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<br />3. pier Scour for EXDosed Footinas <br /> <br />Often the pier footings and/or pile groups become exposed to the <br />flow by scour, This m"y occur either from long term degradation, <br />contraction scour, local scour or lateral shifting of the stream. <br />Computations of local pier scour depths for footings or pile caps <br />exposed to the flow based on footing or pile cap width appear to <br />be too conservative. I'or example, calculations of scour depths <br />for the Schoharie Cr, bridge failure were closer to the measured <br />model and prototype scour depths when pier width was used rather <br />than footing width. Even in this case where the footing top was <br />at the elevation of the bed surface the calculated depths were 47 <br />percent larger than thE' measured (22ft vs. 14 and 15 ft) (31) . <br />It appeared that the footing decreased the potential scour depth, <br /> <br />A recent model study 01' scour at the Acosta bridge at <br />Jacksonville, Florida by Sterling Jones (32) found that when the <br />top of the footing was flush with the stream bed local scour was <br />20 percent less than for other conditions tested. The other <br />conditions were bottom of the footing at the bed surface, the top <br />of the footing at the water surface with pile group exposed and <br />top of footing at mid depth, In a generalized study he found <br />that a footing with a :.ip extending upstream of the p~er reduced <br />pier scour when the top of the footing was located flush or below <br />the bed but scour holes became deeper and larger in proportion to <br />the extent that the footing projected into the flow field. <br /> <br />Based on his study he nade che following recommendation for <br />calculating pier scour if the footing is or may be exposed to the <br />flow (32). <br /> <br />"It is recommended that the pier width be used for the value <br />of 'a' in the pier scour equations if the top of the footing <br />is at or below the streambed (taking into account <br />contraction scour;. If the pier footing extends above the <br />stream bed, make a second computation using the width of the <br />footing for the value of a and the depth and averaqe <br />ve10citv in the f~ow zone obstructed bv the footinq for the <br />'v I and I V I respec:ti vel v in the scour equation. Use the <br />larqer of the two scour computations" <br /> <br />To determine 'V' obstructed by the flow J.ones gave the following <br />equation: <br /> <br />Vf <br />= <br /> <br />V, <br /> <br />y '/ <br />ll1 (10. 93-,-i-1) Ill: (10,93 Ok' +1) <br />Ks 5 <br /> <br />(10 ) <br /> <br />54 <br />