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HomeMy WebLinkAboutSDP201000086 Calculations 2010-12-15COLLINS 200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902 434.293.3719 PH 434.293.2813 FX www.collins- engineering.com HEC -22 ANALYSIS OF ON -GRADE GRATE INLETS Page 1 of 9 December 15, 2010 HEC - 22 ANALYSIS OF ON - GRADE GRATE INLETS The Lockwood Townhouses at Hollymead on -grade DI - storm inlets were analyzed per the HEC -22 Urban Drainage Design Manual requirements of Section 4 Pavement Drainage. The alleyway inverted section was analyzed as a shallow swale section with S determined by the following equation: S = S Sx2 x S x1xl x. A value of 0.02 was used for Sxi and S the resulting S value was 0.01. A Manning's n value of 0.013 was selected for the smooth asphalt alleyway. The velocities of the channel were analyzed as a v- shaped channel in the Flowmaster program. A side slope of 0.02, a Manning's value of 0.013, the channel slope (alley slope at the inlet), and the total flow in cfs to the inlet were used to calculate velocities. With these inputs, the interception capacity of the inlets on- grade could be calculated. The proposed development uses T -D1 -7 grate inlets, which were found to be most similar to the P -1 -1/8 (P -30) inlets tested and modeled by the Federal Highway Administration based on shape of bars, orientation of bars, and size of grate. The P-1-1/8 inlet was used in this analysis. Section 4.4.3.3. describes the fundamentals of interception capacity of a grate inlet on- grade: At low velocities, all of the water flowing in the section of gutter occupied by the grate, called frontal flow, is intercepted by grate inlets. Only a small portion of the flow outside of the grate, termed side flow, is intercepted. When longitudinal slope is increased, water begins to skip or splash over the grate at velocities dependent on the grate configuration." Also, the capacity and efficiency of grates increase with increased slope and velocity if splash -over does not occur. This is because frontal flow increases with increased velocity, and all frontal flow will be intercepted if splash -over does not occur." The procedure outlined in section 4.4.4 was followed in the analysis of the intersection capacity of the Lockwood Townhouses inlets on- grade. The total spread of water, T, was solved for using Appendix 9C -1 Chart — Flow in Triangular Gutter Sections. The T spread value computed for each on -grade grate inlet was found to be within the allowable spread value of 10' for the 4in /hr storm. The ratio of frontal flow R was solved for using Chart 5B with a length, L, of 3' -4" (based on the T -DI -7 grate geometry). The R value for each on -grade grate inlet was found to be 1.0 because velocities proved to be less than the splash over velocity for the P -1 -1/8 grate with the given length. This meant that all inlets at the 4 in /hr and 6.5 in /hr storm intercepted all frontal flow. This is consistent with the statement in section 4.4.4 that, When the velocity approaching Page 2 of 9 the grate is less than the "splash- over "" velocity, the grate will intercept essentially all of the frontal flow." The ratio of side flow intercepted to total side flow, R was solved for using Chart 6B using the length, L, of 3' -4 ", the velocity from Flowmaster, and the S, of 0.01. The R, values varied from inlet to inlet, but were higher in the 4 in /hr storm than in the 6.5 in /hr storm. This R value accounts for the overflow that is carried from one inlet to the next. These values were always less than 1 cfs, in both the 4 in /hr and 6.5 in /hr storms. The frontal flow to total gutter flow ratio, E was determined using Chart 2B with a S,, /S, value of 1, a width of 2' -11 '' /z' ", the T value calculated previously, and W/T calculated from these terms. The efficiency, E, of the grate was calculated per equation 4 -20: E = R E. R, (1 E.) The E values calculated were found to range from 0.83 to 0.66 and were higher in the 4 in /hr storm than in the 6.5 in /hr storm. These values were multiplied by the Q to the inlet to determine Q intercepted and Q bypass. The Q bypass flows were added to the downstream inlet flow and summed for analysis of the downstream inlet. Calculations for the on -grade grate inlets at 4 in /hr and 6.5 in /hr follow this narrative. Page 3 of 9 STR 12 — 4 IN/HR ANALYSIS --4 5Th 5Th 12_ C 4 "Vhf ° u i G S = 6 . 3l q a. o cis jX - 0. 0 t-- - td 0 115c,- A . RC_. t 4 .= -- T ( -) for V 51101-<- v5c n - Cha 513 5x= 5x, L _ s--141 a . 5 CeS tea. •.o C lowrvlAs .r w, V Channel Q 0.50 cis char, lei S= 0.63ti, r, 0 (sp l asth $ 3 s t blam 1126 0.ot, L 3' -L C a i5V0 IZ6 E k t ---E W 2 -1112` Tr - laPt W 0•q3, G = G (0.1z) (o. ar5' t -o, rc) Page 4 of 9 STR 8 — 4 IN/HR ANALYSIS 3 i R iS (ca 4 ;r' /hr ? D tx — 1 c - v, 03C> O.ot Aft. ge _t Ch + 5L l,= 3 -y 1, /Fi cwwttsi-er w/ V a .5t f 5 `l chatret , 0.02 5,64. _. 1.0 St> e%, 0,ot 3 n ( (SptcS 1 Fi o c c C, o l 1 3 , 1 1 l v s1ft6 s =0,!t Cs, C heAA-k Pe W= 2 1 - 11,2', 3 3 1 _1 E O- R S (1 - Eo) 1 (O.tor4 4 b.1%(t- U.(i +) 5,13 0; - t d x 0:33 - \O. - 43 cF Page 5 of 9 STR 6 — 4 IN/HR ANALYSIS 3 R "Yti r - p (I= 0 it-l °gs ( 6.a - 4 cfs crto S = 15.030(s) 0.ot U3r1 AN `9C-t i ! Chang 5 3 (F buxnasi L- 3' -4" Ch net v = a.(4 fps sk_v 5t n= 0,03 t. s tash vet = 6.3 t - ' Ch (r B L : 3' -y ('?S t"- (6, cf40- CWA+ a w= 22 -I tit" T =`t' = U.3 ?,, = Eon O,(o' v b Q t - t 0,19-0 - Q; -GCE = 09yxOtoq =EQ (s5c{S ScR Lij Page 6 of 9 STR 12 - 6.5 IN/HR ANALYSIS O.$1 O. 0311 ©.01 Aff `tc -t Chcv4 --5 y 2 - '42 k'p5 CPtou,Y,n4.stC.c ) 4-: c .a Chap+ es = LA) : - 2' - il` /2" T . %.t , '1r 0 3'4- E 0.91 t ; (o.1+") + b. lel C a;= QE - a.kix o.4 - 4 =0.62 eCs-1 ( STC l Page 7 of 9 STR 8 — 6.5 IN/HR ANALYSIS Lc, j "%I C 1. (-7 S 0.03U (k At'? 9C -( T` C'- s `t cps C{,ts\a,v,aS c.( C hi+ a Eo =O.5 C t..o o %)4- o.,i -O, QE (. (s2 (0,C1.4o (. off -C_IS ( f) Page 8 of 9 STR 6 - 6.5 IN/HR ANALYSIS 5T @ U , 5 V Yl A'l f' Q 1. (B' Cfs ( • 55 V b cr) - O. 3oce S.;R 1 x=o. Chal+ Sts - 1 U 5 C Ps Cfotomc:.s () Cha,4 (a 0. c;u — l t /2" T= I1 came, . 0542 +off STR 4 _ Page 9 of 9