222 W MARINE VIEW DR 2016-01-01 MF Import

Fflf� n�itx�r� tz ;,v December 2, 1994 File No. 24-92-009-007-01 COMPARISON OF C[TY OF EVERETT STANDARD DETENTION VAULT TO CORRUGATED METAL PIPE VAULT Enginrcrr Plonnert Sur.�ryon Reid Afiddle�on 1903133rd Ace. lV. Suite 301 P.O. Box 6638 Lynnwood, WA 93036-0638 Phone 206 775.1/3{ FNc 206 775•3839 City of Everett stormwater regulations beneral!y require the installation of a stormwater setding basin to improve the quality of stormwater prior to dischazge to the environment. Often this basin is converted to an equivalent underground vault to provide additional usable surface area on a particulaz site. Clurent regulations were developed azound a rectangular underground vault At the Sterling Asphalt Batch Plant in Everett, a vault conswcted of ]arge (8 foot diameter) Corrugated Metal Pipe (CMP) has been proposed to substitute fer the permitted underground concrete vault. Existing groundwater conditions in the area of the vault indicate the CMP vault could be significantly less expensive to install. The permitted vault is 20 feet wide, 125 feet long, and holds a permanent sturage of water 7 feet high (no "live" storage is provided in the vault). The substitute CMP vault in made of two 8 foot diameter pipes run in pazallel (half ihe stormwater flows down each pipe). Water height in each pipe is 7 feet, and they aze 170 feet long each. The following examines the equations which aze the basis for City of Everett requiremenu and compazes their application between the permitted rectangular vault and the proposed CMP vault The goveming equavon in the "Depaztment of Ecology Stormwater A' tnagement Manual for the Puget Sound Basin" for presetding basins is called the "Camp-Hazen" equation and is restated below: � E= L— e� °) (pg. III-4-R DOE) &. Equation 8.10 "Stormwater Management" This equation is also found in "Stonnwater T4anagement" by Sanielista and Yousef. The equation is a probability expression for determining the probability of pazticle removal, understanding that a normal in-line presettling basin is subject to small and lazge flow conditions. This equation is not stricdy applicable ro the Sterling Asphalt installaticn, since high flo�v volumes (in excess of the 6-month, 24-hour storm peak) are bypassed azound the presettling basin (actually installed as a vault at Sterling Asphalt). However, a discussion of the equation and its applicability at Sterling follow to demonstrate the appropriateness of the change to corrugated metal pipe for the presettling vault.