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<br />Live-bed scour in sand ~ed streams with a dune bed configuration <br />fluctuates about the eq~ilibriun scour depth, The reason for <br />this is the variability of the bed material sediment transport in <br />the approach flow when the bed configuration of the stream is <br />dunes. In this case (d~ne bed configuration in the channel <br />upstream of the bridge), maximum depth of scour is abo~t 30 <br />percent larger than equilibrium depth of scour. <br /> <br />The maximum depth of scour is the same as the equilibrium depth <br />of scour for live-bed scour with a plain bed configuration, with <br />antidunes occurring upstream and in the bridge crossing the <br />maximum depth of scour from the limited research of Jain and <br />Fisher (17) is about 10 percent greater than the equilibrium <br />depth of scour. <br /> <br />For a discussion of bed forms in alluvial channel flow the reader <br />is referred to Chapter 3 of "Highvlays in the River Environment" <br />(7). Equations for estimating local scour at abutments or piers <br />are given in Chapter 5 of this publication. These equations were <br />developed from laboratory experiments and limited field data for <br />both clear-water and live-bed scour, <br /> <br />G. LATERAL SHIFTING OF A STRE&~ <br /> <br />Streams are dynamic, Areas of flow concentra~ion cc~~i~ually <br />shift bank lines, A meandering stream has its "S" shaped plan <br />form continually moving laterally and downstream. A braided <br />stream has its various channels continually changing. <br />Incidentally, the deepest natural scour occurs when two channels <br />of a braided stream carre together or when the flow comes together <br />downstream of an islanc or bar, This has been observed to be 5 <br />times the downstream flow depth, <br /> <br />A bridge is static. It fixes the stream at one place in time and <br />space. A meandering stream continues to move laterally and <br />downstream, eroding the approach embankment and affecting <br />contraction and local scour because of changes in flow direction. <br />A braided stream can s~ift its channels under a bridge, and have <br />two channels come toget.her at a pier or abutment, thus increasing <br />scour. Descriptions of stream morphology are given in "Highways <br />in the River Environment" (7) and in Hydraulic Engineering <br />Circular 20 (6). <br /> <br />Factors that affect lateral shifting and the stability of a <br />bridge are the geomorphology of the stream, location of the <br />crossing on the stream, bed and bank materials, flood <br />characteristics, the characteristics of the bed material and <br />washload discharge, <br /> <br />20 <br />