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<br />'J <br />, <br /> <br />, <br />t <br />i~ <br /> <br />" <br /> <br />,{ <br />~. <br />.t <br />c;J, <br />< <br />~ <br /> <br />l <br />,,"I <br />~. <br />I, <br /> <br />~t" <br /> <br /> <br />I <br />,.. <br />fc <br />;"0' <br />t <br /> <br />i <br />c <br />" <br />i' <br /> <br />~; <br />~ <br />~ <br /> <br />~ <br /> <br />Box Elder Creek Hydrology <br /> <br />addressing the future urbanized conditions. However, only floodplains based on future <br />watershed conditions were mapped. <br /> <br />"Loss rates should be varied when computing different frequency floods. Urbanization <br />effects must be reflected in the loss rates. The Soil Conservation Service (SCS) curve <br />number method is one of the methods that can be used." CUHP allows for varied loss <br />rates and the effects of urbanization on the loss rates. <br /> <br />"Time of concentration or lag computations must reflect the effects of increases in <br /> <br />velocities due to channel modifications and urbanization." The CUHP model can be <br /> <br />adjusted as needed to account for changes in urbanization and charnel modifications. <br /> <br />"Rainfall duration must be large enough to capture all excess rainfall as well as provide <br />reasonable runoff and sediment volumes when performing storage analysis." CUHP uses <br />the design storm which is flexible enough to utilize rainfall durations appropriate for the <br />given watershed situation. <br /> <br />"Stream flow routing methods should be able to analyze the attenm,tion and translation of <br />hydrographs." The District's model does this by utilizing UDSWM, the runoff block of <br />the EP A Stormwater Management Model (SWMM) as modified by the Omaha District, <br />U. S. Army Corps of Engineers (CaE) to route, attenuate and translate runoff <br />hydro graphs generated by CUHP. <br /> <br /> <br />., <br /> <br />Finally, CUHP is on FEMA's list of approved hydrology models. In summary, the <br />District's CUHP rainfall-runoff model, used in conjunction with UDSWM, is an <br />appropriate model to be used for the Box Elder Creek watershed in this case. A brief <br />summary of the history of the development ofCUHP is provided in Attachment 4. <br /> <br />CTC8.doc <br /> <br />6 <br />