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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The following guide is offered for conducting worst case analyses:

Water Surface Elevations

Information may not be available on the water surface elevations of the stream
at some bridges. This can be compensated for by using procedures developed by
the U.S. Geological Survey for'many states. These procedures provide for
estimating depths of flow by using hydrologic area, drainage area, flood
frequency and error of estimate. Using these procedures, a conservative depth-
discharge relationShip can be determined. This relationship can then be used
to develop rough estimates of scour.

Long Term Aggradation and Degradation

Long term stream bed profile changes will usually be difficult to assess. The
main information sources are the records and knowledge of bridge inspectors,
maintenance personnel, or others familiar with the bridge site and the behavior
of the stream and other streams in the general area. If aggradation or
degradation is a problem, there will usually be some knowledge of its
occurrence in the area. Cross sections of the stream at the bridge site, for
example, when taken by bridge inspectors over a periOd of time may indicate a
long term trend in the elevation of the stream bed. Field inspections should
be made at locations whe~e the streams are known to be active and where
significant aggradation/degradation or lateral channel movement is occurring.
Further discussion on long term stream bed elevation changes is included in
Chapters 2, 3, and 4. Particular attention should be given to bridges at
problem sites, as noted earlier in this section. Such bridges should be
reviewed in the field. Additional information on conducting field reviews is
included in Chapter 6.

Plan Form Changes

Assessing the significance of plan form changes such as the shifting location
of meanders, the formation of islands, and the overall pattern of streams,
cannot usually be accomplished in the office. Records and photographs of
bridge inspectors and maintenance personnel may provide some insight into the
nature of the stream for the initial office assessments. Historical aerial
photographs of the stream can be extremely valuable in this analysis.
Ultimately, an engineering judgement must be made as to whether possible future
or existing plan form changes represent a hazard to the bridge, and the extent
of field work required to evaluate this condition.

Contraction Scour

Contraction scour may be calculated using the equations in Chapter 4 where the
amount of overbank and main channel flow is known or can be estimated. The
worst case approach would involve estimating the largest reasonable amount of
overbank flow on the floodplain beyond the bridge abutments and then
calculating contraction scour on this basis. More detailed analyses are
recommended for bridges at problem sites, especially where a large difference
in the water surface elevations may exist upstream and downstream of the
bridge.

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