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Perhaps most important of all input parameters are the cross- sections. To model the characteristics of the major thalweq, the study area was divided into 15 reaches described cnrlier wi th " total of 94 cross-sections. Of these, 44 were field surveyed cross-sections. Surveyed cross-sections arc indicated by a star on the floodplain drawings. This amount of definition was made necessary by the lack of any natural water course and the relatively flat cross-slopes coming down to the thalweg. In the absence of a clearly defined and consistent valley-section, any local topographic details can have significant affects on flood water limits, therefore necessitating the additional field surveyed cross-sections to include as many of these details as possible. These In mathematically describing the basin, the effects of irriga- tion ditches and existing road culverts were igna~ed. As stated previouSly in this study, the irrigation ditches presently affect minor runoff pattQrns. However, as the areas upstrean of th", ditches between tho West Vine Basin and the paudre River develop, storm runoff overflows of the ditches wil) most likely occur locally at o~ ncar the points of runoff inflow concentration points; thus eliminating .';ign; f;<:'nnt "",aunts of storr" !"u!'_off i",portation to or exportation from the West Vine Basin. ~herefore, any excess capacity in the irrigation ditches was ~eglected. Th~ existing crainagc culverts are generally of such small size or have been so poorly maintained that they convey only a tiny fraction of even the 2 year event. For this reason they were not incorporated into the model. V.C. Divergent Flows Reaches R,U,M,T & Y have divergent flows. Since HEC-2 calCUlates the water surface from downstream to upstream,it cannot be used to directly solve for the amount of flow lost in a diverging channel. By manipulating divisions ~nd reach discharges and making m~ximum use of the p~og=am's capability to det~rmine overbank flow, it w~s pOSSible to evaluate these diversions. Various methods were used to analyze each flow split area. -30- methods arR described as follows: 1. Reach(>s T and Y l<edclles ']' & Y in the undeveloped ar<-'<1 of Section 10 Were cdsiest. t,o deal with. Tn t.hose reaches, the flow was split by a mid-stream ridge which occurred at a specific cross-section. The overbank and main channel discharge components were analyzed for several cross-sections upstream from the split and probable discharges were determined. Reach U In reach U, flow overtops either the C & S rail- road track or the Fort Collins Irrigation Ditch bank for a length of approximately 1200 feet. The ground falls away quickly from the levee created by the tracks and the ditch bank, leaving 2. nO backwater to inhibit overflow. Tt) model this condition, the left overbanks of the five appropriate cross-sections were located along the ridge line of this levee. Very short left bank distances between sections were used to approximate the lack of backwat,er. A s",,,i.,,,s of computer runs using different discharge rates were then made, starting with critical depth at the downstream section. At each section that had overbank flow, the remaining channel flow was determined. The remaining channel flow rate was then compared with total discharge rates from the series of '-'o"lp\ltf'r rUn~. Thf' run wi.th '" rl;schaT']p r'lt.P th<lt most closely matched the remaining channel flow rate was used to plot the next downstream water surface elevation. In this manner, the resulting floodplain was plotted upstream to do~strcam. 3. Reach M , e h M,~ sligM1y <liffprent overbar.k condition ,n T_ile existed. For over 1000 feet Vine Drive is overtopped, -~1-