By numerical simulations of turbulent flow in isolated poolriffle units with various riffle heights, four different types of vortices were found and named as follows: surface rollers (SR), corner rollers (CR), ramp rollers (CR), and axial tails (AT). Surface rollers are shaped on the flow surface due to submerged hydraulic jump or any obstacles in the forced poolriffle units. Corner rollers are shaped close to the corners near the walls at the pool head. Ramp rollers are formed at the bed of the channel on the ramp into the head of the pool. All kinds of vortices stretch in the streamwise direction as they travel to the downstream, which they are called axial tails. The simulations showed that all four types of vortices interact with each other, combine, amplify or cancel out each other as they travel downstream. The strength of vortices and how they interact result into different types of flow patterns. The surface rollers combine with corner rollers to make a jet like plunging flow near the pool bed. In other cases with lower riffle heights, ramp rollers tend to push the flow up, which in turn leads to higher turbulence near the bed and higher velocity near the flow surface (skimming flow). Moreover, if both surface rollers and ramp rollers have the similar strength (e.g., vorticity) and scale, the streamwise velocity profile has a peak around the middle of the flow, and minimum velocities near the bed and free surface. This flow pattern was named as “rifting flow.” Based on these findings, a new hypothesis is proposed called ‘vortex-resistance,’ which states that the turbulent structures, by increasing the eddy viscosity and changing the pressure domain, act as an obstacle that steers the flow. Plunging and skimming flow can thus be understood as the products of different types of turbulent structures. These findings provide new clarifications to long-standing questions related to the hydraulics of pools and riffles.

No 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 (