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Where the unit runoff generated by a parcel differs from the assigned amount for that land use <br />category by more than 20%, the P.W.D. may adjust the parcels storm water drainage fee in <br />accordance with the following procedure: <br />a. Calculation of unit runoff for the parcel shall be determined by the methods outlined in the <br />Soil Conservation Service Technical Release No. 55, utilizing a 2" total rainfall amount and <br />antecedent moisture condition II. <br />b. If calculated unit runoff is shown to differ from the assigned amount for that land use <br />category by 20% ar more, the number of assigned REU's for that parcel shall be adjusted <br />by multiplying by the ratio of the calculated unit runoff to the standard unit runoff. <br />c. A parcels storm water drainage fee shall be subject to increases as well as decreases by this <br />procedure. <br />d. Because single family and duplex fees are not based upon actual parcel acreage, no <br />adjustments for unit runoff differences will be made for those land uses. <br />(3) Procedure for Calculation of Credits for Wet Ponds <br />A parcel may be credited for up to fifty (50%) percent of the starm water drainage fee for on site <br />measures which are owned and maintained by the applicant which effectively reduce the outflow <br />of sediment/nutrients from the site. Credit percentage shall be based on one-half of the actual <br />percentage of sediment removal efficiency, as determined by the following procedure, rounded to <br />the nearest 5%; except that no credit will be given for sediment removal efficiencies of less than <br />20%. <br />A. Calculation of Credits for Wet Ponds <br />a. Determine total site acreage and percent of site that has an improved or impervious <br />surface. <br />b. Calculate the annual depth of runoff from the following equation: <br />�r = P (.�s Im + . is� —s.234 �.zs— . ig�s Im) .s9� <br />Where Dr = annual depth of runoff in inches. <br />Im = percent of site impervious area, expressed as a decimal. <br />P= annual depth of precipitation = 29 inches. <br />c. Calculate annual volume of runoff in acre-feet: <br />Vannuai = site acreage x Dr/12 <br />d. Determine pond capacity below outlet elevation in acre-feet. <br />e. Calculate capaciry inflow ratio (CIR), where: <br />CIR= ponding capacity/Vannuai <br />0 <br />