FLOOD03064

• County: Statewide
• Basin: Statewide
• Title: Evaluating Scour at Bridges
• Date: 11/1/1990
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Floodplain Report/Masterplan

CHAPTER 3

DESIGNING 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 lOO-year flood) with little
risk of 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 design of a bridge, These studies should address
both the sizing of the waterway opening and the
designing of the foundations to resist scour. The scope
and depth of the analysis should be commensurate with
the importance of the highway and the ccnseq~ences of
failure.

o Due consideration must be given to the limitations and
gaps in existing knowledge when using currently
available formulas for estimating scour. The designer
needs to apply engineering judgment in comparing results
obtained from scour computations with available
hydrologic and hydraulic data to achieve a reasonable
and prudent design. Such data should include:

a. Performance of existing structures during past
floods,

b, Effects of regulation and control of flood
discharges,

c. Hydrologic characteristics and flood history of the
stream and similar streams, and

d. Whether the bridge is structurally continuous.

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

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