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<br />Shenandoah Improvements Feasibility Report <br />City of Lino Lakes, MN <br />WSB Project No. 018901-000 Page 7 <br />elevation reductions were greatest at the project location but decreased in effect for <br />ponds further upstream. <br />The ot her major component of stormwater design for this project was in water quality <br />improvements. This modeling seeks to understand what pollutant loading and <br />removals can be expected based on the watershed characteristics and stormwater <br />basins in the system. P 8 software was used for water quality modeling. This involved <br />creating an existing conditions P8 model from scratch as there was not one already <br />generated for this drainage area. Pollutant loading was determined based on <br />subwatershed areas and impervious s urface fractions. Pollutant removals were <br />determined based on pond as -built conditions. Under existing conditions, the <br />drainage system upstream of the ditch removes nearly 22,500 lb of TSS and <br />approximately 48.5 lb of TP each year. <br />For proposed conditions, the P8 models were updated to include the proposed <br />improvements with each concept. For the sake of water quality modeling, these <br />BMPs were assumed to function as infiltration basins, where captured pollutant <br />volumes are fully removed from the stormwater s ystem. Existing soils onsite are <br />likely silty sands, providing an infiltration rate of approximately 0.45 in/hr based on <br />the MN Stormwater Manual. <br />However, based on past soil borings and County groundwater data, it is assumed <br />inconsistent and minimal infiltration will be achievable in the basins due to the <br />shallow depth of the water table in the area. Modeling conservatively assumed an <br />infiltration rate of 0.01 in/hr, the minimum allowable in P8. Actual infiltration <br />averaged over the year may b e higher than this based on water table fluctuations. <br />Additionally, P8 modeling does not factor in pollutant removals from <br />evapotranspiration and vegetation uptake, both of which will be emphasized with <br />proposed improvements. Therefore, it is likely that p ollutant removals, TP in <br />particular, will be higher than the results given but are difficult to quantify. The <br />results of the modeling described above are summarized in the proposed concepts <br />below. <br />6.0 Concept 1 <br />Concept 1 is based directly on the proposed impro vements from the 2010 Rice Lake <br />Subwatershed Assessment developed by the Anoka Conservation District. There are <br />three wetland basins of roughly similar sizes that receive water from the ditch during <br />medium to high flow events. Water is allowed to equalize between the basins and <br />draws down very slowly over time via evapotranspiration and slow infiltration through <br />the hydric soils. Captured water is kept in the basins after ditch levels draw down by <br />virtue of one -way flap gates or duckbill valves on the inlet s from the ditch.