Scour is the hole left behind when sediment (sand and rocks) is washed away from the bottom of a river. Although scour may occur at any time, scour action is especially strong during floods. Swiftly flowing water has more energy than calm water to lift and carry sediment down river.
Three types of scour affect bridges:
1) Local scour is removal of sediment from around bridge piers or abutments. (Piers are the pillars supporting a bridge. Abutments are the supports at each end of a bridge.) Water flowing past a pier or abutment may scoop out holes in the sediment; these holes are known as scour holes.
2) Contraction scour is the removal of sediment from the bottom and sides of the river. Contraction scour is caused by an increase in speed of the water as it moves through a bridge opening that is narrower than the natural river channel.
3) Degradational scour is the general removal of sediment from the river bottom by the flow of the river. This sediment removal and resultant lowering of the river bottom is a natural process, but may remove large amounts of sediment over time.
