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6.2.2 Water Surface Profiles e One-of-the-btgye~L i"'Pd(;L~ ur Lhe LTEC de~iYII pru(;t:~~ i~ LI,t: requi,.."ellL fur water surface profiles. Instead of one water surface profile, the LTEC design process requires water surface profiles for each discharge for existing conditions and for each alternative design condition. Unfortunately, most of the existing computer programs are not geared to the LTEC requirements. Existing programs include the CaE HEC-2 and the USGS E431. The FHWA program (HY -4) does not provi de suffic i ent water surface elevation definition to be used for risk analysis. HEC-2 has a special bridge routine as well as a general bridge routine, but the general bridge routine is often more applicable to design problems than the special routine. The E431 program has incorporated the FHWA backwater procedure and should provide reliable water surface profiles upstream of a bridge. One problem with either program is that a tremendous amount of printout must be scanned to pick up a few key elevations that are necessary for risk analysis. Also, items like overtopping depths and velocities are not outputs of these programs. FHWA is currently sponsoring a study with USGS to develop an updated bridge backwater program that will combine the best algorithms of E431 and HEC-2 and will provide convenient printout for a risk analysis. 6.2.3 Stage-Discharge Relationship If a computer is used to determine water surface profiles, rating curve information for each alternative can readily be obtained. The E431 program does not print the upstream water surface elevations right at the ~ bridge section, but it does print the key numbers needed to calculate the ~ elevations as follows: Where: WSBR; WSU - HF WSBR ; water surface elevation just upstream of bridge WSU ; water surface elevation at the approach section HF ; friction head loss between the approach section and the bridge The program prints HF for the natural condition, but it leaves a blank for HF in the bridge condition, therefore: HF ; DISTU(Q/KU)2 DISTU ; distance from bridge to the approach section where WSU is computed (usually DISTU ; Bridge Length) KU ; the upstream conveyance for the bridge condition If just a few alternatives (say four or five) are being considered, the rating curves should be plotted for each as indicated in figure 6.2a. Then overtopping discharges and overtopping depths can be determined graphically as illustrated. e 28