Coherent structures in open channel flows with bed load transport over an hydraulically rough bed

Excess sediment production in the upper parts of catchments may result in important impacts over morphodynamics of gravel bed-rivers. By changing morphodynamics, sediment overfeeding may induce important changes in the structure of near-bed flow, mainly in what concerns exchange of momentum and mass between flow within the roughness elements and flow in the upper regions. It is not well-known how turbulent statistics, including those characterizing the bursting cycle, are affected by bed load transport, for mobile but geometrically similar beds. This study addresses this issue. It is aimed at evaluating the impacts of sediment transport on flow hydrodynamics, namely on statistics of turbulent coherent structures. In order to accomplish the proposed objective, laboratory tests were undertaken. Two-dimensional instantaneous flow velocity fields in the stream-wise and vertical directions were measured with Particle Image Velocimetry.

Two laboratory tests simulated a framework gravel bed with sand matrix and a framework gravel bed with sand matrix but with sediment transport imposed at near capacity conditions. The framework consists of coarse gravel whose diameters range between 0.5 cm and 7 cm and is kept immobile under the imposed flow conditions. The mobile sediments are sand with a mean diameter of 0.9 mm. For both tests, the quadrant threshold analysis technique was employed and shear stress distribution statistics were analyzed and discussed in what concerns their contribution and persistence.

In the case of mobile conditions, sweep events tend to govern the flow in the near-bed region. Relevant differences between mobile and sub-threshold beds are found in the wake of roughness elements, mostly for sweep statistics. In the presence of bed-load, ejection events decrease their participation in the shear stress production processes. This decrease in the ejection events contribution is partially balanced with an increase in the frequency of inward events.