Decelerating bores in channels and estuaries

Abstract

ABSTRACTDecelerating 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.