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I I I I I I I I I I I I I I I I I I I IV. HYDRAULIC ANALYSIS assumed backwater elevation is not exceeded. The actual sizing of these culverts and spillways is not needed for this analysis and should, there- fore, be addressed in a later phase of planning. The flow rates developed in the hydrologic analysis were then used in the hydraul ic analysis to determine the peak water surface profile and flood plain 1 imits resulting from a 100-year return frequency storm within the Dad Clark Gulch drainage basin. As mentioned earl ier, the existing small check dams within Dad Clark 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 plain 1 imits until such time as the check darns are washed out by flood overtopping. Computer runs were made in the upstream direction only for flow in the sub- critical to critical depth mode, although there were indications that super-critical flow could occur in some of the steeper reaches and this would result in higher velocities, lower water surface profiles and corre- spondingly narrower flood plains than those calculated at some locations. Flood plain cross sections were taken from 1 inch = 40 feet scale topo- graphic maps. These cross sections were digitized and incorporated into the HEC-2 Water Surface Profiles computer program developed by the Corps of Engineers Hydrologic Engineering Center, Davis, Cal i-fornia, which has been adapted for use on the Jack G. Raub Company's General Automation 16-440 computer (refer to References E, F and G). The HEC-2 computer program utilizes the Manning formula for analyzing the fl ood pl ain hydraul ics. A Manning's "n" val ue of 0.035 was used both for the main channel and the overbank areas. Manning's "n" val ues ranging between 0.015 and 0.017 were used for the culvert crossings. Calculations of head losses due to gradual changes in the channel cross section utilized a contraction head loss coefficient of 0.1 and an expansion head loss coefficient of 0.3. At bridge or culvert-type road crossings these co- efficients were increased to 0.3 and 0.5, respectively. In the special bridge routine a pier shape coefficient of 1.05 (twin cylinder piers without diaphram) was used for the Highline Canal supporting structure, and 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 cross- ing and the canal crossing. A coefficient of discharge of 3.0 was assumed for use in the weir flow equation. Add it iona 1 st ud ies were performed to determ ine the max imum water surface profile and flood plain 1 imits for backwater conditions which could result from the future construction of culvert-type roadway crossings at various locations along the water course. Floodway limits and water surface ele- vat ions were al so eval uated for those areas where future encroachments within the overbank portions of the flood plain could cause a rise in the peak water surface el evation of up to, but not more than, 0.5 feet above the 100-year storm water surface elevation without encroachments. The existing culvert-type road crossings at County Li.ne Road and the Highl ands Ranch access road 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 would peak out at an elevation three feet below the minimum roadway elevation at the crossing. Spillways would also be provided where necessary to assure that the maximum On completion of the HEC-2 computer runs, the computed water surface profile and 100-year flood plain 1 imits were plotted on the Flood Hazard Area Del ineaUon drawings provided in the Appendix of this report. Back- water conditions resulting from tentative culvert-type road crossings have also been plotted. The flood plain 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 Append ix. The culvert-type road cross ing backwater cond it ions referenced above are tabulated separately in Table 4. 8