Horizontal shear flows over a streamwise varying bathymetry

The Eastern Scheldt storm surge barrier is an icon of Dutch hydraulic engineering. Downstream of the barrier, large local erosion pits (scour holes) have formed adjacent to the applied bed protection after its construction. It was expected during the design phase that these would develop, but both the magnitude of the scour hole depth as well as the present continuation of the scour hole development were not foreseen. In this thesis, the fundamental fluid mechanical behaviour of the flow around the scour holes was studied through a combination of field data analysis, mathematical modelling and laboratory experiments.

Field data showed that the flow near the barrier is characterized by large transverse differences in streamwise velocity, and the flow is thus classified as a horizontal shear flow. As such a flow develops over a locally varying bathymetry in streamwise direction, it showcases non-intuitive behaviour, with potentially large consequences for the ongoing scour development. As a horizontal shear flow develops over a streamwise oriented increase in flow depth, its streamlines either converge or diverge in the horizontal plane. Associated with either horizontal convergence or divergence is the absence or presence, respectively, of vertical flow separation. In case of horizontal convergence and suppression of vertical flow separation, the bed shear stress is shown to be significantly higher compared to a flow that horizontally diverges and vertically separates. The rate of horizontal convergence was shown to be dependent on the increase in flow depth; thus, a positive feedback mechanism is revealed where the presence of a local increase in flow depth sustains or even enhances further development of such a feature. It was demonstrated through mathematical modelling that in case of horizontal convergence and vertical attachment, a streamwise oriented increase in flow depth led to an intensification of the turbulence structures.

The findings from this study were used to explain the observed ongoing growth of the scour holes near the Eastern Scheldt storm surge barrier. Besides, it was hypothesized how the phenomena as revealed in this thesis would apply to similar configurations as the Eastern Scheldt storm surge barrier. The results from this thesis may form part of the knowledge base from which design guidelines or (numerical) design tools for protection against scour around hydraulic structures are developed.