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, <br />;i Allowable outflow is the total runo£f in cfs leavinq the site in <br />' the existing (pre-development) condition and is usually computed <br />� by the Rational Method. <br />�;; . <br />The total volume of water to be stored in the retention basin (VT) <br />may then be computed by the equation <br />UT = VS x acres x developed runoff coefficient <br />�. The assumptions in this method are,.that storrp water rises in the <br />' �retention/detention basin at a constanr rate to fill the basin to' " <br />the peak volume, and that the maximum allowable outflow is reached <br />only at the oeak volume. <br />A listing of the retention/detention equations for both a 10-year <br />and 25-year frequency storm is shown below. <br />This table lists only two outlet conditions, an ori.fice with head <br />pressure and a constant flow (or pump) outlet. tlowever, other types <br />of outlets may be used since t}�ese eguations are used only for <br />calculating required storage volumes. For example, if a weir out- <br />let is used, the "orifice with head" equations apply. The size/ <br />configuration of the outlet structtsre is calculated by relating <br />the allowable outflow to tl�e equations that define the outlet. <br />Consideration for adequate maintenance must also be given to the <br />selection of the outlet structure. <br />i <br />;�.. <br />I � <br />I <br />I AREA <br />I <br />i <br />, I1SE FOR <br />j � s�o�iatitsn <br />COU:ITY <br />TYPE OF OUTLET pEAK STORAGE HAXIMUM 5T(1qA6E pEAK SiOfiAGE MA% <br />TIHE i11IHUTES) VOLUHE �FT�/AC) TIHE SMIHUTES) YOL <br />OHIFICE HITH HEADI -�� 2I5�5 <br />T••25� Y'�o <br />COHSTANT FLOM iT•-2S♦ �5—�� <br />� Qo <br />48 <br />Y� • <br />7607 T <br />�::s- - +� noT <br />yf • <br />]60)T 60 QoT <br />1�3- '. <br />T•-25♦ t)06 <br />�o <br />T•-25� 180� <br />�o <br />Y� - <br />/lt9 T �0 OoT <br />TiT,' - <br />y� . <br />1729 T 60 Ooi <br />T2T- ' <br />