2320 RAILWAY AVE INTERWEST CONSTRUCTION INC 2023-06-12

INTER WEST — EVERETT CONTRACTOR'S YARD INTERIM FILL PROJECT Sitts & Hill Engineers, Inc. CONSTRUCTION STORMWATER POLLUTION PREVENTION PLAN Job Number 17,733 Section 10 — Engineering Calculations Sediment Pond The sediment pond has been sized in accordance with BMP C241 (see Appendix A). The project site ' contains 11.44 acres. A WWHM model was run assuming 100% impervious coverage for the purposes of pond design. A summary of the resulting runoff flow rates is provided below (the "mitigated" event represents the filled site): Flow Frequency Flow(afs)— Prede%reloped Mitigated 2 Year 0.2452 4.8575 5 Year 0.3630 6.5720 10 Year = 0.4436 7.8257 ' 25 Year = 0.5476 9.5501 50 Year 0.6262 10.9400 100 Year = 0.7057 12.4232 The required surface area at the top of the riser was then calculated as follows (where SA is required surface area and Qz is the 2-year runoff discharge): 4.8575 SA=2x Qz =2x—10,120SF 0.00096 0.00096 The primary spillway (riser) must pass the 10-year event with a head of 1 foot. An existing Type 1 catch basin is proposed as the pond's primary spillway. Adequacy is verified using the nomograph included on the following page (the orifice area available on a type 1 catch basin is greater than that available on an 18-inch diameter riser). 1 The emergency overflow spillwayis sized to ass the 100- ear storm event at a flow depth of 0.64 feet. p Y P This is calculated in the following equation (where Qloo is the 100-year runoff discharge, H is the flow depth, and L is the length of the spillway): Qioo 12.4232 L= 15-2.4H= 15-2.4x0.64=6.02ft,use 6ft 3.21H 3.21 x 0.64 ' The dewatering orifice is sized using the following equations (where k is the orifice area in square feet, D is the orifice diameter in inches, A, is the pond surface area in square feet, h is the height of the riser in ' feet, T is 24 hours, and g is the acceleration of gravity). AS(2h)o.s 10,120(2 x 3.5)o.s AO 0.6 x 3600Tg0-5 0.6 x 3600 x 24 x 32.20-5 — 0.091 SF D = 13.54 Ao = 13.54 0.091 = 4.08 inches, use 4 inches SI1TS & HILL ENGINEERS, INC. 22 TACOMA, WASHINGTON