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<br />6.2.4.2 Short-cut Method for Determining Overtopping Times <br /> <br />~ort-cUX-metnod for estlmatlng overtopplng tlmes is based on the assumption e <br />that hydrographs maintain approximately the same shape for all bridge conditions. <br />Then for a known overtopping discharge and depth, the overtopping time can be <br />estimated from a single family of either discharge or stage hydrographs, <br />whichever is more readily availble. Furthermore, since stage hydrographs may <br />be assumed parallel curves for various flood levels, overtopping times can be <br />estimated from a single hydrograph curve for any major flood. <br /> <br />Stage hydrographs are readily available at continuous gaging stations, while <br />discharge hydrographs are computed more readily for ungaged sites. Figure 6.4 <br />illustrates the use of a stage hydograph in conjunction with overtopping <br />depths to estimate overtopping times. The procedure is to scale a distance <br />equal to the overtopping depth from the peak of the stage hydrograph and to <br />measure time as the distance between the legs of the hydrograph. There is no <br />need to draw a whole family of parallel stage hydrographs since anyone would <br />give the same results using this procedure. Figure 6.5 illustrates the case of <br />a family of discharge hydrographs used in conjunction with overtopping discharges <br />to estimate overtopping times. The procedure is to draw a line through the <br />family of discharge hydrographs at the overtopping discharge and measure <br />overtopping times between the legs of the respective hydrographs along this <br />1 i ne. <br /> <br />e <br /> <br />. <br /> <br />e <br /> <br />32 <br />