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<br />STEP 7. PLOT THE TOTAL SCOUR DEPTHS AND EVALUATE THE DESIGN <br /> <br />A. Plot the Total Scour Depths <br /> <br />On the cross-section of the stream channel and floodplain at the bridge <br />crossing plot the estimate of 1) long term bed elevation change; 2) contraction <br />scour; and 3) local scour at the piers and abubnents. Use a distorted scale so <br />that the scour determinations will be easy to evaluate. Make a sketch of any <br />plan form changes (lateral stream channel movement due to meander 8igration, <br />etc) that might be reasonably expected to occur. <br /> <br />1. Long term elevation changes may be aggradation or degradation. <br /> <br />2. Contraction scour is then plotted from and below the long term aggradation <br />or degradation lines depending on the methods of analysis (Method 1 or <br />Method 2.) <br /> <br />3. Local scour is then plotted from and below the contraction scour line <br />depending on the method of analysis (Method 1 or Method 2). Plot not only <br />the depth of scour at each pier and abutment but also the width. The. width <br />can be determined by assuming the bottom of the.scour hole is 5 feet wider <br />than the pier and using the angle of repose of the bed IIIIterial COIIIllOnly <br />assumed to be 200 for sand bed stream to get the side slope of the bole. <br /> <br />B. Evaluate the Total Scour Depths <br /> <br />o Are the scour depths reasonable and consistent with previous <br />experience? <br /> <br />o Do the local scour holes from the piers or abutments intersect or come <br />close to each other between spans? If so, Method I should be <br />considered for the scour analysis. Further, the length of the bridge <br />opening should be reevaluated and the opening increased or the number <br />of piers decreased as necessary. <br /> <br />o Are there other factors (lateral movement of the stream, scour hole <br />armoring, stream flow hydrograph, velocity and discharge distribution, <br />moving of the thalweg, shi.fting of the flow direction, channel changes, <br />type of stream, etc.) to be considered? <br /> <br />o Evaluate cost, safety etc. <br /> <br />o Determine if countermeasures may be needed and if they are cost- <br />effective and should be incorporated into the-design. Can they be used <br />at a later date if needed but before total failure of the support system <br />occurs? (See Chapter 7.) <br /> <br />42 <br /> <br />I <br />I <br />t:> <br />"'.1 <br />'\-." <br /> <br /> <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />A-I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />