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i , , CHAPTER VI HYDRAULIC A.~ALYSIS ofthclo'atersurfaceprofileproceed,illst"ps, frolnonecrosssectionto the next. The HEC-2 prog,,,,,, is also capable of handling the effect of th" various types of hydraulic structures which restrict the flow, A. General Additional detailed hydraulic calculations were nl<1de at road ero"sing~ where the drainage structure capacitywa. small in cOIllparison [0 the floli' Over the road. Rand calculations \o'ere also required in the areas where the water surface prOfile is rapidly varied. Water surface profiles have been computed for the study reach for 5., 10-, 50-, 100- and 500-yea< frequency flOOds. The hydraulic characteristics of the study reach were documented based on three separate field investigations. Culvert and bridge geometry, channel and overbank roughness coefficients, flow obstructions, and the overall characteristics of the area were assessed during these investigations. The Manning's roughness coefficients used in the hydraulic analysis were documented in a separate report prepared as part of the technical addendum. Photographs were inclUded in the report to support the recommended vslues. Reco~ended roughness coefficients ranged frOm 0,030 to 0.045 for the channel and hom 0.025 to 0.065 for .he ovnbank areas. The results of the flOOd computations are presented in Table-6. The elevation. computed at the Cross sections listed in Table-6 wcre used to draw the "ater surface profiles shown on Plates II through 15. The IOO-year "atet surface profile "as then used to determine the flOOded area from the IOO-YearfloodshOl<nonPlates I through 10, Figur,,-4.hOwsan indexrnap for the flooded area plates. Plates 16 and 17 show a nUmb~r of typical croS" sections of the stream and valley ~n the study ar~a. The"ntersurfaeeelevatio""pn'sent<'dinTJble_6"cebasedo"compu- tations ,.hic" assu"", no reduelio" in the bcidge convcyance capabilities d"" to debris. Water su,face elevations O1,'y in"H'",e if si8"ifit:,,"t dt:bris blockagesllOuldoccur. B. Method of Analysis The water surfaCe elevations for ~he 5-, 10-, 50-, 100-, and 500-year flo~ds were c~~puted using the Corps of Engineers HEC-2 Step Backwater Computer Pr~gr~. This program utilizes 4 solution to tbe one-dimensional energy e~uation to deter~ine the shape of the pr~file ~et~een the control section where the "ater surfaCe elevati~n is known or can be assumed. The pr~c"dure for a steady flow profile calculation is called the "Standard SteP Method." In this method, the distance from a do;.tIlstream or upstream poine, where the conditions are known, to the poine where the back"atet effects are to be dete~ined, is divided into teaeh~s by cross Sections at fixed locations "long the river. Starting trQ~ one connol point. calculatiQn~ -43- -44-