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inconveniences, and lost business opportunities. Therefore, a higher standard is warranted for the design of the bridge foundations to protect th.. from scour than is usually required for sizing of the bridge waterway area. This concept is reflected in the following design procedure which is to be applied to the preliminary bridge design sized to accommodate the design discharge. B . DES I liM PROCEDURE The design procedure for scour outlined below in the following steps is recommended for the proposed type, size, and location (TS and L) of substructure units: 1. Select the flood event(s) with return periods of 100 years or less that are expected to produce the most severe scour conditions. Experience indicates that this is likely to be the overtopping flood. Check the 100 year flood, the overtopping flood (if less than the 100 year flood) and other flood events if there is evidence that such events would create deeper scour than the 100 year or overtopping floods. 2. Develop water surface profiles for the flood flows in step 1, taking care to evaluate the range of potential tailwater conditions below the bridge which could occur during these floods. The FHWA WSPRO/HY7 Bridge Waterways Analysis Model (8) is recommended for this task. 3. Using the design procedures in Chapter 4 estimate total scour for the worst condition from steps 1 and 2 above. 4. Plot the total scour depths obtained in step 3 on a cross section of the stream channel and flood plain at the bridge site~ 5. Evaluate the answers obtained in steps 3 and 4. Are they reasonable, given the limitations in current scour estimating procedures1 6. Evaluate the bridge TS and L on the basis of the scour analysis perforled in steps 3-5. Modify the TS and L as necessary. o Visualize the overall flow pattern at the bridge site for the design conditions. Use this mental picture to identify those bridge elements most vulnerable to flood flows and resulting scour. o The extent of protection to be provided should be detennined by: - The degree of uncertainty in the scour prediction method. - The potential for and consequences of failure. - The added cost of making the bridge less vulnerable to scour. (Design measures for original construction are almost always less costly than retrofitting scour countermeasures at a later date.) 12 f, I I I I I I I I I I I I I I I I I I I ..~-', , , "