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<br />Velocity--The mean velocity of flow through bridge constructions (VA) is <br /> <br />computed by the relation <br /> <br />Va - Q/A <br /> <br />Where: Q is discharge of stream and <br /> <br />A is the cross-sectional area of stream at the downstream side of the <br /> <br />bridge. <br /> <br />The gross waterway area is used in computing the mean velocity of flow, <br /> <br />without subtracting the area of piers; this is the procedure followed by <br /> <br />Bradley (1973), when computing the effects of the bridge conntraction. To <br /> <br />estimate the potential for scour, the net waterway opening should be used. <br /> <br />Contraction ratio--The effect of a constricted bridge opening and the approach <br /> <br />embankments of the passage of main channel and overbank flow is measured by <br /> <br />the bridge contraction ratio (M) (Bradley, 1973) <br /> <br />M - Qb/Qt <br /> <br />Where Qb is the unconstricted discharge. <br /> <br />The value of the ratio is one if there is no contraction of flow. <br />