Plunging Flow and Coherent Turbulent Structures in a Straight Pool‐Riffle

Abstract

Riffle-pools are a commonly observed undulating bed morphology in gravel-bed rivers, with the deeper areas of the river called pools and the shallower parts called riffles (D. M. Thompson & MacVicar, 2022). Researchers have observed a range of hydraulic phenomena at field sites with this morphology that are not seen in more uniformly shaped channels. One of the earliest observations, for example, noted a rapid increase in near bed velocity in a pool as flood stage increased and proposed a "velocity reversal" hypothesis in which the near bed velocity in the pool was thought to exceed that in the riffle at high flood stages (Keller, 1971). If observed in other pools, this mechanism would have provided a simple physical explanation for pool scour and maintenance, but field measurement of this mechanism is challenging and evidence has been mixed (Byrne et al., 2021;Clifford & Richards, 1992;Milan, 2013). As instrumentation improved, it became possible to measure turbulent fluctuations. Researchers noted high turbulent intensities and sometimes highly coherent turbulent structures, which led them to consider ways in which turbulent scour might impact pool formation (