5.5. SR 08-06-2007 - Laserfiche WebLink

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Work Scope OBJECTIVE Wenck's objective is to provide the City with a report that not only satisfies the nondegradation requirements of the MPCA but also provides a tool that the City can use to incorporate into its future development policies and ensure protection of water quality. The result of this study will show the effectiveness of current development and stormwater management standards and, if necessary, provide a framework to revise the standards if the study shows that nondegradation is not satisfied. The study consists of three key steps: 1. An assessment of change in land use and imperviousness in the City from 1988 to current (2004) and out to 2020 conditions 2. An associated pollutant load change with the land use change for total phosphorus (TP), total suspended solids (TSS), and water volume. 3. Identification of appropriate BMPs to return to 1988 water quality conditions or better. Once these steps have been completed, the City's SWPPP will be updated appropriately. The following approach has been developed for the City of Elk River to address their nondegradation requirements including the loading assessment and nondegradation report. MODEL SELECTION Wenck proposes development of a spreadsheet model for the City of Elk River. As stated previously in the "Approach" subsection of the "Qualifications" section, the spreadsheet model will provide a clear-cut method in calculating the loading assessment and allow Wenck staff to emphasize addressing potential changes to stormwater management in the City. The spreadsheet model will utilize the US EP A Simple Method. Event mean concentrations are widely available from many literature sources. The Wenck project team, however, is very familiar with reasonable concentrations for a variety of land uses through our water resources work and experience with other nondegradation studies. Similar to event mean concentrations, runoff coefficients are applied for different land uses to predict an annual runoff volume. Simple multiplication of the concentration by the volume yields the loading rate from a particular land use. Load reductions for the current (2004) and 2020 conditions will be determined based on City stormwater management standards. For those areas that developed according to the development review standards, a load reduction percentage will be applied based on the results of the model. The load reduction percentage for regional ponds not created as part of a development will also be incorporated into the model. GENERAL APPROACH Throughout the study, the Wenck Project Manager will consult with City staff, local water authorities, and MPCA staff to coordinate efforts and get buy-in on the methods and results. The first step of the Wenck approach includes developing the loading assessment for the three time periods (1988, current, and 2020). The second step includes applying the necessary load reductions for BMPs that have been implemented by the City. The third step in the process includes analyzing the loading assessment and T:\0598\OOINonDegradation Proposa1\Work Scope.doc 11 ,~Wenck