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<br />that large particles will decrease scour is not clearly <br />understood. <br /> <br />The size of the bed material also determines whether the <br />scour at a pier or abutment is clear-water or live-bed scour. <br />This topic is discussed later in this chapter. <br /> <br />Fine bed material (silts and clays) will have scour depths as <br />deep as sandbed streams. This is true even if bonded <br />together by cohesion, The affect of cohesion is to influence <br />the time it takes to reach the maximum scour. with sand bed <br />material, the tim8 to reach maximum depth of scour is <br />measured in hours and can result from a single flood event. <br />with cohesive bed materials it may take days, months, or even <br />years to reach the maximum scour depth, the result of many <br />flood events. <br /> <br />7. Angle of attack of the flow to the pier or abutment has a <br />significant affect on local scour, as was pointed out in the <br />discussion of pier length. Abutment scour is reduced when <br />embankments are angled downstream and increased when <br />embankments are angled upstream. According to the work of <br />Ahmad, the maximum depth of scour at an embankment inclined <br />45 degrees downstream is reduced by 20 percent, whereas, the <br />maximum scour at an embankment inclined 45 degrees upstream <br />is increased about 10 percent, <br /> <br />8. Shape of the nose of a pier or an abutment has a significant <br />affect on scour. Streamlining the front end of a pier <br />reduces the strength of the horseshoe vortex, thereby <br />reducing scour depth. Streamlining the downstream end of <br />piers reduces the strength of the wake vortices. A square- <br />nose pier will have maximum scour depths accut 20 percent <br />greater than a sharp-nose pier and 10 percent greater than <br />either a cylindrical or round-nose pier. <br /> <br />Full retaining abutments with vertical walls on the <br />streamside (parallel to the flow) will produce scour depths <br />about double that of spill-through abutments. <br /> <br />9. Bed configuration effects the magnitude of local scour. In <br />streams with sand bed material, the shape of the bed (bed <br />configuration) as determined by Richardson et al (15) may be <br />ripples, dunes, plane bed and antidunes. The bed <br />configuration depends on the size distribution of the sand <br />bed material, flow conditions, and fluid viscosity. The bed <br />configuration may change from dunes to plane bed or antidunes <br />during an increase in flow for a single flood event. It may <br />change back with a decrease in flow. The bed configuration <br />may also change with a change in water temperature or change <br />in suspended sediment concentration of silts and clays. The <br />type of bed configuration and change in bed configuration <br /> <br />17 <br />