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<br />average velocity in the flow zone below the top of <br />the footing <br />distance from the bed to the top of the footing <br />the grain roughness of the bed. Normally taken as <br />the D84 of the bed material, <br /> <br />The values of Vf and Yf would be used in the CSU equation given <br />above. <br /> <br />where <br />Vf = <br />Yf = <br />ks = <br /> <br />4. pier Scour for Exoosed pile Grouos <br /> <br />Sterling Jones (32) also conducted experiments to determine <br />guidelines for specifying the characteristic width of a pile <br />group (Figure 4.8) that are or may be exposed to the flow when <br />the cylinders are spaced laterally as well as longitudinally in <br />the stream flow. He concluded the following: <br /> <br />"Pile groups that project above the stream bed can analyzed <br />conservatively by representing them as a single pier width <br />equal to the projected area of the piles ignoring the clear <br />space between piles, Good judgement needs to be used in <br />accounting for debris because pile groups tend to collect <br />debris that could effectively clog the clear spaces between <br />pile and cause the pile group to act as a much larger mass," <br /> <br />For example, five l6-inch cylindrical piles spaced at 6 feet <br />(Figure 4.8) would have an "a" value 6.67 feet, This composite <br />pier width would be used in Equation 3 to determine depth of pier <br />scour. The correction factor "k," in Equation 3 for the multiple <br />piles would be 1,0 regardless of shape. The depth of scour for <br />exposed pile groups will be analyzed in this manner except when <br />addressing the affect of debris lodged between piles. If debris <br />is evaluated, it would be logical to consider the multiple <br />columns and debris as a solid elongated pier. The appropriate <br />Lja value and flow angle of attack would then be used to <br />determine k2 in Table 4.4 <br /> <br />5. Multiole columns <br /> <br />Multiple columns (as illustrated as a group of cylinders in <br />Figure 4.7) skewed to the flow the scour depth depends on the <br />spacing between the piers. The correction factor for angle of <br />attack would be smaller than for a solid pier. How much smaller <br />is not known. Raudkivi (9) in discussing effects of alignment <br />states "..the use of cylindrical columns would produce a <br />shallower scour; for example, with five-diameter spacing the <br />local scour can be limited to about 1.2 times the local scour at <br />a single cylinder." <br /> <br />55 <br />