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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />C. A HEC-2 model was developed based on the instructions provided in Reference 2. A <br />copy of the personal computer version was obtained from Dr. James Guo at the University <br />of Colorado-Denver. <br /> <br />Lengths for the channel, left- and right- overbank areas were measured directly from the <br />blue-prints. Preliminary Manning's n-values estimates were provided by Mr. Larry Lang, <br />based on a field trip to the site. 100-year floodpeaks were obtained from the Floodplain <br />Information Report. <br /> <br />E. The model was run and the computed 100-year water surface elevations (WSE) were <br />compared to the values in the Floodplain Information Report (Table 3). Adjustments were <br />made to the model until the 100-year WSE of the reconstructed model were within 0.5 <br />feet of the Floodplain Information Report. Adjustments included modifying: <br /> <br />I. Manning's n-values for channel and left- and right-overbanks. <br /> <br />2. Cross-section shape, particularly the main channel bottom width and the overbank <br />elevations. <br /> <br />3. Location of left- and right-banks. <br /> <br />The above information was to develop the base flood HEC-2 model. <br /> <br />Proposed Project - The approach taken to size the proposed channelization and ponds is <br />presented below. This information was subsequently input into the base flood HEC-2 model to <br />develop a model for the proposed project: <br /> <br />A. The sediment/debris pond was located at station 696+00, per the meeting of June 14, 1993 <br />(Reference 3). To create the necessary "ponding" and settle-out sediment and debris, a <br />control section was sized consisting of three components: <br /> <br />1. A constriction in the main channel to create a backwater from minor flows in the <br />river. The constriction was sized to pass the base flows (Q = 164 cfs, Reference <br />3) at a depth of 4.5 feet (see Calculation sheets 1 to 3, Appendix D). <br /> <br />2. An overflow at the control section was sized to pass the 100-year flood, using the <br />weir equation, assuming NO rise in the lOO-year water surface elevation. <br /> <br />3. A settling basin was assumed excavated upstream of the control section. For <br />HEC-2 modelling purposes, any excavation below the natural channel bed was not <br />considered effective flow area. <br /> <br />A schematic of the sediment pond area is presented in Figure I. <br /> <br />5 <br />