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. @en

Flow separation and its control have been the subject of intensive research for decades. Flow separation occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction. Numerous strategies exist to control flow separation, and in this study we demonstrate experimentally that vertical flow separation over steep slopes in shallow free-surface flows may be suppressed by contracting the flow horizontally upstream of the slope. We found that, unexpectedly, introducing lateral non-uniformity in the upstream flow field could suppress vertical flow separation for steep slopes up to 1 in 2. This study reveals the possibility of two different flow states over steep slopes; (i) a vertically attached flow combined with horizontal convergence, and (ii) a vertically detached flow combined with horizontal divergence. A detailed analysis of the dynamics of the two different flow states is presented. Although a predictive relation determining the transition point between the two flow states was not found in the current study, the observed phenomena were shown to be strongly related to the magnitude of the lateral gradient at the upstream edge of the slope. The results demonstrate a significant influence of the vertical flow state - separated or attached - on the shear stress at the confining boundaries of the flow.@en

The Eastern Scheldt storm surge barrier is one of the primary flood defense structures in the Netherlands. Since its construction, large scour holes have developed downstream of the applied bed protection at both sides of the inlet that have grown much deeper than foreseen during the design phase. Flow velocity data collected at the location of the scour holes revealed a pronounced contraction of the tidal flow towards the deepest part of the scour hole, in combination with a suppression of vertical flow separation when horizontal velocity differences are present and large enough. To investigate these phenomena we set up a laboratory experiment. First results of this experiment show strong similarities between the flow observed in the field, and the flow observed in the laboratory. Therefore, this set-up can be used to further investigate the phenomena that are observed at the Eastern Scheldt and other similar configurations.

@en

Scour in the vicinity of a hydraulic structure may compromise its geotechnical stability or that of nearby structures. Much fundamental research has been dedicated to the understanding and prediction of these processes, in order to enable the efficient design of mitigation measures. While most of these efforts consider laterally uniform flows, nonuniformity is the rule rather than the exception. This study presents field observations near the storm surge barrier in the Eastern Scheldt estuary, The Netherlands, where significant scour holes have developed. The tidal flow through this semi-open barrier exhibits characteristics of a shallow jet. A pronounced contraction of this jet toward the nearby scour hole during particular stages of the tidal cycle is revealed, attributed to potential vorticity conservation; the depth-averaged vertical vorticity increases proportionally to the depth increase. Measurements show a vertically uniform velocity profile in the scour hole that is attributed to a reduction of the adverse pressure gradient due to the contraction, that suppresses boundary layer separation from the upstream slope of the scour hole. A positive feedback mechanism is thus revealed; lateral velocity gradients lead to relatively high near-bed velocities in the scour hole which enhances erosion—causing an even stronger horizontal contraction—maintaining the scouring potential. If the upstream flow field is laterally uniform (observed around slack tide), horizontal contraction does not occur, and the boundary layer separates from the upstream slope. The scour depth will then tend to an equilibrium. The results infer the importance of the nonuniformity of the horizontal flow field.@en