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7 Water Quality Benefit <br />The proposed improvements do not require Runoff Treatment (Core Element #5) or Flow Control <br />(Core Element #6) in accordance with the SWMMEW (Ecology, 2019). In addition, this retrofit <br />project is not required to meet the new/redevelopment standards. Ecology has established a <br />procedure to quantify the water quality benefit realized through retrofit projects such as this one <br />(Design Deliverables for SforrriwaferProjects with Ecology Funding, Ecology; Revised June <br />2018). <br />Because the size and environmental constraints within the project site could control the size and <br />capacity of a BMR, a baseline is quantified by comparing a ratio of the size of the proposed <br />retrofit BMP to the size of a BMP design to meet new/redevelopment criteria. Once this is <br />determined, the Equivalent New/Redevelopment Area for the retrofit project is determined by <br />multiplying the ratios by the contributing area. <br />As the project objective is to provide stormwater runoff treatment for Total Suspended Solids <br />(TSS) and provide for reduction in other pollutants within the urbanized areas of the City, the <br />selected BMPs provide high runoff treatment and low flow control. Therefore, only the runoff <br />treatment water quality benefit is calculated. <br />The following analysis is an assessment of the Water Quality Benefit proposed by the project. <br />This analysis will be refined with future design efforts. <br />7.1. Runoff Treatment Comparison <br />The Flow Control Ratio was developed following procedure ERT -1: Analysis of Size of traditional <br />Flow Rate or volume -based Runoff Treatment BMP. <br />RatiolRT-z = Design flow rate or volume for proposed retrofit treatment BMP <br />Design Flow rate or volume to meet new/redevelopment criteria <br />If RatiOERT-1 } 1, then set RatiOFRT_1 = 1 <br />Design flow rates for the water quality storm were output from the XPSWMM model. Biofiltration <br />swales were sized using this design flow rate and the design parameters from the SWMMEW <br />(Ecology, 2019). These calculations are performed with the aid of an Excel spreadsheet, The <br />resultant calculation is a swale length required to meet new/redevelopment runoff treatment <br />criteria. Design flow rates for the proposed retrofits were back calculated using the available <br />footprint of each proposed swale. The biofiltration swale sizing calculations are included in <br />Appendix C. <br />In some cases, the biofiltration swales will be able to achieve more treatment than needed with a <br />Ratiomr-1 > i, therefore RatioFst-, = 1. The ratios for each biofiltration swale are shown in <br />Appendix D. <br />8 <br />