Decelerating bores in channels and estuaries

Abstract

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.

Physical modeling of vanishing bores in open-channel flows

“…The passage of the decelerating undular bore did not set any pebbles into upstream motion on the mobile gravel bed (Experiment VB1). Similar to the arrested bore experiments (Li and Chanson 2018), the bore deceleration and weakening were associated with a reduction of front height with further distance travelled upstream. However, the decelerating undular bore did not stop propagating upstream on the gravel bed in the current experiments.…”

“…The physical modeling was performed in a 15 m long and 0.5 m wide rectangular tilting channel with transparent tempered glass sidewalls and a PVC bed (Figure 2), previously used by Li and Chanson (2018). Herein, the channel bed was smooth for x < 1.50 m and x > 13.0 m, where…”

“…When a bore propagates upstream against a supercritical inflow in a sloping open channel, it may decelerate and evolve into a stationary hydraulic jump, i.e. an arrested bore (Chanson 2011b;Li and Chanson 2018;Viero et al 2017). During the whole transforming process, the bore Froude number progressively decreases but remains larger than unity.…”

Hydrodynamics of tidal bores: turbulent propagation and sediment transport

“…The coupling between free-surface fluctuations, velocity fluctuations and turbulent Reynolds stresses was also a focus in most recent experiments (e.g. Leng & Chanson, 2016a;Li & Chanson, 2018a). However, previous experiments paid little attention to the effect of large bed slope on tidal bore propagations, especially the transformation from a tidal bore into a stationary hydraulic jump.…”

“…A tidal bore process is a rapidly-varied flow phenomenon associated with large impulses of turbulent Reynolds shear stresses, especially in the near-bed regions (Leng & Chanson, 2016a;Li & Chanson, 2018a). Turbulence impulses play a vital role in the sediment particle dislodgement at the inception of bedload motion (Diplas et al, 2008;Celik et al, 2013).…”