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 2
BASIC CONCEPTS AND DEFINITIONS OF SCOUR

A. GENERAl

Scour is the result of the erosive action of running water, excavating, and
carrying away material from the bed and banks of streams. Different materials
scour at different rates. Loose granular soils are rapidly eroded under water
action while cohesive or cemented soils are .ore scour resistant. However,
ultimate scour in cohesive or cemented soils can be as deep, or deeper, than
scour in sandbed streams. Scour will reach its maxi.u. depth in sand and
gravel bed materials in hours; cohesive bed materials in days; glacial tills,
sand stones and shales in months; limestones in years and dense granites in
centuries. Massive rock formations with few discontinuities can be highly
resistant to scour and erosion during the lifetime of a typical bridge.

Designers and inspectors need to carefully study site specific subsurface
information in evaluating scour potential at bridges, giving particular
attention to foundations on rock.

8. TOTAL SCOUR

Total Scour at a highway crossing is comprised of three components. These
components are:

1. A~gradation and Degradation. These are long term stream bed elevation
c anges due to natural or man induced causes within the reach of the river
on which the bridge is located. Aggradation involves the deposition of
material eroded from other sections of a stre.. whereas degradation
involves the lowering or scouring of the bed of a stream.

2. General Scour and Contraction Scour. This type of scour involves the
removal of material from the bed and banks across all or most of the width
of a channel. General scour can result from a contraction of the flow, a
change in downstream control of the water surface elevation, or the
location of the bridge in relation to a bend. In each case, the scour is
caused by increased velocities and resulting increased bed shear stresses.
The most common form of general scour at a bridge is caused by the approach
embankments to the bridge encroaching onto the floodplain and/or into the
main channel with resulting contraction of the flow. This type of general
scour is commonly known as contraction scour.

3. Local Scour. This type of scour involves removal of material from the
channel bed or banks and is restricted to a minor part of the width of a
channel. This scour occurs around piers, abutments, spurs, and
embankments and is caused by the acceleration of the flow and the
development of vortex systems induced by the obstructions to the flow.

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