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<br />4. Flow depth has a direct affect on scour depth. An increase in flow depth <br />can increase scour depth by a factor of 2 or larger for piers. With <br />abutments the increase is from 1.1 to 2.15 depending on the shape of the <br />abutment. <br /> <br />5. Velocity of the approach flow increases scour depth. The larger the <br />velocity the deeper the scour depth. There is also a high probability that <br />whether the flow is tranquil or rapid (subcritical of supercritical) will <br />affect the scour depth. Most research and data is for flows with Froude <br />Numbers much less than one (Fr. < 1 ). <br /> <br />6. Size of the bed material in the sand size range has no affect on scour <br />depth. larger size bed material if it will be moved by the approaching <br />flow or by the vortices and turbulence created by the pier or abutment .nll <br />not affect the ultimate or maximum scour but only the time it takes to <br />reach it. Very large particles in the bed material, cobbles or boulders, <br />may armor plate the scour hole. The size of the bed material also <br />determines whether the scour at a pier or abutment is clear-water or live- <br />bed scour. This topic is discussed later in this section. <br /> <br />Fine bed material (silts and clays) will have scour depths as deep or <br />deeper than sandbed streams. This is true even if bonded together by <br />cohesion. The affect of cohesion is to determine the time it takes to <br />reach the maximum scour. With sand bed material the maxi~ depth of <br />scour is measured in hours. With cohesive bed materials it may take <br />days, months, or even years to reach the maximum scour depth. <br /> <br />7. Angle of attack of the flow to the pier or abutment has a large affect on <br />local scour as was pointed out in the discussion of the affect of pier <br />length above. The affect on piers will not be repeated here. Wit~ <br />abutments the depth of scour is reduced for embankments angled downstre.. <br />and is increased if the embankments are angled upstream. According to the <br />work of Ahmad, the maximum depth of scour at an embankment inclined 45 <br />degrees downstream is reduced by 20 percent, whereas, the scour at an <br />embankment inclined 45 degrees upstream is increased about 10 percent. <br /> <br />8. Shape of pier or abutment has a significant affect on scour. With a pier, <br />streamlining the front end reduces the strength of the horseshoe vortex <br />reducing scour depth. Streamlining the downstream end of piers reduces the <br />strength of the wake vertices. A square-nose pier will have maximum scour <br />depths about 20 percent larger than a sharp-nose pier and 10 percent larger <br />than a cylinder or round-nose pier. Abutments with vertical walls on the <br />streamside and upstream side will have scour depths about double that of <br />spill slope abutments. <br /> <br />9. Bed configuration effects the magnitude of local scour. In streams with <br />sand bed material the shape of the bed (bed configuration) as determined by <br />Simons and Richardson (1963) may be ripples, dunes, plane bed and <br />anti dunes. The bed configuration depends on the size distribution of the <br />sand bed material, flow conditions, and fluid viscosity. The bed <br />configuration may change from dunes to plane bed or anti dunes during an <br />increase in flow. It may change back with a decrease in flow. The bed <br /> <br />8 <br /> <br />'I <br />I <br /> <br />I' <br />t I <br />I <br />I <br />I <br />I <br />I <br />I <br />~(I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />( I <br />I <br />