Observations and Analysis of the Horizontal Structure of a Tidal Jet at Deep Scour Holes

Abstract

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.

Suppression of vertical flow separation over steep slopes in open channels by horizontal flow contraction

“…The transition from a vertically attached and horizontally converging flow state towards a vertically separating and horizontally diverging flow state occurred for milder lateral gradients in streamwise velocity. This is in line with the reasoning of Broekema et al (2018). Although the analogy between the vertical stretching and horizontal compression of the conveyance cross-section with the vertical stretching of a single vortex is still recognized in the observed flow field ( § 4.1), it was shown in § 4.2 that for the selected cases there are no significant differences in the development of the velocity difference over the mixing layer ( U) and the mixing layer width (δ).…”

“…freedom (Tobak & Peak 1982;Simpson 1996;Cherry, Elkins & Eaton 2008;Gao et al 2015). Field observations of a flow with large lateral gradients in the streamwise velocity over a bed topography similar to that of figure 1, revealed complex separation phenomena (Broekema, Labeur & Uijttewaal 2018). For sufficiently large lateral gradients of the streamwise velocity, vertical flow separation did not occur, whereas it was observed if the lateral variation of the mean flow field was limited.…”

“…The experimental work was conducted in a 14 m long, 0.4 m deep and 0.4 m wide glass-sided flume (figure 2). Broekema et al (2018) related the suppression of vertical flow separation to the magnitude of the lateral velocity gradient, which is therefore an important variable in the experiment. The lateral non-uniformity in the streamwise velocity was imposed by an obstruction on one side of the flume, located upstream of the slope, with a maximum lateral blockage of half the flume width.…”

“…We now briefly consider the case that inspired this experimental study; the large-scale local erosion that is observed downstream of a storm surge barrier in the Netherlands (Broekema et al 2018). Field observations have shown the occurrence of large lateral gradients in the streamwise velocity, in combination with a strong horizontal convergence towards the deepest point of the erosion in combination with an absence of vertical flow separation.…”

“…The bed level in the flume was uniform in the lateral direction. The upstream water depth, d u , was approximately 0.12 m, and the downstream water depth, d d , approximately 0.27 m. These values were chosen to achieve an increase in flow depth which is the same as in the field case considered in Broekema et al (2018). The discharge Q in was chosen such that the maximum Froude number Fr = 0.4, which is sufficiently low to minimize the effects of surface disturbances, and that the Reynolds number Re = 3 × 10 4 , which is sufficiently high to ensure that the flow is fully turbulent.…”