Decelerating bores are commonly seen in shorelines, estuaries and rivers in forms of swash run-up, tidal bores, tsunami bores. A decelerating bore propagating upstream can gradually change its shape, finally becoming an arrested bore, i.e. a stationary hydraulic jump. New experiments on decelerating bores against an adverse slope were conducted. Observations highlighted various types of arrested bores: fully breaking jumps, partially breaking jumps and non-breaking undular jumps. Measurements were repeated at least 25 times to obtain ensemble-averaged data with regards to instantaneous median and fluctuations of free-surface elevation, velocity components and turbulent shear stresses. An abrupt rise of free-surface elevation and immediate decrease in stream-wise velocity were observed during the passage of a decelerating bore. The arrival of decelerating bores induced some drastic increase of instantaneous free-surface fluctuations and all velocity components. Large-amplitude Reynolds stresses and extreme Reynolds stress fluctuations occurred in the same phase during and after the passage of decelerating bores. Histogram analysis of instantaneous normal and tangential Reynolds stresses suggested a preponderance of relatively smaller amplitudes. The upstream propagation of decelerating bores increased the probability density of large normal and tangential Reynolds stresses, yielding extrema vastly exceeding the critical threshold for inception of sediment motion.
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...
When an undular bore is propagating upstream against a subcritical flow in a flat channel, its leading front may weaken and vanish. The present study physically investigated the whole decaying process of vanishing bores in a relatively large facility. The invert was made of mobile or fixed natural river gravels. Detailed sampling of water surface elevations and velocities was performed in combination with high-resolution photo and video recordings. Despite some increased turbulence characteristics linked to the bore front arrival, no sediment motion was observed underneath the vanishing bores. The quantitative results provide a unique benchmark data set for further study of the later-stage bore propagations.
The hydrodynamic instabilities of open-channel flow past sequential lateral cavities were physically modelled in a relatively large facility. The range of free-surface oscillation was about one third of the mean water depth. High-frequency sampling of free-surface elevations confirmed the existence of fluctuations at a quasi-constant frequency, less than that of gravity standing waves. The trajectories of neutrally-buoyant particles were recorded within a cavity. The data showed a wide range of residence times with complicated motion patterns. The current study emphasised the effect of full-height sidewall baffles on open-channel flow instabilities while further analyses need to be performed to provide reference for the design of baffled waterways.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.