Flow and turbulence redistribution in a straight artificial pool

Abstract

Multiple hypotheses have been advanced to explain the occurrence of pools in gravel bed rivers. These hypotheses were developed without a hydrodynamic model of how open channel flow is affected by pools, and it is not clear why and when the flow phenomena they describe might occur. Laboratory experiments are warranted to improve our understanding of how a gradual convective deceleration and acceleration of the flow, without flow separation, redistributes flow and turbulence in an open channel. Experiments are conducted in a 1.5 m wide flume with a 0.25 m deep, 7.29 m long straight pool, entry and exit slopes of 5°, vertical side walls, and gravel sediment (D50 = 9.9 mm). Three‐dimensional velocity components are recorded at 50 Hz using Nortek Vectrinos. Velocity and Reynolds stress profiles in the channel centerline agree with previous results in nonuniform flow and include increased Reynolds stress during deceleration and high velocity near the bed during acceleration. Lateral flow convergence occurs where depth is increasing, which demonstrates that convergence is induced during flow deceleration and does not require a lateral flow constriction. Turbulence during deceleration is characterized by sweeps angled toward the sidewall of the channel, an effect that could lead to the formation of a nonuniform pool depth through lateral gradients in the deposition of mobile sediment. A conceptual model of pool hydrodynamics is proposed that includes increased turbulence, near‐bed acceleration, and lateral flow convergence as linked aspects of convective deceleration and acceleration due to depth changes in the pool.