Evaluation of riprap failure impact on the downstream scour hole

Scouring around bridge piers is considered to be a significant process in rivers because it can alter bridge loading and consequently its stability. Riprap is often deployed as scour countermeasure, and while it does protect the pier from local scouring, it doesn’t completely solve the scouring problem because it deflects the scour hole downstream of the bridge. Riprap is flexible, and flood events can induce five significant failure mechanisms - shear, edge and winnowing failure under clear-water conditions and bedform-induced and bed-degradation-induced failure under live-bed conditions. On the other hand, the thick riprap layer can withstand a partial failure of the layer with the capability of armouring the scour hole. This paper investigates the mechanisms of riprap partial collapsing and its effects on the development of a downstream scour hole. Experiments were conducted on the physical model of scouring around bridge piers protected with riprap built in the Department of Hydroscience and Engineering laboratory under the University of Zagreb. Experimental setup included different pier shapes (rectangular and circular piers) in order to examine the influence of the pier as well as the influence of riprap geometry in different flow conditions.

Acknowledgments

This work has been supported in part by Croatian Science Foundation under the project R3PEAT (UIP-2019-04-4046).