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8 <br />I <br />The "Extran" computational block, contained in the SWMM <br />program, is used to simulate performance of detention <br />structures. The "Extran" block receives hydrograph input <br />from the "Runoff" blocks previously described in this <br />Section. "An arbitrary stage -area -volume relationship is <br />input to represent the characteristics of detention <br />ponds. Routing is performed by ordinary level -surface <br />(1) <br />reservoir methods ". The level -surface reservoir method <br />(Level Pool Routing) assumes that the reservoir has a <br />horizontal surface with outflow characteristics which can <br />be numerically defined. The "Extran" program generates <br />outflow and stage hydrographs for the detention <br />structures. <br />Control structures are used to define the outflow <br />characteristics of a detention storage facility. <br />Section 4 of the DCSS, specifies that control structures <br />shall be catch basins with restrictor devices used for <br />controlling outflow from a facility to meet the desired <br />performance standards. The types of restrictor devices <br />used consist of orifice and/or multiple orifice <br />configurations which are sized and located to meet the <br />desired discharge performance levels. <br />19 <br />