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<br />Occasional damage to high~ay approaches from rare floods <br />can be repaired rather quickly to restore traffic <br />service, On the other hand, a bridge which collapses or <br />suffers major structural damage from scour can create <br />safety hazards to motorists as well as large social <br />impacts and economic losses over a long period of time. <br />Aside from the costs to the highway agency of <br />replacing/repairing the bridge and constructing and <br />maintaining detours, there can be significant costs to <br />communities or entire regions due to additional detour <br />travel time, inconveniences, and lost business <br />opportunities, Therefore, a higher hydraulic standard <br />is warranted for the design of bridge foundations as a <br />protection against scour than is usually required for <br />sizing of the bridge waterway. This concept is <br />reflected in the following design procedure which is to <br />be applied to the bridge design sized to acccmmodate the <br />design discharge, <br /> <br />B. DESIGN PROCEDURE <br /> <br />The design procedure fer scour outlined in the following steps is <br />recommended for the pre,posed bridge type, size, and location <br />(TS&L) of substructure units: <br /> <br />1. Select the flood event(s) "ith =eturn Deriods c: 100 vears <br />or less that are expected to produce the most severe scour <br />conditions. Experience' indicates that this is likely to be <br />the overtopping flood which mayor may not be equal to the <br />lOO-year flood. Check the lOa-year flood, the overtopping <br />flood (if less t~an the laO-year flood) and other flood <br />events if there is evidence that such events would create <br />deeper scour than the 100 year or overtopping floods. <br /> <br />2, Develop water surface profiles for the flood flows in step <br />1, taking care to evaluate the range of potential tailwater <br />conditions below the bridge which could occur during these <br />floods. The FHW1. microcomputer soft'dare WSPRO, "Bridge <br />Waterways Analysis Model" (23), or the Corps of Engineers <br />HEC 2, are recomnended for this task. <br /> <br />3. Using the design procedures in Chapter 4, estimate total <br />scour for the worst condition from steps 1 and 2 above. <br /> <br />4. Plot the total scour depths obtained in step 3 on a cross <br />section of the si:ream channel and flood plain at the bridge <br />site. <br /> <br />5. Evaluate the ans\lers obtained in steps 3 and 4. Are they <br />reasonable, cons~dering the limitations in current scour <br />estimating procedures? The scour depth(s) adopted may <br /> <br />24 <br />