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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CHAPTER 3
DESIGNIII6 BRIDGES TO RESIST SCOUR

A. DESIGN PHILOSOPHY AND CONCEPTS

Bridges should be designed to withstand the effects of scour from a superflood
(a flood exceeding the 100 year flood) without failing. This requires careful
evaluation of the hydraulic, structural, and geotechnical aspects of bridge
foundation design.

The guidance in this chapter is based on the following concepts.

o The foundation should be designed by an interdisciplinary team of
engineers with expertise in hydraulic, geotechnical and structural
design.

o Hydraulic studies of bridge sites are a necessary part of the
preliminary design of a bridge. These studies should address both the
sizing of the waterway area and the designing of the foundations to
resist scour. The scope and depth of the analysis should be
commensurate with the i.portance of the highway and the consequences of
failure.

o Due consideration .ust be given to the limitations and gaps in existing
knowledge when using currently available fo~las for esti..ting scour.
The designer needs to apply engineering judgment in c~aring results
obtained from scour computations with available hydrologic and hydraulic
data to achieve a reasonable and prudent design. Such data might
include:

a. perfonnance of existing structures during past floods;
b. effects of regulation and control of flood discharges; and
c. hydrologic characteristics and flood history of the stream and
similar streUlS.

o The principles of economic analysis and experience with actual flood
damage indicates that it is almost always cost-effective to provide for
a foundation that will not fail, even from a very large flood event.

Occasional damage from rare floods to highway approaches can usually be
repaired rather quickly in order to restore traffic service. On the other
hand, a bridge which collapses or suffers major structural damage from scour
can create safety hazards to motorists as well as large social impacts and
economic losses over a long period of time. Aside from the costs to the
highway agency of replacing/repairing the bridge and constructing and
maintaining detours, there can be significant costs to communities or entire
regions due to additional travel time and distances on the detours,

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