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d) Specific details about outlet facility(s). <br />e) Calculation of peak outflow rate for 5-year and 100-year design rainfall, using <br />modified rational method with t� of 30 min. or more. <br />3. Calculation Procedure. <br />a) Divide site into areas which drain to each drainage outlet. <br />b) Select design rainfall. <br />c) Route rainfall through pond using modified rational method. Choose t� at least <br />30 min., and use "C" Value from Rossmillers Equation Graph. Determine peak <br />outflow rate by summing all areas. <br />d) Check other rainfall events to determine "worst" case. <br />e) Calculate pre-development rates for parcel using "C"=20 and assuming no on- <br />site retention. <br />� If peak 5-year outflow less than predevelopment--10% credit. <br />If peak 100-year outflow less than predevelopment--additional 15% credit. <br />4. Exceptions, Special Cases. <br />a) Off-site water drains to outlet--owner has right to drain this water through his <br />site without detention. Grant credits if he provides sufficient control for his <br />op rtion of the total flow. <br />b) Outlet facility owned by city or others--No Credits. (See Criteria B) <br />c) Shared Ponding Situation: All ponds have an outlet. If outlet is on another's <br />land, no credits (as in b) above), except if the pond level is normallv below <br />outlet, (requires historic documentation) this case, if no overflow occurs in a 5- <br />year event, parcel gets 10% credit. If no overflow occurs in 100-year event, <br />additional 15% credit. <br />C. WATER QUALITY CREDITS <br />1. Criteria: When a parcel provides on-site treatment facilities which function to <br />improve the quality of runoff exiting the site, up to 50% of the drainage fee may be <br />credited depending of treatment effectiveness. <br />2. Required infarmation by applicant: <br />a) When treatment facility is a"wet" pond. <br />(1) Area of site draining to pond and percent impervious. <br />(2) Volume of pond below outlet elevation. <br />b) When treatment facility not a pond, applicant to furnish sufficient <br />documentation to ascertain the effectiveness of the facility in removing <br />suspended solids. <br />3. Calculation procedure for wet ponds: <br />a) Divide site into areas draining to each outlet or facility. <br />b) Calculate the average annual runoff for each area using the following equations: <br />�r p (.�s Im+.is�—s.234 �.zs-.ig�s Im).59� <br />Where Dr= annual runoff depth <br />P= annual rainfall= 29 inches <br />Im % improved, expressed as a decimal <br />c) Calculate annual runoff volume (V annual) =Dr x area acreage. <br />