Effects of bed degradation on riprap protection at bridge piers

Scouring around bridge piers results from a combination of local, general and contraction scour mechanisms. To counteract local scouring of sediment, armouring countermeasures such as riprap are commonly used. The riprap protects the bed from erosional forces close to the pier. General and contraction scour result in degradation of the whole channel bed. This study aims to assess the impact of this degradation on riprap layers installed to prevent local scouring. Experiments, conducted in a laboratory flume, were used to identify changes to riprap stability and failure mechanisms caused by a steady reduction in the sediment bed level. Both a uniform and a non-uniform bed material were tested. It was observed that degradation of the sediment bed allowed the riprap layer to be undermined and riprap stones to subside into the sediment bed. After substantial degradation of the channel bed the riprap stone layer disintegrates and the stones no longer protect the pier from local scour.