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City of Orono <br />Surface Water Management Plan February 2019 32 <br />CITY OF ORONO – SURFACE WATER MANAGEMENT PLAN <br />5 WATER RESOURCES ASSESSMENT AND <br />IMPLEMENTATION PLAN <br />5.1 General Assessment Procedures <br />The general procedure and scope of this SWMP includes the development of a planning <br />document to identify 1) stream and shoreline restoration sites, 2) outlet control structure repair <br />or replacement sites, 3) localized storage, volume reduction, and water quality improvement <br />sites, 4) wetland restoration sites, and 5) natural upland restoration sites. The sites listed here <br />should be used for planning purposes and are intended to highlight any potential large-scale <br />issues. <br />The following summarizes the major activities associated with plan development: <br />1. The existing City utility and storm sewer mapping was researched to determine existing <br />drainage patterns and locations of catch basins, culverts, storm sewer, outlet structures, <br />treatment basins, and other pertinent drainage features. Additionally, existing storm water <br />models from recent developments and the District were analyzed. <br />2. Lidar topographic mapping was obtained and correlated with the existing storm drainage <br />data to determine and model the existing drainage patterns. However, the elevation <br />contours provided with lidar mapping is only approximate and should not be considered <br />adequate for final design of storm drainage systems. A definite margin for error in the <br />determination of the watershed limits exists and the parameters used in estimating <br />stormwater flows would need to be revised with more detailed localized topographic <br />information for specific improvements. <br />3. Key areas of concern were identified for field inspection using topographic mapping and <br />storm sewer data. Field inspections and surveys were then performed to identify culvert <br />sizes and invert elevations of critical culverts in the drainage system, as well as road top <br />elevations. <br />4. Each minor drainage area flowing to a collection point was identified and mapped on a <br />master drainage area drawing. Eighteen subwatershed collection areas were identified as <br />part of this project. <br />5. Drainage area maps were plotted and used to review existing drainage patterns and <br />determine reasonable alternatives for future storm drainage improvements. Many factors <br />were considered in this planning/design process including, flooding potential, potential <br />water quality benefits, operation and maintenance, and cost. <br />Surface runoff and storm drainage design is primarily dependent upon the permeability of <br />existing surfaces, travel time and rainfall intensity. The SCS Runoff Curve Number Method, as <br />developed by the United States Department of Agriculture (USDA) Soil Conservation Service <br />(SCS), was used to estimate runoff. Curve Numbers were computed for each subwatershed to <br />reasonably reflect the degree of existing industrial, commercial and residential development. <br />The subwatershed delineations and node identifications are shown on the Subwatershed Maps <br />provided in Appendix B. Based on the subwatershed and routing analysis, a potential future <br />stormwater management system was developed.