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<br />Flow Factors <br /> <br />Discharge--Streamflow data at a bridge site are usually determined form <br /> <br />records obtained at Geologica.l Survey gaging stations. At some sites, <br /> <br />indirect measurements of flo~ls, or results of special studies such as bridge <br /> <br />site analyses, are required. <br /> <br />Recurrence interval--The recu.crence interval of the design flood may be <br /> <br />determined from streamflow re,:ords or historic data. The best statistical <br /> <br />analysis is based on a series of annual maximum floods (historic data) so that <br /> <br />the computed flood recurrence interval is the average interval of time within <br /> <br />which the given flood will be equalled or exceeded as an annual maximum. The <br /> <br />analysis of the flood data is generally based on procedures described by the <br /> <br />U.S. Water Resources Council :1976). <br /> <br />Depth--The mean (average) depoh of flow at a bridge (da) may be computed by <br />the relation <br /> <br />da AIL <br /> <br />Where: .A is the cross-sectio~al area of stream normal to flow, and <br /> <br />L is length of bridge at design water surface, adjusted for skewness. <br /> <br />The hydraulics of the bridge ~pening is determined at the most constricted <br /> <br />location in the channel, generally at the downstream side of a bridge <br /> <br />(Hatthai, 1976). <br /> <br />o <br />