Dynamics of shallow lateral shear layers: Experimental study in a river with a sandy bed

Abstract

[1] Shallow lateral shear layers forming between flows with different velocities, though essential for mixing processes in natural streams, have been examined only in laboratory settings using smooth, fixed-bed channels. This paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer, in contrast to the two-dimensional structure observed in the laboratory, is highly three-dimensional. The study included pronounced transverse pressure gradients, which influenced shear layer structure compared to flume experiments. It also introduces an analysis that complements conventional theory on mixing layers. The lateral velocity gradient between the flows downstream from a splitter plate placed in the river, the principal controlling factor, was adjusted for three experimental runs to determine the influence of different gradients on shear-layer dynamics. In each run, detailed three-dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate. Although experimental results agreed with conventional mixing-layer theories with respect to turbulence, the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum. After re-examining the governing equations, we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget: mean and turbulent lateral fluxes of momentum, turbulent kinetic energy, and dissipation rates. The study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams.

Dynamics of Flow at Concordant Gravel Bed River Confluences: Effects of Junction Angle and Momentum Flux Ratio

“…Previous field, experimental, and numerical research succeeded in producing several conceptual models of flow structure at confluences (Mosley, ; Best, ; Best & Roy, ; Ashmore et al, ; Paola, ; Bradbrook et al, ). However, studies focusing on theoretical generalizations are rare and have examined only a few relations between bulk flow variables under idealized hydraulic conditions (Chu & Babarutsi, ; Creēlle et al, ; Hager, , , ; Modi et al, ; Sukhodolov et al, ; Taylor, ; Uijttewaal & Booij, ). The lack of general theory in this area of research leaves room for differences of opinion about mechanisms driving the dynamics of flow at river confluences.…”

“…Turbulence is another aspect in confluence research that has gained interest in recent decades. The capability of measuring nearly instantaneous velocity fluctuations under field and laboratory conditions and to model flow dynamics with eddy resolving numerical methods has fuelled investigations of turbulence at confluences (Biron et al, , ; Bradbrook et al, ; Constantinescu, ; Rhoads & Sukhodolov, ; Sukhodolov et al, ; Sukhodolov et al, ; Sukhodolov & Rhoads, ). Observations in the field have highlighted the importance of several different types of vortical structures, lateral flapping of the mixing interface, and periodic upwelling (Biron et al, ; Bradbrook, Lane, Richards, Biron, & Roy, ; Constantinescu et al, ; Constantinescu et al, ; Rhoads & Sukhodolov, ).…”

“…Rhoads and Sukhodolov () and Sukhodolov, Krick, et al () showed that those observations can be interpreted using analogy with shallow mixing layers, wakes, and jets, thereby providing opportunities for generalizations framed in conventional theories of shallow and free flows (Herrero et al, ; Uijttewaal, ). These theoretical approaches introduce parameterized models of secondary and turbulent flow into fundamental partial differential equations, which are then integrated analytically (Sukhodolov et al, ; Uijttewaal & Booij, ). Although these models were successfully used to explain stabilization of shallow mixing layers and wakes past bluff bodies, they remain limited to the case of parallel flows and further research is needed to consider angled flow configurations.…”

“…In a field study, Biron et al () showed that the zone of flow stagnation and the mixing layer are characterized by superelevation; a lateral tilt is present at the edge of the mixing layer, and a zone of superelevation is also observed on the tributary side at the downstream junction corner. Although there were some attempts in relating topographic heterogeneity of a free surface (pressure field) to the dynamics of shallow mixing layers (Sukhodolov et al, ), this important aspect of confluence hydrodynamics remains relatively unstudied.…”

Scour patterns around isolated vegetation elements

“…In the past, hydraulic 38 engineers traditionally regarded the riparian and aquatic vegetation as a source of additional re-tify their morphometric characteristics. As a secondary objective, it is also aimed to provide an 68 interpretation of the coherent flow structures, which are generated around each obstacle, by es-…”