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<br />CHAPTER 4 <br /> <br />ESTIMATING SCOUR AT BRIDGES <br /> <br />I. INTRODUCTION <br /> <br />In this chapter methods and equations are given for determining <br />total scour at a bridge; i.e., long-term aggradation or <br />degradation, contraction scour and local scour. At the end of <br />the chapter example problems are given. <br /> <br />Before the various scour forecasting methods for contraction and <br />local scour can be applied, it is first necessary to 1) obtain <br />the fixed-bed channel hydraulics, 2) estimate the long-term <br />profile degradation or aggradation, 3) adjust the fixed-bed <br />hydraulics to reflect these changes, and 4) compute the bridge <br />hydraulics. <br /> <br />II. DESIGN APPROACH <br /> <br />The seven steps recommended for estimating scour at bridges are: <br /> <br />STEP l. <br />STEP 2 . <br />STEP 3 . <br />STEP 4. <br />STEP 5. <br />STEP 6. <br />STEP 7. <br /> <br />Determine scour analysis variables, <br /> <br />Analyze long term bed elevation change. <br />Evaluate the scour analysis method, <br />Compute the magnitude of contraction scour. <br />Compute the magnitude of local sccur at abutments. <br />Compute the magnitude of local scour at piers. <br /> <br />Plot the total scour depths <br /> <br />The procedures for each of the steps, including recommended scour <br />equations, are discussed in detail. <br /> <br />III. DETAILED PROCEDURES <br /> <br />A. STEP 1. DETERMINING SCOUR ANALYSIS VARIABLES <br /> <br />1. Determine the magnitude of the discharges for the floods in <br />Step 1 of the Design Procedure, Chapter III, including the <br />overtopping flood when applicable. If the magnitude of the <br />SOO-year flood is not available, use a discharge equal to <br />1.7 X Ql~' Experience has shown that the incipient <br />overtopplng discharge often puts the most stress on a <br /> <br />31 <br />