The entrainment of riprap elements placed around a bridge pier is investigated, using previous research relative to the Shields' entrainment function for turbulent rough flow, and its modification due to the presence of a bridge pier. A novel failure definition is offered first, based on the loss of a riprap element in the immediate surrounding of the pier. Then, the effects of approach flow conditions and pier geometry are separated from the effects of relative riprap size and number of riprap layers placed concentrically around the pier. This concept is verified with preliminary experiments conducted for a sediment mixture to result in a plot in which the individual effects of riprap and sediment diameters, sediments layers, relative bridge pier diameter, density ratio between sediment and fluid phases, and relative approach flow depth are related to the entrainment upstream velocity under which riprap failure occurs. This diagram contains also a division line between the two failure modes observed, namely riprap sliding into the adjacent sediment erosion, and riprap rolling out from the conglomerate. This paper may thus be considered a first step for the definition of a generalized riprap failure equation in relation to bridge piers.
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