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i <br /> .-a) Divide site...into areas -which: drain to- each drainage outlet . <br /> b) Select design. rainfall. I <br /> c) Route: rainfall.-through pond using modified- rational method. <br /> Cchoose .tc -at least 30 min. , -and use "C" Value from Rossnillers• <br /> Equation Graph. Determine peak outflow' rate by summing all areas . <br /> d) Check other rainfall events to determine "worst" case. <br /> e) Calculate pre-development rates for parcel ,usirig "C"=,20 and assur. <br /> ing no on-site retention. <br /> f) If peak 5-year outflow -less• than predevelopment--10" credit: <br /> If peak 100-year outflow less than predevelopment--additional 1� <br /> credit. <br /> 4. Exceptions, Special Cases . <br /> a) Off-site water drains to outlet--owner has right to drain -this wat <br /> through his site without detention. Grant credits .if he provides <br /> sufficient control for his portion of the total flow. (This will <br /> require applicant to provide information about watershed beyond hi <br /> parcel) . <br /> b) Outlet facility owned by city or others--No Credits . (See Criteria <br /> c) Shared Ponding Situation: All ponds have an outlet . If outlet is <br /> on another's land, no credits (as in b) above) , exceot if the por <br /> level is normally below outlet, (requires historic documentation) . <br /> this case, if no overflow occurs in a 5-,year event, parcel gets 1C <br /> credit . If no overflow occurs in 100-,year event, additional 15- c <br /> C. Plater Quality Credits • <br /> 1. Criteria: When a parcel provides on-site treatment facilities which funct <br /> to improve the quality of runoff exiting the site, up to 50% of the drainz <br /> fee may be credited depending of treatment effectiveness . <br /> 2. Required information by applicant: <br /> a) i:'hen 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) ,''hen treatment facility not a pond, applicant to furnish sufficient <br /> documentation to ascertain the effectiveness of the facility in remc <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 usin; <br /> the following equations : ,sar <br /> Dr=P ( . 7S I + - 1S) -5 . 23-1 ( .25- ..1S75 Im) <br /> .here Dr=annual runoff dept;i <br /> P=annual rainfall = 20 inches <br /> Im ":-Troyed, expressed as a decimal . <br /> C) Calcu.l:tc annual ru^o:: :o l., �e (h annual = Dr <br /> I`, x area acreage . <br /> d) Calculate or verity pond vol;z:�e in acre- feet below outlet ele%atl�n. <br /> C) Calculate , CaDacity- inflow ration (Cl2) <br /> CIR=Pond V01L1c/V <br /> ' nnual <br /> f) Read sediment removal efficiency from Brune 's Trap Efficiency Curve . <br /> (See policy) . <br /> - � I <br />