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Home My WebLink AboutSW8051123_COMPLIANCE_20060126STORMWATER DIVISION CODING SHEET POST -CONSTRUCTION PERMITS PERMIT NO. SW �;kS�/�3 DOC TYPE ❑ CURRENT PERMIT ❑ APPROVED PLANS ❑ HISTORICAL FILE COMPLIANCE EVALUATION INSPECTION DOC DATE YYYYMMDD WET DETENTION POND ANALYSIS FILE NAME PROJECT # SW8 051123 PROJECT NAME Fox Hollow Apartments S IWQSIPOND1051123 WK1 REVIEWER L Lewis DATE 26-Jan-06 Receiving Stream New River Class C NSW Drainage Basin White Oak Index No WOK02 19-1 Site Area 5 61 acres Drainage Area I I �244371 00 Isquare feet Area in Acres 561 IMPERVIOUS AREAS Buildings Parking SW TOTAL SURFACE AREA CALCULATION % IMPERVIOUS 46 800lc Des Depth 55 TSS 90 VOLUME CALCULATION 4734000 square feet 4028000 square feet 2673400 square feet square feet square feet square feet square feet Ali 11!11,14354 00,I square feet Rational ?* Des Storm 1 00 inches Rv= 0 471 Bottom 33 msl Perm Pool 11H!,l� kl 13 50'' msl Design Pool 39 63 msl Storage met @elev 3962 msl Req Volume hl l'I;bI,N 9595 cf Vol Prov at DP ililllhlllill 1161 9652cf ORIFICE CALCULATION Avg Head = 054 ft Flow Q2, cfs 0 056 cfs Flow Q5, cfs 0 022 cfs No of Orifices Diameter, inches I`I i'I I'i I' 126, weir H x W Drawdown = 1 1'11'IJldllll l'I'b! ' 0 7 days C 3 30919697 Rational C Rational Cc= Oil SAIDA Ratio 2 91 % Req SA 7106 sf Prov SA 76361'1 sf * place a "1" in the box if Rational is used FOREBAY Perm Pool Volume= 19037 Req Forebay Volume= 38074 Provided Volume= 410300 Percent= F7 216%1 Elevation 3850 3963 Area 763500 944900 Q2 Area = 2 26 Q5 Area = 090 Orifice Area 1 23 Q= 0 030 Ilililiril 1,11,111,111fjj in x Surface Area @DP = Falling Head Drawdown = L= -0 108 COMMENTS Surface Area , Volume and Orifice are within Design Guidelines Inc Vol 965246 Ace Vol 000 965246 sq inches sq inches sq inches cfs 6'I'113fiE`;i in 9449 sq ft 5 1 days R �IVED 0 JAN 17 2006 DWQ ,ROJ # S iIDS Q Z3 1 PROJECT DESCRIPTION Design Calculations Stormwater Treatment Facilities for FOX HOLLOW APARTMENTS located in Onslow County T eVt'47 J U j f -10 6 This project consist of a 5 99 ac tract for apartments for the elderly 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(reserved) Total Total Area Drainage to Pond Percent Impervious = b Compute Pond Surface Area 1,%III pit, o� O ssr :I 181 8 V 4 r �lrz�a� Existing Proposed TQt 47,340 SF 0 SF •f G+v G+jv� A+G t+4 G G GG G i�i ��I1+w 40,280 SF AG}}!1f } +G Ti �♦ v,,tt v`rk4' 26,734 SF Y+{JG }�'CJ •��+¢ O {y �G vi 4 GGui G iS�}Y'IV kh {J...J 1.1 0 114,354 114,354 SF V vy QSF �. w4. G wv GL�'v. 4v}r'vL.Ji"S 0 114,354 114,354 SF 0 468 or 46 8 % %:° g �1; SF Using Surface Area to Drainage Area Ratio Chart for 90% TSS Removal (copy attached) and assuming°�,}a t} V v f r Y }nv`H sw,v;foot pond depth SA/DA Yh<An f,�z% (interpolated) Therefore Surface Area = SA/DA x Drainage Area - 7,106 SF say s v;; Tx� SF c Compute Minimum Pond Dimensions Assume rectangular shape with 3 1 Length (L) to Width (W) Ratio Length = 151 34 feet Width = 50 45 feet Site conditions, however, dictate irregular shaped pond Hydr Length A$A feet so, Hydr Width 53 0 feet (avg) Actual L/W = 3 5 (irregular shape) 3 DETERMINE VOLUME TO BE CONTROLLED FROM 1" STORM (NCDENR REQUIREMENT) Using Schuler "Simple Method" Rv = 0 05 + 0 009(I) where I = percent impervious Rv = 0 471 in/in Control Volume = Design Rainfall x Rv x Drainage Area Control Volume = 9,595 CF say n' " ;`t CF 4 DETERMINE VOLUME TO BE CONTROLLED FROM 25 YR STORM (CITY REQUIREMENT) Assume 25 Yr-1 hour Storm = 2 95 in/hr Using Schuler "Simple Method" Rv = 0 05 + 0 009(I) where I = percent impervious Rv = 0 471 in/in Control Volume = Design Rainfall x Rv x Drainage Area Control Volume = 28,305 CF say 5 DETERMINE REQUIRED STORAGE DEPTH 1" Storm 25 Yr Assume 3 1 side slopes Assume 3 1 side slopes solving for depth (d) by trial & error solving for depth (d) by trial & error d= 093 d= 279 01 6 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 3-1/2 day period D=(Q/0026(h)s)5 Where Q = (Control Volume / 3 5 days x 86,400 sec/day) = 0 032 cfs h= d / 2 = 0 47 Solving for D D= 134 Use ,v+},v� —� ' pipe t 11, F Actual Drawdown with actual drawdown pipe diameter (using orifice equation) Q= 0 028 cfs Draw Down Period = 4 03 days OK, < 5 0 7 SELECT OUTLET STRUCTURE DIAMETER Size outlet structure such that depth of 010 flow <1 0 feet Using Rational Formula (Q=CIA) where )�} '4 Z <S Cperv= Ccum = 053 110 = v` Y �dn n .4d' in/hr A = 5 61 ac Q 10 = 19 47 cfs Assuming 1/2 circumference is used as weir and using Francis Formula (Q=3 33LH' 5) to solve for L such that H = 1 0 L = 5 85 Feet Min 42 inch diameter outlet structure Use 48 inch diameter outlet structure 8 DETERMINE Q10 STAGE Q 10 = 19 47 cfs Using Francis Formula (0=3 331-H' 5) solve for H H 10 = 0 95 feet 9 DETERMINE Q100 STAGE (ELEV OF EMERGENCY SPILLWAY) Using Rational Formula (Q=CIA) where Ccum = 053 1100—��„4��, �,inlhr A = 5 61 ac Q 100 total = 25 27 cfs 0100 net = 25 27 - 10 52 =14 75 cfs Q 100 net= 1475 Using Francis Formula (Q=3 33LH' 5) solve for H H 100 = 0 79 feet 10 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)213 where Q = Q100 14 75 cfs Cw=30 Length = ' „{ ; feet H = 0 30 feet 11 FOREBAY SIZING Compute Actual Pond Volumes (using average end method) Avg Incr Incr Cum Main Bay -YQIume . Elevation Area(SEArea (SF) Depth (FT l Vol (CT? Vol (CFl Pond Bottom 3300 gas Bottom Veg Shelf 36,83 2,902 1,885 383 7,220 7,220 Normal Pond 3850 61337 4,620 167 7,715 14,934 Main Bay Volume = 14,934 CF Forebay Volume. Elevation Area -(SF] Area -(SF) Depth (FT) Vol (CE Vol (CF) Pond Bottom 3300 194 Normal Pond 3850 1.298 746 550 4,103 4,103 Forebay Volume = 4,103 CF Total Pond Volume = 19,037 CF Forebay Volume equates to 21 6 % of Total Volume Temp. Storage Volume . Normal Pond Elevation Area 3850 (SF) Area (SFI Depth (FT l Vol (CF) Vol (CF) 7,635 Temp Pond Level aw 0109E 8,542 1 13 9,652 9,652 Temp Pool Volume = 9,652 CF 25 Yr Storage Volume . Normal Pond Elevation Area 3850 {SF} Area (SF] Depth (FT l Vol (CF) Vol_(CF) 7,635 25 Yr Pond Level 41,32 12,41$ 10,027 282 28,275 28,275 25 Yr Pool Volume = 28,275 CF 100 Yr Storage Volume. 25 Yr Pond Level 100 Yr Pond Level 100 Yr Pool Volume = 12 SUMMARY Pond Elevations Elevation Area (SF) Area (SE Depth (FT] Vol (CF} Vol (CE� 41 32 12,418 4214 15,009 13,758 079 10,898 10,898 39,172 CF Top Berm (minimum) H= 0 30 100-Yr Flood H100 = 0 79 25-Yr Flood H25 = 169 1-inch Storage storage depth = 1 13 Permanent Pool pond depth = 5 5 Pond Bottom Relative Actual 772 42 41 (say 43 0) 742 42 11 (emerg weir elev) 832 41 32 663 39 63 (primary weir elev) 550 38 50 (invert out elev) .v` pvv 13 WET DETENTION BASIN SUPPLEMENT Basin Bottom Elevation 3300 Perm Pool Elevation 3850 Temp Pool Elevation 3963 Perm Pool Surface Area 7,635 SF Drainage Area 5 61 AC Impervious Area 2 63 AC Perm Pool Volume 19,037 CF Temp Pool Volume 9,652 CIF Forebay Volume 4,103 CIF SA/DA 2 908 Diameter of Orifice 1 25 Design Rainfall 1" Design TSS Removal 90%