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

B. SCHEDULING OF WORK TO CONSTRUCT SCOUR COIJNl'EIIOSURES

The engineering scour evaluation study should address the degree of hazard to be
expected at a scour critical bridge so that priorities and schedules can be
prepared for necessary installations ,of scour countermeasures. In some cases the
hazard may be obvious, as where an inspection reveals that a spread footing for a
pier has been partially undermined. Immediate action is warranted. In other
cases, the need for immediate action is not so apparent, and considerable
judgement must be exercised in deciding upon the best course of action. An
example of the latter case is where a stream meander is gradually encroaching
upon a bridge abutment. Judgments must be made in this instance on the rate of
change of the meander and the probable effect as the main channel flow is
directed toward the abutment.

Problems are commonly encountered with this type of condition involving gradual
changes when the engineer waits too long to take action. As the degree of
encroachment and scour hazard increases, the number of alternative effective
countermeasures is decreased and costs of correction are corresponding increased.

C. MONITORING. INSPECTING. AND CLOSING SCOUR CllmCAI. BRIDGES

As noted in Chapter V special attention should be given to monitoring scour
critical bridges during and after flood events. The plan of action for the
bridge should include special instructions to the bridge inspector, including
guidance as to when the bridge should be closed to traffic. Guidance should be
given to other DOT officials on when bridge should be closed. The intensity of
the monitoring effort should be related to the degree of the scour hazard as
determined from the scour evaluation study. The following items are recommended
for consideration when developing the plan of action monitoring effort:

1. Information on any existing rotational movement of abutments and piers or
settlement of foundations.

2. Information on rates of stream bed degradation, aggradation, or lateral
movement based on analysis of changes in streaa cross sections taken during
successive bridge inspections, sketches of the stream plan form, aerial
photographs, etc.

3. Recommended procedures and equipment for taking measurements of stream bed
elevations (use of rods, probes, weights, etc.) during and after floods.

4. Maximum permissible scour depths, flood flows, water surface elevations etc.
and beyond which the bridge should be closed to traffic.

5. Reporting procedures for handling excess scour, larger than normal
velocities, water surface elevation or discharge that may warrant bridge
closure. Who makes closer dicisions and how are they implemented?

6. Instructions regarding the checking of stream bed levels in deep channels
where accurate measurements cannot be made from the bridge (use of divers,
electronic instruments such as sonar, radar, etc.).

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