7805 40TH AVE W 2018 Plans 2022-11-30 (2)

'1-7(1__-74).-- ---'\ 1'S'ZTRFAArAN i l'CFI-1 Fig?1i t, 7 V, .S! ta'1;:i'%s t•il I.,r'Syr s 2'J1.'�oc.j 1' 7 '.'i.1.1,:i ).f.:1 F,•'lii.l.'i'r n:t .. t.t. 1+' "� i ''1 howl u.,4u:, iT a `'r 11.0:„f sired Iarsr �v�lotas lrcaa \s I:'. ��.£�Gt 11r:t 0 1-1 \0,!.i c rt. Sj;. Ity ;1.1 ',.ra 13.): ; z l rto:': U l;ti lilt l 1 tir1 4441144 .. 1 !r � M ,,,Ac:�, Fling cSEkilall 11t',2 A iLs 1 S;Fs 3 +i ;Fw �k „= . `X ? Fusin to st alt`;?_ IOW AV \1' =U.t1,Act_.i 1.588 SF do ' 1 1 .rb H t..>t.,WI t p. t '< 3 I tic,1 1 11, ,',c�a, .�0.3- Ail ri. 1 J.?_3!ti) r V. TZ .� �v� „II:,t +t' ry, ,' �u+ .",�" ''w`;""` �14 VI ft,‘ .5003 c.c.i:y.v..111 !"3 r,7 rsi , 0 ,1.>t E.l'.' .;t ,�.' Z .nfi.MS � •a b��t 1T 4F 1i�t� .`hF4 t t��v ,1HiiF w-- 1X I i 'C` -`' 1 4 w t 1 .at jam°1.,A'. a s Observations on-site during rainfall events have shown that a larger amount of upstream stormwater from the existing road surface is collected by CB#2, rather than upstream at catch basin#6, as was designed for in the SSP.These observations are consistent with the overall topography of the existing roadway.The existing roadway has a grade of about 6.4%north and a 2% cross-slope to the east,with the crown at the middle of the roadway.Stormwater flows more quickly down the steeper slope with a lesser amount of water traveling cross slope and being collected in CB#6. As a result a significant amount of water from the existing road would have bypassed CB#6 and be conveyed by CB#2 through the detention/water treatment system, rather than bypass the detention system as designed for and allowed for in Code. The drainage revision attached to this memo is to address the existing observed flow paths described above,to achieve the Code requirement that all new and replacement impervious be detained and treated. For areas that would bypass the new system, equivalent,existing untreated areas can be added to the new system resulting in a net total equaling or surpassing the new area.This is being done as detailed in the following by: - Sending stormwater through the drainage system in a way that more closely matches the analysis and assumptions that went into sizing the detention system to satisfy the design and code requirements,and - Prevent excess water from entering the detention system which would require more water to be treated than originally design or required by code, resulting in poorer overall treatment system performance and long-term maintenance concerns for the owner. Moving CB#7 to the location shown in the curb line and using an open grate puts the effective CB in a location where it can collect the upstream flow which was designed to be collected by CB#6 and is not being collected by this CB. By collecting the road water at CB#7 this collects the water for the bypass system as designed and prevents some of the bypass water from the existing street runoff described