FLOOD06847

• County: Statewide
• Basin: Statewide
• Title: Bridge Scour and Stream Instability Countermeasures
• Date: 7/1/1997
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

because of a large flood, a shifting thalweg, or from other actions of the stream or human
activities. Where the investment in a highway crossing is not in imminent danger of being
lost, it is often prudent to delay the installation of countermeasures until the magnitude and
location of the problem becomes obvious.

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Thus, for stream instability countermeasures, a "wait and see" attitude may constitute the
most economical approach. Retrofitting can be considered sound engineering practice in
many locations because the magnitude, location, and nature of potential instability problems
are not always discernible at the design stage, and indeed, may take a period of several
years to develop.

4.2 Countermeasure Design Approach

The bridge scour and stream instability countermeasures matrix (Table 1) helps define the
set of specific countermeasures that are best suited to specific site conditions. The
countermeasures matrix is intended, primarily, to assist with the selection of an appropriate
countermeasure. Consideration of potential environment impacts, maintenance,
construction-related activities, and legal aspects can be used to refine the selection. The
final selection criteria, and perhaps the most important, are the initial and long-term costs.
The countermeasure that provides the desired level of protection at the lowest total cost may
be the "best" for a particular application.

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The following principles should be followed in designing and constructing stream instability
and bridge scour countermeasures:

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· The initial and long-term cost should not exceed the benefits to be derived.
Permanent countermeasures should be used for important bridges on main roads
and where the results of failure would be intolerable. Expendable works may be
used where traffic volumes are light, alternative routes are available, and the risk
of failure is acceptable.

· Designs should be based on studies of channel trends and processes and on
experience with comparable situations. The environmental effects of the
countermeasures on the natural channel both up- and downstream should be
considered.

· Field reconnaissance by the designer is highly desirable and should include the
watershed and river system up- and downstream from the bridge.

. Evaluation of time-sequenced aerial photography is a useful tool to detect long-
term trends.

· The possibility of using physical model studies as a design aid should receive
consideration at an early stage.

. Countermeasures must be inspected periodically after floods to check
performance and modify the design, if necessary. The first design may require
modification. Continuity in treatment, as opposed to sporadic attention, is
advisable. The condition of the countermeasure should be documented with
photographs to enable comparison of its condition from one inspection to another.

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