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<br />CHAPTER'll <br />HYDRAULIC A~ALYSlS <br /> <br />Step Backwater Computer Program (Reference 3). This program utilites a solution <br />to the one dimensional energy equation to determine the shape of the ora file be- <br />tween control section ~here the water surface elevation is known or can be assumed. <br />The procedure for a steady flow profile calculation is called the "Standard <br />Step Method." lnthislllCthod, the distance from a do;mstrcamor upstream <br />point where the conditions are known, to the point where the backwater effe~ts <br />are to be determined, is divided into reaches by crOSS sections at fixed loca- <br />tions a10ng the creek. Starting from One control point, calculations of the <br />water surface profile proceed, in stepa, from one CrOSS section to the next. <br />The HEC-2 program is also capable of handling the effect of the various hydraulic <br />structures, such as bridges and culverts, wbicb restrict tbe flow. <br /> <br />A. General <br /> <br />The hydraulic analysis entailed computation of water surface profiles <br />within the study reacb fot tbe 10-, 25-, 50-, 100-, and 500-year frequency <br />flows. These elevations arc listed in Table-4. <br /> <br />T~o field investigations were conducted to determine the hydraulic <br />characteristics of the study reach. Bridge hydraul.c characteristics, channel <br />and bank roughness coefficients, and the overall features of tbe area were <br />determined during these investigations. A separate report was prepared as <br />part of the technical addendum wbich documents tbe Manning's roughness coeffi- <br />cients used in tbis analysis. Photographs of typical reaches were .ncluded <br />to support tbe finsl selected values. The values of roughness coefficients <br />ranged from 0.045 to 0.080 for the cbannel and frnm 0.045 to 0.060 for tbe <br />overbank areaS. <br /> <br />The results of the flood computationa ar~ presented in Table-4. The <br />flooded areas from a IOO-yeat freqoency flOOd are presented on Plate 1 through <br />4. The flood profiles are shown on Plates 5 through 9. Plates 10 and 1\ show <br />a number of typical cross sections of tbe strea~ and valley in thc study area. <br /> <br />Tbe water surface elevations presented in Table~4 are based on c~~putations <br />.Jbich assume no reduction in the btidge conveyancc capabilities due to debris. <br />Water sorface elevat.ons may increase if significant debris blockage should <br />occur. <br /> <br />A detailed description of the procedures used ,n the hydraulic analysis <br />is incloded in the technical addendum. <br /> <br />B. Proceduus <br /> <br />The initial phases of thchydraulic atlalysisc<>ncerned thedeterm inatioo <br />of the potent.al for divided flows. ~e roads and railroads which traverse <br />tbe Coal Creek floodplain provide significant barriers to flood tlows such <br />tllat'later is diverted and separated in certain areas. Wherewaterwassepa- <br />~at~d into distinct flo'l paths, a separate water surfaee profile "as computed <br />for cacb path. <br /> <br />C. Flooding Characteristics <br /> <br />Based on the 100-year peak flows, a briet descriptien of the characteristics <br />of flooding in the study reach follows: <br /> <br />The water surface elevations for tbe 10-, 25-, 10_, 100-, and ';o()-y~,H <br />reCllrrCnrc intervsl floods were c<>mputed "sing th"Corps of EnginHrs HEC-2 <br /> <br />~s tbc flOOd approaches Perry Street (Pla,e-4), tbe toad ~nd railroad <br />embankments will obstruct the flow. Tbe capacity of the Perry Street bridge <br /> <br />-)9- <br /> <br />-40- <br />