1020 SE EVERETT MALL WAY CHEVRON 2018-01-01 MF Import

,n . j �.. I PUBL/C WORKS E ETT I � � i �MoxarrauM 1 I TO: Paul McKee � FROM: Jane Zimmerman I'� DATE: December 9, 1997 SUB,j�CT: Drainage Calculation/Plan Review, Everett Mall Chevron Station The standard for water quality treatment in the Silver Lake drainage basin is a wetpond in series with a biofiltration swale, wi[h the we[pond being the most ef1'ective component of the system. To meet Ciry redevelopment standards, die area available must be maximized for stormwater treatment capabilities. Although it doesn't appear feasible to provide a wetpond in series with a swale on this site, a more effective syatem could be provided t}tan is proposed. I believe that provision of a minimally sized wetvault (approximately 164 cubic fee[ of dead storage per acre of impervious surface, see section 3-3.6.1 of rhe City's stormwater manual) in series widi a wedand treatment cell is the minimum combination of facilities necessary to meet City redevelopment standards - provided flie wetland treatment cell is designed to maximize the treatment volume given the area available in the southeast corner of the site. The we[vault can be provided anywhere in the system between the exis[ing control manhole and the wedand treatment cell. It may be wise to locate the wetvault upstream of the pump cliamber so tliat sediment removal is accomplished before the srormwater is pumped (�vhich should decrease required pump maintenance and increase pump life). City standards allow a maximum avera�e dead storage depd� of 2A feet in the treatment cell, with a maximum depth of 2.5 feet. This maximum average depth of 2.0 fee[ must be used in this design in order to maximize the treatment volurne in the treatment cell. Ciry standards do not require a fence around a facility widi a maximum depth less than 3 Peet, as long as the side slopes are no steeper than 3H:1V. If a fence isn't required but the developer cl:ooses to provide one, the developer is free to choose [he fence material and height. The plans must show a planting detail for the wetland treatment cell which meets item i/10 of the �vetpond standards found in section 3�.5.1 of the Ciry's Stormwater Management Manual.