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Physical Modelling of Scour Around Bridge Piers Caused by Flood Waves

Author(s): Oscar Link, Sebastian Henríquez, Bernd Ettmer

Linked Author(s): Oscar Link

Keywords: Bridge pier scour, physical modelling, dimensional analysis, unsteady hydraulics, loose boundary hydraulics

Abstract:

Scour around bridge piers of complex geometry and around pier groups cannnot be estimated following standard procedures, except for ideal conditions. Moreover, during flood waves, parameters typically involved in scour estimation, such as the flow intensity (U / Ucr with U the average flow velocity and Ucr the critical velocity for initiation of sediment motion), vary widely in time and cannnot be used in a straight forward manner for estimation of scour with existent scour formulas. Under such circumstances, the local scour problem should be studied in a physical model at reduced scale, less than 1:25 ratio. Typically in physical models of river flows, Froude similitude is required. For models with sediment transport geometric scaling of the sediment particles is however an issue when the model sediment achieve sizes as small as to present cohesive forces, typically in cases with sand in nature. Thus, the scalability of the scour process is by far not trivial, and has not been resolved yet. Thus, we present a dimensional analysis of the problem, identifying a consistent scaling criterion for the flow and for the sediment. This criterion is tested through carefully designed laboratory experiments, conducted at the Laboratories for Hydraulic Engineering of the University of Concepción, Chile, and at the Leichtweiß-Institut für Wasserbau, Technische Universität Braunschweig, Germany. Test was conducted in three different flumes having different sizes and piers of different diameters, with five sediments of different sizes and densities. Our results show that the dimensionless effective flow work, W* is a suitable criteria for scaling the effect of river flows on the local scour process around a pier in a given sediment. The dimensionless sediment diameter D* is also a suitable criteria for scaling sediments in order to achieve similar scour depths under similar flow conditions. (2629, 65, 309)


DOI:

Year: 2017

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