Numerical-experimental modelling of local scouring downstream of protected bridges in alluvial river beds.

Beatriz Nacher-Rodriguez, Francisco J. Vallés-Morán, Ángel Balaguer-Beser, Mayte Capilla

Tuesday 30 june 2015

14:35 - 14:50h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 6A. Sediment - Erosion

Local scouring due to flooding in the vicinity of structures in river basins is one of their main causes of failure. This fact has severe social and economic consequences, including loss of human life. Standard procedure to protect these structures is to place elements that are able to withstand the action of the flow, such as rip-rap or concrete slabs, which implies a change in the river bed rigidity. When it comes to the case of bridges, alongside the change in rigidity, contraction of the flow also occurs due to the narrowing and obstruction caused by piles and abutments. Combination of both phenomena results, in real cases, in scour holes downstream the protected area. Their dimensions can be large enough to affect foundations, and even may lead to the collapse of the structure. To study this phenomenon, a simplified case of the real problem has been analysed by means of laboratory experiments, carried out in a sand bed open channel. Different contraction ratios and flow characteristics are covered. The effect of the protection is assessed conducting the same experiments with and without a rigid, non-erodible element in the contracted area. When comparing both cases, it can be observed how the erosive mechanisms downstream the structure are significantly different. In those cases with a protected bed, scour holes are larger than those in the cases without rigid zone. Furthermore, it has been determined that, although shear stresses in this zone are not (theoretically) large enough for incipient motion of particles, important scour is actually being developed. This means that flow contraction altogether with a change in bed stiffness increases the vulnerability towards erosion of this material. Data obtained from this experimental campaign is used to calibrate a mobile-bed numerical model based on a high-order well-balanced finite volume scheme, which provides an accurate solution in the vicinity of the structure. This program, that models sediment transport by means of classic formulation, is capable of successfully simulate erosion due only to flow contraction. As future modifications, mathematical expressions that change the resistance to erosion of the material affected by the structure will be implemented, so the model will be able to represent erosion and deposition in cases of protected bridges.