Flow reversal over a natural pool–riffle sequence: a computational study

A computational study is presented on the hydraulics of a natural pool–riffle sequence composed of mixed cobbles, pebbles and sand in the River Lune, northern England. A depth‐averaged two‐dimensional numerical model is employed, calibrated with observed data at the field site. From the computational outputs, the occurrence of longitudinally double peak zones of bed shear stress and velocity is found. In particular, at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope, in addition to a secondary peak at the pool head. As discharge increases, the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope, showing progressive equalization to the surrounding hydraulic profiles. Concurrently, the secondary peak, due to channel constriction, appears to stand at the pool head, with its value increasing with discharge and approaching or exceeding the primary peak over the riffle. The existence of flow reversal is demonstrated for this specific case, which is attributable to channel constriction at the pool head. A dynamic equilibrium model is presented to reconstruct the pool–riffle morphology. A series of numerical modelling exercises demonstrates that the pool–riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments. It is concluded that channel constriction can, but may not necessarily, lead to competence reversal, depending on channel geometry, flow discharge and sediment properties.