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<br />In application of the CSU equation with multiple columns, the <br />pier width "a" is the ':otal projected '.iidth of all the colunns in <br />a single bent, normal ':0 the flml angle of attack, For example, <br />three 24-inch cylindrical columns spaced at 10 feet would have an <br />"a" value ranging between 2 and 6 feet, depending upon the flow <br />angle of attack. This composite pier width would be used in <br />Equation 3 to determinlo depth of pier scour. The correction <br />factor Uk," in Equatior 3 for the multiple column would be 1,0 <br />regardless of column shape. The depth of scour for a multiple <br />column bent will be analyzed in this manner except when <br />addressing the affect of debris lodged between columns. If <br />debris is evaluated, i~ 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 kz in Table 4,4 <br /> <br />Additional laboratory studies are necessary to provide guidance <br />on the limiting flow a:1g1es of attack for given distance between <br />multiple columns beyond which multiple columns can be expected to <br />function as solitary rn~mbers with minimal influence from adjacent <br />columns. <br /> <br />6. Pressure Flow .ocour <br /> <br />Pressure flow at a brijge occurs when bridge decks in~ersects <br />the flow or are subnerged, Flume studies a~ Cclcra~c St3~e <br />University were conducted in the spring of 1990 with a bridge <br />deck partly submerged, a single pier in the flume, with different <br />distances from the str:am bed and the deck and different flow <br />velocities. There was no sediment transport upstream of the <br />bridge (clear water sc~ur) (33). \Vithout the deck submerged there <br />was no contraction SCOJr and local scour occurred. \Vith the deck <br />submerged there was contraction scour and scour depths at the <br />pier were increased by a factor of two or three. The magnitude of <br />the contraction and local scour, as was to be expected, depended <br />on the velocity of the approach flow and the distance from the <br />deck to the bed. For the same approach velocity, contraction <br />scour and local pier scour increased as the distance from the bed <br />to the deck decreased, Analysis of the results of these <br />experiments and possible additional laboratory study will be <br />necessary to define the impact of bridge submergence on <br />contraction and local scour, <br /> <br />7. width of Scour Holes <br /> <br />The top width of a scour hole in cohesionless bed material from <br />one side of a pier or footing can be estimated from the following <br />equation: <br /> <br />56 <br />