SCOUR AT BRIDGES

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material from around the abutments and piers which support the bridge and bed and bank

material of the stream the structure crosses. Both scour at highway structures and stream

migration (instability) can cause a bridge failure.

All material in a streambed will erode. It is just a matter of time. However, some material such

as granite may take hundred's of years to erode. Whereas, sandbed streams will erode to the

maximum depth of scour in hours. Sandstone, shales, and other sedimentary bedrock

materials, although they will not erode in hours or even days will, over time, if subjected to the

erosive forces of water, erode to the extent that a bridge will be in danger unless the

substructures are founded deep enough. Cohesive bed and bank material such as clays, silty

clays, silts and silty sands or even coarser bed material such as glacial tills, which are

cemented by chemical action or compression, will erode if subjected to the forces of flowing

water. The erosion of cohesive and other cemented material is slower than sand bed material,

but their ultimate scour will be as deep if not deeper than the scour depth in a non-cohesive

sandbed stream (Briaud et al. 1999). It might take the erosive action of several major floods

but ultimately the scour hole will be equal to or greater in depth than with a sand bed material.

This does not mean that every bridge foundation must be buried below the calculated scour

depth determined for non-bedrock streams. It does mean that so-called bedrock streams must

be carefully evaluated.

Scour at bridge crossings is a sediment transport process. Long-term degradation, general

scour, and local scour at piers and abutments result from the fact that more sediment is

removed from these areas than is transported into them. If there is no transport of bed

material into the bridge crossing, clear-water scour exists. Transport of appreciable bed

material into the crossing results in live-bed scour. In this latter case the transport of the bed

material limits the scour depth. Whereas, with clear-water scour the scour depths are limited

by the critical velocity or critical shear stress of a dominant size in the bed material at the

crossing.

7.1.2 Costs of Bridge Failure from Scour

Hydraulic factors (scour/ice/debris) cause 60 percent of bridge failures in the United States

(Shirole and Holt 1991). In the United States there are over 580,000 bridges in the National

Bridge Inventory. These numbers include federal highway system, state, county and city

bridges. Approximately 84 percent of these bridges are over water. Bridge failures cost

millions of dollars each year as a result of both direct cost necessary to replace and restore

bridges, and indirect costs related to disruption of transportation facilities. However, of even

greater consequence is loss of life from bridge failures. Chang, in a 1973 study for the

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