Failure Behavior of Riprap Layer at Bridge Piers under Live-Bed Conditions

Experiments conducted under live-bed conditions show that a riprap layer at a cylindrical bridge pier will fail in either one of the following two modes: Total disintegration or embedment. The former refers to the break-up of the entire riprap layer where the stones are washed away by the flow field generated at the pier. The latter relates to the embedment of the riprap layer where it is buried in the sediment bed. The study proposes a criterion to demarcate the limiting condition between the two types of failure. It also identifies that embedment failure is a more common failure mode of riprap layer under live-bed conditions. The causes of embedment failure are twofold: (1) bed feature destabilization; and (2) differential mobility. Bed level fluctuations caused by the propagating bed features resulted in bed feature destabilization, whereas differential mobility is due to the different response of the riprap stones and bed sediments to the flow field. Experimental results also show that the riprap layer can degrade to an equilibrium level for a given flow condition. Finally, the study proposes a semiempirical equation to compute the maximum depth of riprap degradation, which occurs at the upper end of dune regime.