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<br />drainage areas for the study basins. the diverted runoffs can concurrently impact adjacent basins <br />by discharging stormwater runoff to areas that are not connected to the original basin, In <br />addition, the canal systems can overflow or fail dur.ing a flood event which can increase the <br />runoff in a natural drainage basin. The presence of canals and ditches are to be considered for <br />flood flow impacts on a case-by-case basis, <br />Policv Statement: The conveyance of floodwaters through canals is acknowledged only by <br />agreement between ditch company and local governing jurisdiction. <br /> <br />5,4 Non-Tributary Areas <br />The hydrologic response of a watershed may be affected by portions of that watershed which may <br />not contribute surface runoff to a downstream point of interest during all flood events up to the <br />lOO-year flood, This can be the result of ponding where no outlet (channel or pipe) is present, <br />or the result of a "flow-split", which diverts a portion of the runoff into more than one drainage <br />basin. These conditions result in non-tributary watershed areas, Stormwater runoff in urbanized <br />and rural areas can be transferred or routed to areas outside of the design point of a basin through <br />storm sewer systems or other man-made conveyance systems. In some areas of the state, natural <br />topographic sinks or depressions can retain stormwater runoff where present and should be <br />considered for flow reduction when developing a hydrologic model for that area. <br />Policv statement: The drainage area serviced by tllIe storm drainage system is considered <br />at tJle point of discharge. Non-tributary areas are considered when quaIified by meeting <br />a test of detention or have topographic feature withl)ut outfall channel or the watershed has <br />a flow split condition. A watershed is considered non-tributary if the surface runoff from <br />a 24-hour 100-year event would be totally retained. The watershed area would! not be <br />included in a hydrologic model when analyzing the runoff to a downstream point of interest. <br />Flow splits mnst be analyzed on a case-by-case bllS.is. The manner in which a flow split is <br />taken into account in the hydrologic model is dependent on whether failure of the drainage <br />.. ' <br />system is possible. First, determine if the drainage system will fail for events up to the 100- <br />year and the direction of flow. If the flow is tributary to the point of interest, the <br />hydrologic model mnst include the tributary area. If the drainage system will not fail for <br /> <br />5 <br />