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<br />Pier area--The area of bridge piers or pile bents occupying the waterway is <br /> <br />determined at the downstream :!ide of the bridge,. Normally, the piers and <br /> <br />pile bents do not occupy more than about 3 percent of the waterway. The <br /> <br />effect of bridge piers and pile bents on the flow area at the bridge is <br /> <br />described by Bradley (1973) and is measured by the relation <br /> <br />j c Aj / A <br /> <br />Where: j is the ratio of pieJ: area to gross area of waterway, and <br /> <br />Aj is area of piles OJ~ piers in the waterway, and <br /> <br />A is the cross-sectional area of the stream. <br /> <br />Bridge submergence--During extreme floods, or when bridge waterways are <br /> <br />blocked by debris, submergencl! of a bridge may occur. The amount of <br /> <br />submergence (t) is measured all the depth of water above the "low steel" or <br /> <br />bottom of the bridge stringer <br /> <br />described by Bradley (1973). <br /> <br />The effects ~f bridge submergence on flow are <br /> <br />Froude nwnber--The Froude nwnher, a measure of the specific energy of <br /> <br />streamflow, is computed by thE! relation <br /> <br />F VaF' <br /> <br />Where: Va is mean velocity of flow <br /> <br />g is the gravitational constant, and <br /> <br />da is mean depth of st. ream. <br />