FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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C. CHECKLIST OF DESIGN CONSIDERATIONS

General

1. Raise the bridge superstructure elevation above the general elevation of
the approach roadways wherever practicable to provide for overtopping of
approach embankments and the relief it affords from the hydraulic forces
acting on the bridge. This is particularly important for strealS carrying
large amounts of debris which could clog the waterway area of the bridge.

2. Superstructures should be securely anchored to the substructure if buoyant,
drag, debris, and ice forces are probable. Further, the superstructure
should be shallow and open to minimize resistance to the flow where
overtopping is likely.

3. Continuous span bridges withstand forces due to scour and resultant
foundation movement better than si~le span bridges because they give
alternate load paths (redundancy) for unbalanced forces caused by
settlement and/or rotation of the foundations. This design is especially
recommended for bridges where there is a significant scour potential,
because the potential for span collapse is significantly reduced.

4. Local scour holes at piers and abutments should not overlap. The top width
of a scour hole is about 2.75 times the depth of scour. If scour holes
overlap the local scour can be deeper.

5. For pile and drilled shaft designs subject to scour, consideration should
be given, when practicable, to using a lesser number of longer piles or
shafts as compared with a greater number of shorter piles or shafts to
develop bearing loads. In general, this approach will provide a greater
factor of safety against pile failure due to scour at little or no increase
in cost.

Piers

1. Pier foundations on floodplains should be designed in the same manner as
the pier foundations in the stream channel if there is a likelihood that
the channel will shift its location on the floodplain over the lifespan of
the bridge.

2. Align piers with the direction of flood flows. Assess the hydraulic
advantages of round piers, particularly where there are complex flow
patterns during flood events.

3. Streamline pier shapes to decrease scour and mini~ize potential for buildup
of ice and debris. Use ice and debris deflectors where appropriate.

4. Evaluate the hazards of ice and debris buildup when considering use of
multiple pile bents in stream channels. Where ice and debris buildup is a
problem, design the bent as though it were a solid pier for purposes of
e~timating scour. Consider use of other pier types where clogging of the
waterway area could be a major problem.

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