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<br />-3- <br /> <br />II. Adjustments vfuere Parcel Runoff Is Significantly Different <br />From Land Use Standard <br /> <br />Where the unit runoff generated by a parcel differs from the <br />assigned amount for that land use catagory by more than 20%, <br />the P.W.D. may adjust the parcels storm water drainage fee <br />in accordance with the following procedure: <br /> <br />(a) Calculation of unit runoff for the parcel shall be <br />determined by the methods outlined in the Soil Conservation <br />Service Technical Release No. 55, utilizing a 2" total rain- <br />fall amount and antecedant moisture condition II. <br /> <br />(b) If calculated unit runoff is shown to differ from the <br />assigned amount for that land use catagory by 20% or more, <br />the number of assigned REU's for that parcel shall be adjusted <br />by multiplying by the ration of the calculated unit runoff to <br />the standard unit runoff. <br /> <br />(c) A parcels storm water drainage fee shall be subject to <br />increases as well as decreases by this procedure. <br /> <br />(d) Because single family and duplex fees are not based upon <br />actual parcel acreage, no adjustments for unit runoff differ- <br />ences will be made for those land uses. <br /> <br />III. Procedure for Calculation of Credits for Wet Ponds <br /> <br />A parcel may be credited for up to fifty (50%) percent of the <br />storm water drainage fee for onsite measures which are owned <br />and maintained by the applicant which effectively reduce the <br />outflow of sediment/nutrients from the site. Credit percentage <br />shall be based on one-half of the actual percentage of sediment <br />removal efficiency, as determined by the following procedure, <br />rounded to the nearest 5%; except that no credit will be given <br />for sediment removal efficiencies of less than 20%. <br /> <br />1. Calculation of Credits for Wet Ponds <br /> <br />(a) Determine total site acreage and percent of site <br />that has an improved or irrpervious surface. <br /> <br />(b) Calculate the annual depth of runoff from the follow- <br />ing equation: <br />D =P (.75 I +.15)-5.234 (.25-.1875 I ) .597 <br />r m m <br />Where D = annual depth of runoff in inches. <br />r <br />I = percent of site impervious area expressed as a decimal. <br />m <br />P=annual depth of precipitation=29 inches. <br /> <br />(c) Calculate annual volume of runoff in acre-feet: <br />V l=site acreage x D /12 <br />annua r <br /> <br />(d) Determine pond capacity below outlet elevation in acre-feet. <br /> <br />(e) Calculate capacity inflow ration (CIR) , where: <br />CIR=ponding capacity/v 1 <br />- annua <br />