13 Facts About Bridge scour

Bridge scour is the removal of sediment such as sand and gravel from around bridge abutments or piers.

Hydrodynamic Bridge scour, caused by fast flowing water, can carve out Bridge scour holes, compromising the integrity of a structure.

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.

Many of the equations for Bridge scour were derived from laboratory studies, for which the range of applicability is difficult to ascertain.

When considering Bridge scour it is normal to distinguish between non-cohesive or cohesionless sediments and cohesive material.

Live-bed Bridge scour occurs where the upstream shear stress is greater than the threshold value and the bed material upstream of the crossing is moving.

The equilibrium Bridge scour depth is achieved when material is transported into the Bridge scour hole at the same rate at which it is transported out.

Contraction scour occurs over a whole cross-section as a result of the increased velocities and bed shear stresses arising from a narrowing of the channel by a construction such as a bridge.

Local Bridge scour arises from the increased velocities and associated vortices as water accelerates around the corners of abutments, piers and spur dykes.

Local pier Bridge scour begins when the downflow velocity near the stagnation point is strong enough to overcome the resistance to motion of the bed particles.

FHWA recommends design criteria in HEC-18 and 23, such as avoiding unfavourable flow patterns, streamlining the abutments, and designing pier foundations resistant to Bridge scour without depending upon the use of riprap or other countermeasures.

The total scour depth is determined by adding three scour components which includes the long-term aggradation and degradation of the river bed, general scour at the bridge and local scour at the piers or abutment.