Consideration of the Mechanisms for Tidal Bore Formation in an Idealized Planform Geometry

A tidal bore is a positive wave traveling upstream along the estuary of a river, generated by a relatively rapid rise of the tide, often enhanced by the funneling shape of the estuary. The swell produced by the tide grows and its front steepens as the flooding tide advances inland, promoting the formation of a sharp front wave, i.e., the tidal bore. Because of the many mechanisms and conditions involved in the process, it is difficult to formulate an effective criterion to predict the bore formation. In this preliminary analysis, aimed at bringing out the main processes and parameters that control tidal bore formation, the degrees of freedom of the problem are largely reduced by considering a rectangular channel of constant width with uniform flow, forced downstream by rising the water level at a constant rate. The framework used in this study is extremely simple, yet the problem is still complex and the solution is far from being trivial. From the results of numerical simulations, three distinctive behaviors emerged related to conditions in which a tidal bore forms, a tidal bore does not form, and a weak bore forms; the latter has a weakly steep front and after the bore formed it rapidly vanishes. Based on these behaviors, some criteria to predict the bore formation are proposed and discussed. The more effective criterion, suitably rearranged, is checked against data from real estuaries and the predictions are found to compare favorably with the available data

Sediment inception under breaking tidal bores

A tidal bore may develop in an estuary during the spring tide conditions when the tidal range exceeds 5-6 m and the flood tide is confined to a narrow funnelled estuary with low freshwater levels. The tidal bore is of great importance for the geomorphology of the estuarine zone. In this study, some physical modelling was performed to investigate the sediment motion inception beneath a tidal bore on a movable gravel bed. The results show the significant impact of breaking bore propagation on the gravel bed motion. The dominant contribution to sediment transport inception is the longitudinal pressure gradient force, while the transient recirculation motion next to the bed yields to a drag force acting in the upstream direction and contributing to sediment motion. (C) 2012 Elsevier Ltd. All rights reserved

Air Bubble Entrainment in Breaking Bores: Physical and Numerical CFD Modelling

In an estuary, a tidal bore is a hydraulic jump in translation generated at the leading edge of the tidal wave during the early flood tide under spring macro-tidal conditions in a narrow funnelled channel. After formation, the bore is traditionally analysed as a hydraulic jump in translation and its leading edge is characterised by a breaking roller for Fr1 \u3e 1.3–1.5. The roller is a key flow feature characterised by intense turbulence and air bubble entrainment. Herein detailed air-water flow measurements were conducted in breaking bores propagating in a large-size channel. The data showed a relatively steep roller, with a short and dynamic bubbly flow region. The results were used to validate a Computational Fluid Dynamics (CFD) model of breaking bores. The instantaneous void fraction and bubble distribution data showed systematically a lesser aeration region in the physical model, compared to the numerical data. The differences may be linked to some limitation of the CFD modelling

Undular and breaking tidal bores on fixed and movable gravel beds

A tidal bore is a positive surge taking place during the flood tide with a large tidal range and the bore corresponds to the leading edge of the tidal wave propagating upstream. In this study, some physical modelling was performed to investigate the upstream bore propagation over fixed and movable gravel beds. Both undular and breaking bores were tested. The free-surface and velocity measurements were complemented by some observations of particle motion beneath the bore front. In the initially steady flow and beneath undular bores, no sediment motion was observed for the experimental setup. Beneath breaking bores, on the other hand, some upstream gravel bed load motion was observed behind the bore. The gravel bed particles were de-stabilised by the roller toe passage and advected upstream. The unsteady velocity data showed some damping of the transient recirculation on the movable gravel bed

Impact of a large cylindrical roughness on tidal bore propagation

A tidal bore is a hydrodynamic shock, surging upstream in some shallow-water bays and estuaries during the flood tide under large tidal range. This study investigates experimentally the propagation of tidal bores over a large cylindrical roughness element, representative of damaged bridge pier foundation. In the initially steady flow, the large cylindrical element generated a wake region, with extents comparable to steady flow literature. During the tidal bore propagation, the presence of the element had negligible effect on the free-surface properties, but a significant impact in terms of the instantaneous velocity and Reynolds stresses. This resulted in longer transient recirculation both upstream and downstream of the element and larger maximum velocity recirculation magnitudes, as well as enhanced turbulent stress levels and potential bed erosion around the large element, within two diameters from the element centre. The results showed the potential development of a large scour hole around the cylindrical element

Turbulence measurements in tidal bore-like positive surges over a rough bed

A sudden rise of the water depth in an open channel flow creates a surge propagating in the channel called a positive surge. Positive surges can be observed as natural phenomena when a spring tide enters a funnel-shaped estuary under appropriate tidal and bathymetric conditions: the process is called a tidal bore. In this study, the free surface profile and the unsteady turbulent motion of positive surges were studied physically in a relatively large facility under controlled conditions based upon a Froude similarity. The experiments were conducted in a 12 m long 0.5 m wide rectangular channel mostly covered by a rough fixed gravel bed. The metrology included a combination of acoustic Doppler velocimeters (ADVs) and acoustic displacement meters (ADMs). The initially steady flow conditions were controlled by the water discharge, and the steady flow properties were investigated thorougly using a Pitot tube and ADV. The positive surge was generated by the fast closure of a downstream gate and the bore propagated upstream against the initially steady flow. Both free-surface and velocity measurements were repeated a number of times to perform an ensemble average. The free surface characteristics were studied for both undular and breaking bores. The passage of the bore was associated with large free surface fluctuations, particularly in the case of breaking bores. The free surface properties were in agreement with earlier findings. The bore shape was closely linked with the Froude number and found to be independent of the distance travelled by the bore. High frequency (200 Hz) instantaneous velocity measurements were conducted 6.13 m upstream of the downstream gate. In the breaking bore, the data showed a transient longitudinal velocity reversal next to the bed beneath and immediately behind the bore front. In the undular bore, a deceleration of the streamwise velocity was observed without velocity reversal of the ensemble averaged velocity, but with transient negative values of the instantaneous longitudinal velocity. For both breaking and undular bores, the vertical velocity data trend followed closely the time derivative of the instantaneous water depth. Large velocity fluctuations were observed during the passage of the bore and in its wake. The turbulent integral length scales of the longitudinal and transverse velocity components were similar in magnitude in steady and unsteady flows, with slightly larger values in the unsteady flow on the upper measurements (z/do = 0.63). The turbulent integral time scales were larger in the unsteady flow. Further velocity measurements were performed immediately upstream of the gate and provided some insights into the time-variation of the velocity field during the bore generation. The turbulent stresses were larger beneath the bore close to the bed. After the bore front passage, the strongest fluctuations in terms of turbulent stresses were observed on the mid-water column and under the wave crests. Overall this study covered a broad range of unsteady flow conditions which were thoroughly investigated

Effect of a large bed roughness on positive surge propagation in canals

In open channels and water supply canals, the brusque operation of control valves and gates may induce large unsteady flow motions called surges. To date, the literature has focused on the propagation of surges in smooth canals, ignoring the effects of large roughness and debris. Herein, a physical study was conducted under controlled flow conditions to study the turbulent mixing in the close vicinity of a large circular bed roughness element during the upstream propagation of positive surges. Detailed free-surface and instantaneous velocity measurements were conducted with and without the large and flat cylindrical element. For a number of tests, the experiments were repeated 25 times, and the results were ensemble averaged. The data suggested that the positive surge propagation was associated with large instantaneous free-surface fluctuations for all investigated flow conditions. The velocity measurements showed large variations in longitudinal velocity during the surge generation, as well as large fluctuations of all velocity components. The presence of the large bed element modified the velocity fluctuations and unsteady Reynolds stresses in the vicinity of the element. The present results implied the potential for bed scour around the element during surge propagation