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

Abstract

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.

Quantification of the bed load effects on turbulent open‐channel flows

“…The values of σ + and σ − depend on σ h . In this research, σ + b = 2.5 u rms and σ − = −2.5 u rms , which are consistent with Ferreira et al [], Santos et al [], and Franca et al []. This threshold criterion is given as…”

“…A single turbulent event may be detected as a train of separated smaller events. They proposed a new method to modify the results of the quadrant threshold method as shown in Figure 1, which has been used in Santos et al [2014] and Franca et al [2014]. It is based on the observation on the persistence of u ′ .…”

“…They characterized ejection and sweep events over mobile and immobile rough beds. More recently, Santos et al [] also utilized this method and found that sediment motion reduced the frequency of large events and increased the frequency of small events. Similar observations were also reported in Franca et al [].…”

Turbulence in Rivers

“…Finally, the bursting duration for entrainment-threshold beds is smaller than that for immobile beds; in contrast, the bursting frequency for entrainment-threshold beds is larger than that for immobile beds. Santos et al (2014) showed that, generally, the sediment transport of sand decreases the transported momentum and maximum shear stress values but increases their frequency of occurrence in time. The analysis of the probability distribution function of both ejections and sweeps shows an effect of sediment transport in terms of the reduction in the frequency of large events and in the increase of the frequency of small events.…”