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I I I I I 01 I I I I I I I I I I I I I IV. HYDRAULIC ANALYSIS Computer runs were made in the upstream direction for flow in the sub- critical to critical depth mode, and in the downstream direction for flow in the supercritic~ mode. The flow was found to be subcritical at the lower end of the study area where backwater is created by the various culverts. Upstream, the flow tends to oscillate between subcritical and supercritical. The flow rates developed in the hydrologic analysis were then used in the hydraul i c anal ys i s to determi ne the peak water surface profil e and fl ood plain limits resulting from a 100-year return frequency storm within the Marcy Gulch drainage basin. As mentioned earlier, the existing small check dams within Marcy Gulch have been included in the hydraulic analYSis since their presence, although probably temporary in nature, could produce higher water surface profiles and wider flood pI ain 1 imits until such time as the check dams are washed out by flood overtopping. The HEC-2 computer program utilizes the Manning formula for analyzing the flood plain hydraulics. A Manning's "n" value of 0.035 was used both for the main channel and the overbank areas. Manning's "n" values ranging between 0.017 and 0.027 were used for the culvert and canal crossings respectively. Calculations of head losses utilized a contraction head loss coefficient of 0.3 and an expansion head loss coefficient of 0.5 at bridge of culvert-type road crossings. In the special bridge routine a pier shape coefficient of 1.25 (square nose and tail) was used for the Highline Canal supporting structure and also for U.S. Highway 85, D&RGW, and AT&SF struc- tures. A total head loss coefficient of 1.5 was assumed for use in the orifice flow equation between cross sections on either side of each culvert crossing and the canal crossing. A coefficient of 'discharge of 3.0 was assumed for use in the weir flow equation. Additional studies were performed to determine the maximum water surface profile and flood plain limits for backwater conditions which could result from the future construction of culvert-type roadway crossings at various locations along the water course. F100dway limits and water surface elevations were also evaluated for those areas where future encroachments within the overbank portions of the flood plain could cause a rise in the peak water surface e 1 ev at i on of up to, but not more than, 0.5 feet above the 100-year storm water surface elevation without encroachments. Flood pI ain cross sections were taken from 1 inch = 40 feet scale topo- graphi c maps. These cross sect ions were digit i zed and incorporated into the HEC-2 Water Surface Profiles computer program developed by the Corps of Engineers Hydrologic Engineering Center, Davis, California. On completion of the HEC-2 computer runs, ~he computed water surface profile and 100-year flood pI ain 1 imits were plotted on the Flood Hazard Area Del ineation drawings provided in the Appendix of this report. Back- water conditions resulting from tentative culvert-type road crossings have also been plotted. The flood pI ain data used for these plots as well as computed floodway data are tabulated on the Flood Plain and Floodway Reference Data sheets, Tables 3 and 4, which are also included in the Appendix. The culvert-type road crossing backwater conditions referred to above are tabulated separately in Table 4. The existing culvert-type road crossings at U.S. Highway 85, D&RGW and AT&SF Railroads, as well as the Highl ine Canal crossing were modeled using the HEC-2 special bridge routine. Tentative culvert-type road crossings were evaluated separately assuming that the individual culverts would be sized such that the resulting backwater wou1 d peak out at an el ev at i on three feet below the minimum roadway elevation at the crossing. Spillways might also be provided where necessary to assure that the maximum assumed backwater elevation is not exceeded. The actual sizing of these culverts and spillways is not needed for this analysis and should, therefore, be addressed in a later phase of planning. 9