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<br />Hydraulic An"lysis <br /> <br />Hydrnulic <In"ly$is in this study entails primarily the com-- <br />putation of the baCKwat~T within the ~tudy reaches of the river. The <br />hasicdata for the study was obtained from different governrnental agen- <br />cies and field investigations. Topo~raphic mapping with a sca.lc of 1 <br />inch equal to 200 feet and 2 feet contour interval ~ere supplied by the <br />Colorado Waler Conservation Buard. Th.. data was reviewed in depth to <br />detenlline its adequacy nnd accuracy. Malor sourc~s of err"r when <br />applying hydraulic theory are inadequacies in d"~a which can result in <br />an improper modeling of flow conditions. Also. calibrating a si~u1ation <br />flo~ model requires adequate data of reasonable accuracy. <br /> <br />running parallel to the floodplain, etc. are needed to model the river <br />terrain as accurately as possible. The model was calibarted using the <br />discharge rating curvcs for Gunniaon River near Gunnis"ll and Tonichi <br />Creek near Gunnison gagin~ st"tion~. The hydraulic roughness parameter <br />for th~ channel IInd overbank for different reaches were estimated from <br />roughness data of oth~r riv~rH with similar characteristics and was <br />further verified by field Observations of the entire river reach. <br /> <br />Digiti"ed cross secti.ons ~ere secured when the topo~raphic <br />maps Wllre compiled. These cros" sections were augmented by field- <br />surveyed cross sections for typical renchea of the river. The location <br />of the croas sections arc shown on the flood area mnps (Plates 23-31) <br />and the flood profiles (Plates 15-22). The cross sections. strenmbed <br />elevations, and water surface elevations for 10-, 50-, 100-, and 500- <br />year frequencies is shown in Table 6, <br /> <br />A detailed description of che hydraulic a""lysis is included <br />in the Technical Addendum. <br /> <br />The water ~urface elevations for the 10-, 50-, 100- and 500- <br />year recurranCe interval floods were computed utilizing the Corps of <br />Engineers HEC-2 step backwater computer program. The computation of <br />water surf lice profiles involves solving the one-dimensional energy <br />equation to determine the shape of the profile between control sections <br />where the water surface elevation is known or can be assumed. The <br />generally applicable procedure for steady flow profile calculation is <br />called the "Standard Step Method". In this method. the distance from II <br />downstream or upstream point where conditions are known, to th~ point <br />where b"ck~ater eHeets are to bc dc_tl':nnined. is divided into r{!aches by <br />cross RectionR at fixed locations along the river and, st~rting from one <br />control, profilc calculations proceed in steps from cross "ection to <br />cross section to the next control. HEC-2 program is capable of IIlso <br />handling the eHect of various structures "-cross the river such as <br />bridgeR. culverts, weirs and "mbankm<'nts. <br /> <br />The basic data to run a backwater study are the geo.."tri" <br /> <br />~a~a. ~~ic~ incluJe cr03a 3ceticns of the ~i.cr ",c~~~rcc .crr~~dic~l~r <br /> <br />to the direction of flow at suitable intervals and at points where the <br />geometry of the river changes. .110n!; with th<, distance bet~een ~h" cross <br />sections. Detailed tnpographic mapH sho~lng floodplain. obstructions in <br />th.. floodplain thst would constrict the fl"", or restrict the floodpla:ln, <br />structure" "cross tho dVH ,,,,d their d..tail~, dikes. road.. and railroad <br /> <br />" <br /> <br />23 <br />