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<br />the model, and the upstream end of Cranks Creek Reservoir formed the <br /> <br />upstream boundary. Cross sections were located to define total volume <br /> <br />in each reservoir, and, where they extended up tributary arms, arti- <br /> <br />ficial flow boundaries were imposed to separate that portion of the <br /> <br />section which conveys flaw from that portion which only stores water. <br /> <br />From these sections geometric data were calculated for nodal points <br /> <br />spaced 1/2 half mile apart, thus forming the basic geometric model for <br />the unsteady flow computer program. <br /> <br />Determining Hydraulic Roughness <br /> <br />Since friction loss was such an important consideration in this <br /> <br />study, the selection of realistic n-values for Manning's equation was <br /> <br />of primary importance. Values of 0.05 for the channel and 0.10 for the <br />overbanks had been used in other studies by engineers familiar with the <br />streams in this area. These values were, however, based on natural <br />conditions. Values of 0.03 and 0.01 had been used for reservoir conditions. <br />Since the flow velocity associated with the dam-break flood was expected <br /> <br />to be more nearly that for natural conditions than that which would be <br /> <br />expected with the reservoir impounded, the natural conditions n-values <br /> <br />were used. These were adjusted into composite values for the entire <br /> <br />cross section and to account for the sinuosity of the valley, which <br /> <br />resulted in a composite n-value of .07 for each cross section. In Cranks <br /> <br />Creek this value was reduced to .05, since the flaw conditions would tend <br /> <br />toward preimpoundment conditions as the water drained out. <br /> <br />9 <br />