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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 />IV. HYDRAULIC ANALYSIS <br /> <br />assumed backwater elevation is not exceeded. The actual sizing of these <br />culverts and spillways is not needed for this analysis and should, there- <br />fore, be addressed in a later phase of planning. <br /> <br />The flow rates developed in the hydrologic analysis were then used in the <br />hydraulic analysis to determine the peak water surface profile and flood <br />plain limits resulting from a 100-year return frequency storm within the <br />Dad Clark Gulch drainage basin. As mentioned earlier, the existing small <br />check dams within Dad Clark Gulch have been included in the hydraulic <br />analysis since their presence, although probably temporary in nature, could <br />produce higher water surface profiles and wider flood plain 1 imits until <br />such time as the check dams are washed out by flood overtopping. <br /> <br />Computer runs were made in the upstream direction only for flow in the sub- <br />critical to critical depth mode, although there were indications that <br />super-critical flow could occur in some of the steeper reaches and this <br />would result in higher velocities, lower water surface profiles and corre- <br />spondingly narrower flood plains than those calculated at some locations. <br /> <br />The HEC-2 computer program utilizes the Manning formula for analyzing the <br />flood plain hydraulics. A ~'anning's "n" value of 0.035 was used both for <br />the main channel and the overbank areas. Manning's "n" values ranging <br />between 0.015 and 0.017 were used for the culvert crossings. Calculations <br />of head losses due to gradual changes in the channel cross section utilized <br />a contraction head loss coefficient of 0.1 and an expansion head loss <br />coefficient of 0.3. At bridge or culvert-type road crossings these co- <br />efficients. were increased to 0.3 and 0.5, respectively. In the special <br />bridge routine a pier shape coefficient of 1.05 (twin cylinder piers <br />without diaphram) was used for the Highline Canal supporting structure, and <br />a total head loss coefficient of 1.5 was assumed for use in the orifice <br />flow equation between cross sections on either side of each culvert cross- <br />ing and the canal crossing. A coefficient of discharge of 3.0 was assumed <br />for use in the weir flow equation. <br /> <br />Additional studies were performed to determine the maXlmum water surface <br />profile and flood plain limits for backwater conditions which could result <br />from the future construct ion of culvert-type roadway crossings at various <br />locations along the water course. Floodway 1 imits and water surface ele- <br />vat ions were al so eval uated for those areas where future encroachments <br />within the overbank portions of the flood pl ain could cause a rise in the <br />peak water surface elevation of up to, but not more than, 0.5 feet above <br />the 100,year storm water surface elevation without encroachments. <br /> <br />Flood plain cross sections were taken from 1 inch = 40 feet scale topo- <br />graphic maps. These cross sections were digitized and incorporated into the <br />HEC-2 Water Surface Profiles computer program developed by the Corps of <br />Engineers Hydrologic Engineering Center, Davis, Cal Hornia, which has been <br />adapted for use on the Jack G. Raub Company's General Automation 16-440 <br />computer (refer to References E, F and G). <br /> <br />On completion of the HEC-2 computer runs, the computed water surface <br />profile and 100-year flood plain 1 imits were plotted on the Flood Hazard <br />Area Del ineation drawings provided in the Appendix of this report. Back- <br />water conditions resulting from tentative culvert-type road crossings have <br />also been plotted. The flood plain data used for these plots as well as <br />computed floodway data are tabulated on the Flood Plain and Floodway <br />Reference Data sheets, Tables 3 and 4, which are also included in the <br />Append ix. The culvert-type road cross i ng backwater cond it ions referenced <br />above are tabulated separately in Table 4. <br /> <br />The existing culvert-type road crossings at County Line Road and the <br />Highl ands Ranch access road as well as the Highl ine Canal crossing were <br />modeled using the HEC-2 special bridge routine. Tentative culvert-type <br />road crossings were evaluated separately assuming that the individual <br />culverts would be sized such that the resulting backwater would peak out at <br />an elevation three feet below the minimum roadway elevation at the crossing. <br />Spillways would also be provided where necessary to assure that the maximum <br /> <br />8 <br />