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1 . lntroduction <br />1.1. Project Purpose <br />The City of Roslyn (City) completed a Stormwater Comprehensive Plan (Plan) in July 2014. The <br />Plan contains structural and non-structural solutions to address stormwater conveyance and <br />water quality problems within the City. The Capital lmprovement Plan (ClP) contained within the <br />Plan identifies various structural solutions to address conveyance issues by replacing pipes and <br />culverts. Nonstructural solutions such as stormwater management facilig inspection, public <br />education and outreach, water quality monitoring and encouraging infiltration and low impact <br />development are aimed at improving water quality within the region. <br />The City received a Water Quality Grant from the Washington State Department of Ecology <br />(Ecology) to identifo ways to improve water quality within the localwatenrvay, Crystal Creek. The <br />goal is to identify Best Management Practices (BMPs) to provide stormwater runoff treatment for <br />Total Suspended Solids (TSS) and provide for reduction in other pollutants within the urbanized <br />areas of the City. <br />The grant has two objectives: <br />Evaluate the possible implementation of stormwater retrofits to provide water quality <br />treatment and determine the most suitable locations, methods, estimated costs, available <br />funding, and phased approach for such retrofits in a citywide study. <br />2. Advance design of stormwater treatment retrofit improvements for the CIP 1 project <br />which is a pipe rehabilitation project currently under design. <br />1.2. Report Purpose <br />This report documents the citywide study to be adopted as an addendum to Cig of Roslyn <br />Stormwater Comprehensive Plan. The CIP 1 treatment project will be advanced in detail design <br />after study completion, This addendum describes the drainage basin, details of the project site, <br />an analysis of the core elements, the alternatives considered, the design analysis, a quantification <br />of the water quality benefit, CIP summaries, financial review of potential funding, and operation <br />and maintenance considerations. <br />1.3.Design Codes and References <br />1.3.1. Department of Ecology <br />The analysis will be in accordance with Ecology's Stormwater Management Manual for Eastern <br />Washington (S\ /[,lMEW, 2019, and Ecology's Design Deliverables for Stormwater Projects with <br />Ecology Funding, June 2018. <br />'1.3.2. City of Roslyn <br />This analysis will be in accordance with the City's Stormwater Regulations listed in Chapter 3 of <br />the Plan.