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7550 MERIDIAN CIR N, SUITE 120 | MAPLE GROVE, MN 55369 PAGE 3 OF 6 <br />INITIAL CHARGE DETERMINATION <br />The method to determine the charge for each parcel generally consists of estimating the annual <br />runoff volume from each parcel in the WMD. The amount of revenue needed for the WMD is then <br />allocated to each parcel in proportion to the estimated annual runoff volume from a parcel compared <br />to the total runoff volume from all parcels. <br />Annual runoff volumes were calculated using a runoff coefficient. The method is utilized by the <br />Minimal Impact Design Standards (MIDS) Calculator, as described in the MnPCA Stormwater <br />Manual 1. The method uses the following equation to calculate the runoff depth (ܦ௥ ), with Table 1 <br />displaying the various runoff coefficients: <br />ܦ௥ =ܲכܲ௝ כܴ௩ <br />Where: <br />x P is the total annual rainfall depth (inches) <br />x Pj is the fraction of annual rainfall events that produce runoff (assumed to be 0.9) <br />x Rv is the runoff coefficient (dimensionless) <br />Table 1.Runoff Coefficients by Cover Type and Hydrologic Soil Group <br />Rv Coefficients A <br />Soils <br />B <br />Soils <br />C <br />Soils <br />D <br />Soils <br />Impervious Cover 0.95 0.95 0.95 0.95 <br />Pervious Cover (Managed turf - disturbed soils) 0.15 0.2 0.22 0.25 <br />All parcels were assumed to have hydrologic soil group type D based on review of the SSURGO <br />soils database. Each land use category was assigned an impervious percentage based on guidance <br />in TR-55 <br />2. For land uses that do not have an impervious percentage factor provided by TR-55, an <br />estimation was made based on aerial imagery review. With these inputs, an annual runoff depth was <br />computed for each land use category (see Table 2). Then, a per-acre charge for each land use <br />classification was calculated by multiplying the total amount of revenue needed ($450,000) by the <br />ratio between the land use’s runoff volume (summation of all individual parcels in the land use <br />classification) and the sum of the runoff volume from all parcels in the WMD. <br />Table 2 lists the estimated initial total per-acre charges by land use. These total amounts can be <br />divided into incremental amounts over multiple years as determined by the Board of Managers. <br />1 Minimal Impact Design Standards, Minnesota Pollution Control Agency, <br />https://stormwater.pca.state.mn.us/index.php/MIDS_calculator, April 8th 2020 <br />2 Urban Hydrology for Small Watersheds Technical Release -55, United States Department of Agriculture – <br />Natural Resource Conservation Services, June 1986.