Inadequate knowledge and understanding of bridge scour increases risk to bridges and the public who use them. Scour processes include local scour, contraction scour, and channel instability. These processes are functions of iInzerreiated factors which must all be analyzed for a bridge site selected for complete scour evaluation. Contraction scour is the general lowering of the channel section due to flow acceleration through a flow section.
Local scour is the erosion due to redirected and contracted flow lines around piers or abutments.
Channel instability may be lateral, as channel migration or bank failure, or vertical, as aggradation or degradation. Likewise, the cause of the instability may be local or many miles removed. Channel instability is the primary process causing bridge failures in some basins and regions, particularly where there has been extensive channelization for flood control or channel mining. Hazards to bridges caused by contraction and local scour are compounded at sites experiencing significant vertical or lateral erosion from channel processes. It is, therefore, important in the evaluation of potential scour to assess not only local and contraction scour but also the role of general channel processes at a site.