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<br />Ys <br />2.8 <br /> <br />= 0.13 [ <br /> <br />11,600 <br />1 7 <br />0.001233 2.86 200 <br /> <br />6 <br />]7 <br /> <br />- 1 <br /> <br />(7) <br /> <br />Ys = 9.20 X 2.8 ft = 25.6 ft <br /> <br />Comments <br />This amount of scour is not unreasonable but can be decreased by <br />using spill-through abutments. with spill-through abutments, using <br />Froehlich's correction factor, the scour would be 55 % of the <br />vertical wall scour or 14 ft. Laursen's correction for spill-through <br />abutments is too conservative. Also, the foundations can be set at a <br />higher depth if the abutment is protected with riprap. <br /> <br />will not use relief bridge because of cost. <br /> <br />STEP 5. LOCAL SCOUR AT ABUTMENTS <br /> <br />Scour at abutments in main channel. <br /> <br />Use Froehlich's equation to calculate scour depths. <br /> <br />..::Ls- = 2. 27 K, <br />Ya <br /> <br />K2 (~) 0.43 <br />Ya <br /> <br />Fr 0.61 + 1 <br />. <br /> <br />K, = coefficient for abutment type <br />K2 = (0/90)0.13 <br />a' = length of abutment intercepting overbank flow <br /> <br />a '=A.lYa <br /> <br />A. = flow area of the approach cross-section <br />obstructed by the embankment. <br />Fr.= Froude number of approach flow upstream of the <br />abutment. <br /> <br />67 <br />