Bridge scour

Bridge scour is the removal of sediment such as sand and rocks from around bridge abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.

In the United States, bridge scour is one of the three main causes of bridge failure (the others being collision and overloading). It has been estimated that 60% of all bridge failures result from scour and other hydraulic-related causes. It is the most common cause of highway bridge failure in the United States, where 46 of 86 major bridge failures resulted from scour near piers from 1961 to 1976.

Water normally flows faster around piers and abutments making them susceptible to local scour. At bridge openings, contraction scour can occur when water accelerates as it flows through an opening that is narrower than the channel upstream from the bridge. Degradation scour occurs both upstream and downstream from a bridge over large areas. Over long periods of time, this can result in lowering of the stream bed.

Stream channel instability resulting in river erosion and changing angles-of-attack can contribute to bridge scour. Debris can also have a substantial impact on bridge scour in several ways. A build-up of material can reduce the size of the waterway under a bridge causing contraction scour in the channel. A build-up of debris on the abutment can increase the obstruction area and increase local scour. Debris can deflect the water flow, changing the angle of attack, increasing local scour. Debris might also shift the entire channel around the bridge causing increased water flow and scour in another location.

The most frequently encountered bridge scour problems usually involve loose alluvial material that can be easily eroded. However, one should not assume that total scour in cohesive or cemented soils will not be as large as in non-cohesive soils, the scour takes longer to develop.

Many of the equations for scour were derived from laboratory studies, for which the range of applicability is difficult to ascertain. Most studies focussed on piers and pile formations, though most bridge scour problems are related to the more complex configuration of the bridge abutment. Some studies were verified using limited field data, though this is also difficult to accurately scale for physical modelling purposes. During field measurements of post scour, a scour hole that had developed on the rising stage of a flood, or at the peak, may be filled in again on the falling stage. For this reason, the maximum depth of scour cannot be simply modelled after the event.

...
Wikipedia