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<br /> _.._.__.,-_........."'"""-'-..~-'--.._-,.__.._._-" - ,-... . -- ....--...--..---... __ _______.c._ ..,,_....... ~... <br /> A-9 <br /> . Calculation Procedure. <br /> . 3. <br /> a) Divide site into areas ~hich drain eo each drainage ouelee. <br /> . b) Select design rainfall. . --- -----. <br /> c) Route rainfall through pond using modified rational n:er.hod. <br /> ( . choose tc' at least 30 J:lin., i111d use "C" Value from Rossnille!"s <br /> Equation Graph. Determine peak outflo\; rate by' sUIilllling all area: <br /> d) Check other rainfall events to determine 1Tworst" case_ <br /> e) Calculate pre-development rates for parcel using "("=.::0 and :15; <br /> ing no on-site retention. <br /> f) . If peak S-year outflow less than predevelopment--lO~ credie. <br /> . If peak lOO-year oueflo\; less than predevelopment--addicional <br /> credi L. <br /> 4. Exceptions, Special Cases. <br /> a) Off-site water drains to ouelet--owner has right to dr;Jin this ,. <br /> through his .s'i te wi thout deten tion. Grant credits if he provide <br /> sufficient control for his portion of the total flow. (This wil <br /> . require applicant to provide iniormation about watershed beyond <br /> parcel). <br /> b) Outlet facility o'med by city or others--No Credits. (See Criter; <br /> c) Shared Ponding Situation: All ponds have an outlet. I f out 1 e e <br /> on another's land, no credits (as in b) above), exceDt if the l' <br /> level is normally belo\; outlet, (requires historic docu:::ene"-tior. <br /> this case, if no overilO\. occurs in a S-year event, parcel gees <br /> credit. If no()verflow occurs 100-year event, addieional 1-' <br /> :. ~n "', <br /> - <br /> C. !Vater Quality Credits <br /> , ( <br /> I. Cri teria: When a parcel provides on-si!.c !.-rca!.r.:cnt. facilities Hhic~- fer. <br /> L to improve the quality of runoff exiting the site, up to 50% of the drai <br /> fee may be credited depending of treatment effecti~eness. <br /> - <br /> ! 2. Required information by applicant: <br /> ; a) I'.'hen treatment facility is a "",et" pond. <br /> (1) Area of site draining to pond and percent impervious. <br /> (2) Volume' of pond belo\; outlet elevation. <br /> :t.. <br /> b) ,fuen treat~ent facility not a pond, applicant to furnish sufficien <br /> I documentation to a5certain the effectiveness of the :':acility in re <br /> , suspended solids. <br /> , <br /> ~- <br /> - Calculation procedure for "et ponds: <br /> 3. <br /> I a) Divide site into areas dr3ining to each outlet: or facilit.y. <br /> I, runoff for CJ.ch o.rc.J. u~in5 <br /> b) Calculate the 3.verJ.ge annual <br /> the follo\;ing equations: .Sqr <br /> ~ Or=P (':75 I + .15) -5.2.3-1 (.~5-.1S75 Ir.l) <br /> , ::\ <br /> lo,... ",here Dr=annu.J.l runoff depth <br /> - <br /> - P=:lnnu:d rainfall = 29 inches <br /> IC'I=o~ improved, expressed asa decical. <br /> c) culate vol~~c (V annual) Dr , . <br /> = 12' x area acreage. <br /> in feet be 1010 <br />