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I I I I I I I I I I I I I I I I I I I 6 To develop the runoff model, the Sand Creek basin was divided into 270 sub- basins and 262 gutter reaches. The basin parameters for present basin conditions were obtained from a quadrangle map having a scale of 1 to 24,000. Gutter di- mensions were based on estimates made during a field investigation. Impervious Water surface profiles were computed using the Corps of Engineers HEC-2 back- water computer program. The cross sections used in the analysis were developed from topographic maps with 2 foot contour intervals. The Sand Creek cross sections cover a 20-mi1e reach of channel and extend from Burlington Ditch (Section 2.1) upstream to Arapahoe County Road No. 18 (Section 124.0). Cross sections were areas were estimated from 1974 aerial photos. To obtain basin parameters for the future basin conditions, a projected land use map was prepared by using present zoning maps in conjunction with present growth trends. The hydrologic effects of this future development were evaluated with the runoff model by utilizing impervious areas consistent with the future development. Table 2-1 of the Urban Storm Drainage Criteria Manual (Ref. 1) was located at close intervals around bridges, culverts, and in other critical areas so that the effects of these structures on the backwater could be better evaluated. Form losses due to contraction and expansion of flow in channel cross sections were computed using coefficients of 0.1 and 0.3 for gradual transitions. For abrupt transitions coefficients of 0.3 and 0.6 were used. used to estimate impervious areas associated with various types of development. Channel roughness factors for various segments of the sections were selected Rainfall values obtained from the Weather Bureau publication previously mentioned were adjusted for depth-area and depth-duration relationships as dis- cussed in the same publication. The 500-year precipitation values were obtained by extrapOlation. Infiltration losses of .5 inch/hour were based on the hourly loss values as published by the Missouri River Division in a comprehensive loss on the basis of examination of available aerial photography. The following Manning's "n" values were assigned: Channel Field and Open Overbanks Tree and Underbrush Areas of Urban Development 0.036 - 0.044 0.035 - 0.045 0.05 - 0.06 0.10 Backwater computations for determination of flood profiles started at the Burlington Ditch weir, where critical depth conditions were assumed to prevail. Losses through bri dgesand cu1 verts were based upon ass umed parti a 1 obstruction of flow areas due to substantial collection of flood debris on the piers and submerged low steel. It is assumed that debris would have the equivalent effect of lowering low steel elevations by 2 feet and increasing the effective width of each pier up to as much as 3 feet, depending upon the shape, spacing, and size of the pier. Area reductions were also made in sections laid out through urbanized areas. The overbank areas of a section were decreased to reflect the study for the entire Missouri River Basin. Backwater studies were developed along Sand Creek using surveyed cross- section data taken during late 1974. The results of these backwater studies were used to develop the unsteady flow routing model for Sand Creek from the Arapahoe-Elbert County line to the mouth. The surface runoff model was broken into ten major subareas for input into the routing mode. Discharge profiles for present and future basin conditions are shown on Figures 1, 2 and 3. Hydraulic Analysis An analysis of the hydraulic characteristics of Sand Creek was carried out to provide estimates of the elevation of the 100-year flood at selected locations. actual amount of unblocked overbank area available to flow. The reduction of flow through the section was necessary to obtain realistic overbank velocities and head loss determinations.