Hydrodynamics of tidal bores: turbulent propagation and sediment transport

Abstract

A tidal bore normally occurs in an estuarine zone, as a combined result of the spring tide and the trumpet-shaped river mouth. During flood tide, the inland flow is amplified by the graduallynarrowing funnelled estuary, forcing the leading edge to grow steeper until forming an upstream propagating discontinuous wave that is the tidal bore. Tidal bores in natural rivers usually contain enormous energy and induce intense turbulent mixing, posing a threat to hydraulic structures and seriously affecting the fluvial environment. The current study of tidal bores focused on the physical modelling of their turbulent propagation and transformation as well as the bore-induced sediment transport. The experiments were conducted in a 15 m long 0.5 m wide rectangular channel, with ten flow conditions encompassing various flow rates, channel slopes and bed materials (smooth PVC, fixed/mobile gravel). High-frequency sampling of instantaneous unsteady free-surface elevation and turbulent velocity was realised using acoustic displacement meters and acoustic Doppler velocimeters at 200 Hz. High-definition video recording and photographic observation were applied to track the bore propagation and transformation along the test section. Ultra-high-speed video camera recordings were undertaken at 1,200 fps, to capture the bore-induced sediment particle motion from the side view. For each series of flow conditions, the experiment was repeated at least 25 times to derive some ensemble-averaged results of flow turbulence properties and sediment particle motion characteristics. Two different kinds of decelerating bores were recorded in the inclined channel: (a) decelerating bores transformed into stationary hydraulic jumps (arrested bores) when propagating against supercritical flows; (b) decelerating bores vanished when travelling opposite to subcritical flows. The arrival of bores induced an abrupt free-surface rise and a rapid decrease of streamwise velocity. The near-bed flow was observed to reverse direction towards upstream beneath breaking bores. The instantaneous free-surface and all flow velocity components experienced some drastic fluctuations during the bore front passage. Large-amplitude Reynolds stresses and extreme Reynolds stress fluctuations occurred in the same phase during and after the passage of decelerating bores.The sediment transport in the bore-induced rapidly-varied flows was investigated focusing on the bed particle motion and diffusion processes. The frame-by-frame analysis of slow-motion videos demonstrated three basic motion modes of individual pebbles: rotation, rolling and saltation. More complicated pebble motion included a combination of 2 or 3 basic modes. Most moving particles were entrained underneath the bore front between the bore roller toe and the first crest. Based upon a Lagrangian approach, more than 550 complete trajectories of the moving bed particles were extracted from the videos recorded at 1,200 fps. These particles were set into motion by bore passages and shortly reposed into the static gr...

Physical modeling of vanishing bores in open-channel flows

“…It is the aim of this study to document in details the free-surface and velocity characteristics, under carefully-controlled flow conditions. This study was part of the PhD project of Li (2020). D r a f t…”