6301 36TH AVE W 2016-01-01 MF Import

114005E8'^Street P425�453A501 Suite345 F425�453�8208 � � � 1 li Bellevue.WA98004 WWW.PACIAND.COM o - e �1 i Memorandum Sediment Pund Sizing A temporary sediment pond will be required since the proposed site requi�es significant grading and retaining wal! construction. The detention/wet pond cannot be utilized as a temporary sediment pond for the initial clearing and grading activities. A proposed sediment pond will be constructed in the east portion of the site, in the Phase 2 development area. The temporary sediment pond was sized according to Vo(ume !1-Construction Stormwoter Pol(ution Prevention of the 2010 City o( Everett Stormwater Managem�nt Manual. Section 3.3. 108MP C241: Temporary Sediment Pond The 10-year, �4-hour peak discharge event was used for the sediment pond sizing based or� post development site conditions. This flow rate (Q,o) is taken from the Intermec — Cot 2 Preliminary Storm Drainage Ana(ysis, dated January 13, 2009 (REVISED February 74, 2010) The required surface area at the top of the riser is equal to 2,083 square feet per cubic feet per second of peak inflow: � (SA) = Q,ox 2,083 = (1b.34 cfs x 2,083) = 34,036 square feet The length to width ration is 3:1 with approximate dimensions of 110' x 3�5'. The sediment pond will have a 4-foot overall depth from top of riser to bottom of pond, as well as 1 foot of freeboard above the riser. The emergency overflow is set rit 1 foot above the riser. The riser has been sized per Figure 3.5 of the aforementioned section of the manual to be 27- inch diameter pipe. The dewatering orifice has been sized per the following equation with "Ao equal to the surface area of the orifice in square inches and "h" is the head of water above orifice (height of riser): Ao = 1.44(h)05 per cfs of inflow = (1.44)(4)°5 = 2.88 square inches Do(orifice diameter) _ (1.13)( Ao)°S = (1.13)(2.88)05 = 1.9 inches 2