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I I I I I I I I I I I I I I I I I I I 3.7 UDSWM2 Modelling A UDSWM2 model was created for each watershed (total of 6) to route the CUHP gener- ated hydrographs through the existing stream network. Conveyance element parameters required for the UDSWM2 model include the conveyance element identifier, channel bottom width, length, invert slope, side slopes, hydraulic roughness, and routing connectivity. A summary of these characteristics is shown in Appendix K and the methods for estimating these characteristics is described in the following sections. 3.7.1 Conveyance Element Identification Number Conveyance elements were numbered from downstream to upstream for each stream as shown in Appendix G. All conveyance elements were identified with even numbers. Design points were located at the upstream and downstream ends of each conveyance element number and identified by the appropriate odd numbers. 3.7.2 Stream Geometry All streams were modelled as trapezoidal cross sections. Bottom widths, lengths, invert slopes, and side slopes were estimated from the 1:24,000 USGS quadrangle maps. In some cases, streams were represented by a base flow channel and an overflow floodplain channel. 3.7.3 Roughness The roughness or Manning's "n" value for each conveyance element was determined using Equation 2 from the UDSWM2 Users Manual (DOFCD, 1985b), which is shown below. n = 0.393 x (S)0038 x (R)-Oo16 (I) where: n = Manning's roughness coefficient S = friction slope (feet/feet) R = hydraulic radius (feet) This equation is the result of research work by Robert D. Jarrett of the USGS (Jarrett, 1984) and is recommended for hydrologic routing in natural channels. For this study, the friction slope was approximated using the invert slope and the hydraulic radius was approximated for lOO-year flood conditions. The Manning's "n" value for the conveyance elements generally ranged from 0.03 to 0.06. DENlOOJ5I~.WPS 3-7