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CHAPTER VI HYDRAULIC ANALYSIS proceed, in steps, from one cross section to the next. The HEC-2 pro~ram is also capable of handling the effect of the various hydraulic struttures, .uch 38 bridges and cul~erts, ~hieh reatrict the flo~. The hydraulic ~n~lysi. in this study entailed the cornpu~ation of ~ater surface profiles ~ithin the study reach for the 10- and loo-year frequency flo~s. The results of the 10- and laO-year frequency flood computations arc presented in Tahle-5. The flooded areas from a 100-yesr frequency flood arc pre.ented on Plates 1 through 4. Th"floodprofiles are shownonPlate85 through 7. Plate 8 8ho~8 a number of typical cross see cion. of the stream ~nd ~alley in the study area. Topographic m3pping, with a scale of I inch equal to 100 feet and 2 foot contour intervals, was provided by Elbert County in cooperation ~ith the Town of Elizabeth. This ~opographie mapping ~as photo-reviaed to reflect the lsnd usecon<litions in the study area aa of June 1979. Channelcrossaectlonswere field survcycd at intcrvals along the study rea~h by Elbert County and the CWCB. Water surface profiles were determined using the cross sections, the channel and bank roughncss coefficients, the bridge hydraulic characteristics, and the flOod flows for the selected frequencies presentcd in ~he hydrologic analysis phase of this study. Water surface elevations presented in Table-j are based on compotations which ties. aSsume no reduction in the State High~ay 86 htidge conveyance capabili- The bridge has adequste freeboard, long spans, and streamlined piers which shoold minimize the accumulacion of debris during flood flo~ periods. A detailed description of the hydraulic analysis i. included in the tech- nical <lddendum. T~o field investigations were conducted to determine the hydraulic charac. teristic. of thc study reach. During the initial <lurvey, photograph. of the ~hannel and valley "'cre taken. Photographs of typical section. ~rc pr"scnted on Pagc~ 12 through 13. FroOl th~se photograph. and the initial field "\lTvey, the Toughness ~harRcteristi~s of the channel and valley were determined. The valucs for the Manning Roughness Cocfficient ~sed in this analy.is ranged fro~ 0.028 to 0.034 for the channel and from 0.040 to 0.085 for the overbank area.. The ~ater surface elevati,)[,s for the 10- and 100-year reCuHeClce intenal floods wer" comput"d usi.nf, th~ Corps of Engineers HEC-2 Step !\a"k~at"r ComplltH rrogr"m. ThiG program utili~c~ .l solution to the ",,,,,-dimensional energy equa- tion to determcne the shape of the profile between control section where the w<lter surface elevation is known or can be "ssumed. The proccd"Tc for a s~eady f1o~ proHle ""lclllation is c31led the "Standard Step Method". In this method the distance from a d~"'nstream or upstream point, where the conditions are known, to the point where the haekwater eHects arc to be determined, is Jividcdintoreach<.'sbycro.ss<.'ctionsatfixedloc.ltionsalcngthe river. S~artinf'; from onc CO:'1lrol poin~, calculation. of the ~"tct ""rface profile -24- -25-