 58,498 SF (�dcla t°si'Z+Ar1 Using Surface Area to Drainage Area Ratio Chart for 90% TSS Removal (copy attached) and assuming foot pond depth: SAIDA (interpolated) Therefore: Surface Area = SAIDA x Drainage Area 2,920 SF say[:k#i SF I r c. Compute Minimum Pond Dimensions Assume rectangular shape with 3:1 Length (L) to Width (W) Ratio: Length = 95.50 feet Width = 31.83 feet Site conditions, however, dictate irregular shaped pond: Hydr. Length = -_> `<`- >> (;£10w:' feet so, Hydr. Width 30.0 feet (avg) Actual L/W = 3.4 (irregular shape) 3. DETERMINE VOLUME TO BE CONTROLLED FROM 11" STORM Using Schuler "Simple Method": Rv = 0.05 } 0.009(I) where I = percent impervious Rv = 0.562 in/in Control Volume = Design Rainfall x Rv x Drainage Area Control Volume = 2,741 CF say =<<<< CF 4. DETERMINE REQUIRED STORAGE DEPTH Assume: 3:1 side slopes solving for depth (d) by trial $ error: d = 0.89 5. DETERMINE ORIFICE SIZE Control Volume must be drawn down over a 2 day - 5 day period. Using orifice equation, solve for Diameter (D) that results in 3-112 day period: D = (Q 10.026 (h)")" Where: Q = (Control Volume 1 3.5 days x 86,400 sec/day) 0.010 cfs h= d12 = 0.45 Solving for D: D = 0.76 Use...3a" pipe Actual Drawdown with actual drawdown pipe diameter (using orifice equation): Q= 0.010 cfs Draw Down Period = 3.56 days OK, c 5.0 6. SELECT OUTLET STRUCTURE DIAMETER Size outlet structure such that depth of Q10 flow <1.0 feet: Using Rational Formula (Q=CIA) where: Cimper = Cpery Ccum = 0.54 Q10 = 5.21 cfs Assuming 112 circumference is used as weir and using Francis Formula (Q=3.33LH"6) to solve for L such that H = 1.0: L = 1.57 Feet Min 24 inch diameter outlet structure Use 36 inch diameter outlet structure 7. DETERMINE Q10 STAGE Q10 = 5.21 cfs Using Francis Formula (Q=3.33LH"5) solve for H: H 10 = 0.48 feet 8. DETERMINE Q100 STAGE (ELEV. OF EMERGENCY SPILLWAY) Using Rational Formula (Q=CIA) where: Ccum = 0.54 ........................... 100 = <'><0inlhr Q100 = 6AB Using Francis Formula (Q=3.33LH"5) solve for H: H 100 = 0.54 feet 9. DETERMINE Q100 STAGE OVER EMERGENCY SPILLWAY (TOP BERM ELEV.) Assume primary spillway blocked (i.e., emergency spillway to accommodate Q100) Emergency spillway shall be a broad crested earthern weir designed to pass 100 year storm. Using broad crested weir equation H = (Q/Cw x L)zra where: Q = Q100 6.18 cfs Cw = 3.0 Length = ................ > 20.. feet H = 0.22 feet 10. FOREBAY SIZING Compute Actual Pond Volumes (using average end method): Avg Incr. Incr. Cum. Main Bay V u e Elevation Area SF Area (SF) Depth (ET] VolC VolC Pond Bottom Bottom Veg Shelf 243 650< 344 2.33 800 800 Normal Pond 26 QD 2 a�fi9. 1,610 1.67 2,688 3,488 Main Bay Volume = 3,488 CIF Forebay Volumg Elevation Area SF Area SF Depth (FT) VolC Vol CF Pond Bottom 22.00 >: ' -« ........................... .......................... Normal Pond 26.00z<<zi>>47:<( 236 4.00 942 942 Forebay Volume = 942 CIF Total Pond Volume = 4,430 CIF Forebay Volume equates to 21.3 % of Total Volume Temp. Storage Volume. • Elevatipn Area (SF Area SF Depth (FT) Vol (CF Vol CF Normal Pond 26.00 3,040 Temp. Pond Level 26 89 3,746 3,393 0.89 3,020 3,020 Temp. Pool Volume = 3,020 CIF 10 Yr Storage Volume: Elevation Area S Area SF Depth FT Vol (CT1 VolCF Temp Pond Level 26.89 3,746 Temp. Pond Level 27 37 ! 145' 3,946 0.48 1,893 1,893 Temp. Pool Volume = 1,893 CF 100 Yr Storage Volume : Elevation Area SF Area SF Depth (FT) Vol CF Vol (CFj Temp Pond Level 26.89 3,746 Temp. Pond Level 27.3 187j 3,967 0.54 2,131 2,131 Temp. Pool Volume = 2,131 CF 11. SUMMARY Pond Elevations: Top Berm (minimum) H= 0.22 100-Yr. Flood H100 = 0.54 10-Yr. Flood Hf0 = 0.48 1-inch Storage storage depth = 0.89 Permanent Pool pond depth = 4 Pond Bottom 12. WET DETENTION BASIN SUPPLEMENT Basin Bottom Elevation 22.00 Perm. Pool Elevation 26.00 Temp. Pool Elevation 26.89 Perm Pool Surface Area 3,040 SF Drainage Area 1.34 AC Impervious Area 0.76 AC Perm. Pool Volume 4,430 CF Temp Pool Volume 3,020 CF Forebay Volume 942 CF SA/DA 4.99199 Diameter of Orifice 0.75 Design Rainfall 1" Design TSS Removal 90% Relative Actual 5.65 27.65 5.43 27.43 (emerg. weir elev) 5.37 27.37 4.89 26.89 (primary weir elev) 4.00 26.00 (invert out elev) Stormwater Treatment & Erosion Control Narrative Advance Auto Advance Auto proposes to construct a commercial (retail) development on a 2.39 acre lot located on NC 24/US 258 (Richlands Hwy), at the intersection with NC 53, in the Jacksonville Township, Onslow County. The site is currently undeveloped. The project site is located approximately 0.11 miles from an UT of Blue Creek (SC;NSW) in the White Oak Basin_ The proposed development activities are high density (percent impervious > 25 %); therefore, stormwater treatment facilities are required. The attached Stormwater Plan includes a 3,040 sf wet detention basin at the eastern side of the site. The pond is sized to accommodate 35,000 square feet of impervious area allocated as follows: Site Offsite Total Buildings 6,791 sf -0- 6,791 sf Streets -0- -0- -0- Parking 25,967 sf -0- 25,967 sf Sidewalks 539 sf -0- 539 sf Other (Dump. Pad) -0- -0- -0- —Reserve-for-Future- -0- 83-sf L 32 Z- 1� t The proposed pond is sized for 90% TSS removal. The pond does not incorporate pretreatment, other than the forebay. The pond does not include a vegetated filter at the outlet. All stormwater emanating from new impervious surfaces that is generated on -site shall be collected and directed to the rear western portion of the site by a system of sheet drainage, grassed swales and storm drainage pipes. The treatment system will require periodic maintenance_ Maintenance frequency shall be dictated by pond depths. Pond depths shall be checked by the owner on a quarterly basis. When measured depths are less than 3.00', the pond shall be manually dewatered (pump) and accumulated sediment removed and properly disposed of. Site topography is gently sloping (0 — 5%) with the lowest elevations towards the notheriy boundary of the tract. Soils are loamy sand/sandy loam. The entire area designated within "Clearing Limits" will be disturbed during initial construction, and will include grading for parking, swales and storm drainage. During construction, all runoff will be diverted by a series of temporary diversion ditches and/or storm drain pipes to the Treatment Pond. The treatment pond is of adequate size and volume to contain sediment from the entire disturbed area during construction. The pond will be dewatered and accumulated sediment removed when the site is adequately stabilized.