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<br />will effect flow velocity, sediment transport, and scour. <br />"Highways in the River Environ~ent" (7) discusses bed <br />configuration in detail, <br /> <br />10. Ice and debris potentially increase the width of the piers, <br />change the shape of piers and abutments, increase the <br />projected length of an abu~ment and cause the flow to plunge <br />downward against the bed. This can increase both the local <br />and contraction scour. The magnitude of the increase is <br />still largely undetermined, Debris can be taken into account <br />in the scour equations by estimating how much the debris will <br />increase the width of a pier or length of an abutment, <br />Debris and ice affects on contraction scour can also be <br />accounted for by estimating the amount of flow blockage <br />(decrease in width of the bridge opening) in the equations <br />for contraction scour. Li~ited field measurements of scour <br />at ice jams indicate the scour can be as much as 10 or 20 <br />feet. <br /> <br />F. CLEAR-WATER AND LIVE-BED SCOUR <br /> <br />There are two conditions for contraction and local scour, These <br />are (1) clear-water scour and (2) live-bed scour. Clear-water <br />scour occurs when there is no movement of the bed material of the <br />stream upstream of the crossing, but the acceleration of the flaIl <br />and vortices created by the piers or abutments causes t~e <br />material in the crossing to move, Live-bed scour occurs when the <br />bed material upstream of the crossing is moving. <br /> <br />Typical clear-water sccur situations include: (1) course bed <br />material streams, (2) flat gradient streams during low flow, (3) <br />local deposits of larger bed materials that are larger than the <br />biggest fraction being transported by the flow (riprap is a <br />special case of this si tuation), (4) armored stream beds where <br />the only locations that tractive forces are adequate to penetrate <br />the armor layer are at piers and/or abutments and (5) vegetated <br />channels where, again, the only locations the cover is penetrated <br />is at piers and/or abutments, <br /> <br />During a flood event, bridges over streans with coarse bed <br />material are often sub~ected to clear-water scour at low <br />discharges, live-bed scour at the higher discharges and then <br />clear-water scour on the falling stages, Clear-water scour <br />reaches its maximum over a longer period of time than live-bed <br />scour, Figure 2,2. This is because clear-water scour occurs <br />mainly in coarse bed material streams. In fact clear-water scour <br />may not reach a maximuM until after several floods. Maximum <br />clear-water scour is about 10 percent greater than the maximum <br />live-bed scour. <br /> <br />18 <br />