Scour hole formation for lateral non-uniform flow in non-cohesive sediments

Abstract

For a project in the province of Zeeland, The Netherlands, a culvert was installed trough a dike to restore the tidal flow. Shortly after commissioning of the facility a scour hole at the end of the bottom protection developed. Although the scour hole was anticipated in the design, the scour hole was developing more rapidly than predicted. Furthermore, the hydrodynamics of the tidal jet flow from the culvert were different than what was expected in the design. It is expected that the uncertainties in predicting the scour hole process lies probably the most in a lack of understanding of the flow conditions in the vicinity of scour holes.

The scour hole depths can be predicted by the Breusers empirical relations. The development of the scour process entirely depends on the flow velocity and the relative turbulence intensity at the transition of the fixed to the erodible bed. The empirical equations do not account for all the hydrodynamic processes in a scour hole. Furthermore, the scour studies are predominantly considering a two-dimensional vertical situation in which lateral uniform flow is assumed. However, in practice lateral non-uniform flow is the rule rather than the exception.

The objective of this master thesis is to research the scour development under laterally nonuniform flow and to obtain more fundamental understanding of the scour process. Physical experiments were conducted in order to investigate the influence of lateral non-uniformities in the flow field on the scour process. In the scale experiments, the lateral velocity differences originated from two parallel streams with different streamwise velocities.

The scour pattern under laterally nonuniform flow is different than the scour pattern under laterally uniform flow. The scour hole in laterally nonuniform flow is deeper and it develops faster compared to the scour hole development in laterally uniform flow under the same hydraulic conditions. Furthermore, the scour pattern under lateral non-uniform flow shows spatial variations in lateral direction. The larger maximum scour depths are not observed in the zone with high flow velocities, but in the mixing layers.

A horizontal contraction of the flow at the scour hole is observed for lateral non-uniform incoming flow, whereas the presence of the scour hole does not affect the horizontal structure of the flow for lateral uniform incoming flow. The vertical structure of the flow showed relatively high near-bed velocities in the scour hole indicating flow attachment to the bed for laterally nonuniform flow. In addition, the vertical structure of the flow appears to become more depth uniform in the scour hole. The vertical structure of the flow is important of the sediment transport in the scour hole. For vertically attached flow, it was indicated that sediment could not be transported from the scour hole in the direction of the bed protection.