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<br />Comments <br /> <br />Would the same depth of scour occur at each pier: NO! <br />Could the pier foundations be set at different depths if there <br />was a substantial saving in cost: Yes. Why: Because it is in a <br />long straight reac:h, has stable banks upstream and downstream and <br />the channel flow :.s uniformly distributed across the width. It <br />only has the deep scour at pier three when there is overbank <br />flow. <br /> <br />STEP 7. PLOT AND EVALUATE TOTAL SCOUR <br /> <br />The plot of the scour ~or this problem is given in figure 4,11. <br />Note that the scour ho~es for the left abutment and pier 3 over <br />overlape if the abutment scour is 33 ft. <br /> <br />Evaluation of scour <br /> <br />1. It is doubtful tha~ with trees, grass and brush that abutment <br />scour will be as deep as 33 feet. But the overbank flow is very <br />large and the bed mate:~ial is sand, Also, construction will <br />disturb the area at the bridge, Thus, the banks will have to be <br />ripraped. Therefore use a guide bank with riprap on the left <br />abutment and riprap the right, <br /> <br />2. Set abutment founda.:ion depths at 7.3 feet plus soil mechanics <br />or ASSHTO STANDARDS WHICH EVER IS LARGER. The 7,3 feet is the <br />contraction scour plus long term degradation, <br /> <br />3, with the left abutment is protected with a guide bank, then <br />the scour holes at the left abutment and pier 3 will not over <br />ape. <br /> <br />3. with a loupe bank scour depths to be given to geotech <br />engineers is the 15.8 Et for pier 1 and 2 and 21.1 ft for pier 3. <br />As discussed earlier the channel being straight and lack of <br />overbank flow on the rLght side makes it possible to set piers 1 <br />and 2 at shallower dep.:hs. <br /> <br />4. An interdisciplinarI team consisting of hydraulic, <br />geotechnical and struc~ural engineer should review this bridge <br />configuration and scour depths. It might be advantageous to <br />widen the bridge openi:1g. But even a wider bridge would require <br />a guide bank on the left side. <br /> <br />B. Other Example Problem. <br /> <br />Appendix F presents th= scour analysis for the Great ?ee Dee <br />River. <br /> <br />72 <br />