Resolution_City of Roslyn_PF Funding Award Agmt

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7. Water Quality Benefit The proposed improvements do not require Runoff Treatment (Core Element #5) or Flow Control (Core Element #6) in accordance with the SWMMEW (Ecology, 2019). In addition, this retrofit project is not required to meet the new/redevelopment standards. Ecology has established a procedure to quantify the water quality benefit realized through retrofit projects such as this one (Design Deliverables for Stormwater Projects with Ecology Funding, Ecology, Revised June 2018). Because the size and environmental constraints within the project site could control the size and capacity of a BMP, a baseline is quantified by comparing a ratio of the size of the proposed retrofit BMP to the size of a BMP design to meet new/redevelopment criteria. Once this is determined, the Equivalent New/Redevelopment Area for the retrofit project is determined by multiplying the ratios by the contributing area. As the project objective is to provide stormwater runoff treatment for Total Suspended Solids (TSS) and provide for reduction in other pollutants within the urbanized areas of the City, the selected BMPs provide high runoff treatment and low flow control. Therefore, only the runoff treatment water quality benefit is calculated. The following analysis is an assessment of the Water Quality Benefit proposed by the project. This analysis will be refined with future design efforts. 7.1. Runoff Treatment Comparison The Flow Control Ratio was developed following procedure ERT -1: Analysis of Size of traditional Flow Rate or volume -based Runoff Treatment BMP. RatiOERT-t = n flow rate or volume for proposed retrofit treatment BMP Design flow rate or volume to meet new/ redevelopment criteria if Ratior•RT_1 > 1, then set RatiOERT_, = 1 Design flow rates for the water quality storm were output from the XPSWMM model. Biofiltration swales were sized using this design flow rate and the design parameters from the SWMMEW (Ecology, 2019). These calculations are performed with the aid of an Excel spreadsheet. The resultant calculation is a swale length required to meet new/redevelopment runoff treatment criteria. Design flow rates for the proposed retrofits were back calculated using the available footprint of each proposed swale. The biofiltration swale sizing calculations are included in Appendix C. In some cases, the biofiltration swales will be able to achieve more treatment than needed with a RatioeaT-1 > 1, therefore RatiOERT.1 = 1. The ratios for each biofiltration $wale are shown in Appendix D.