Publication

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.

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Related concepts (33)
Sediment transport
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks (sand, gravel, boulders, etc.), mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting.
Gravel
Gravel (ˈɡrævəl) is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentary and erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel () and pebble gravel (). ISO 14688 grades gravels as fine, medium, and coarse, with ranges for fine and for coarse.
Bed load
The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or saltating (hopping). Generally, bed load downstream will be smaller and more rounded than bed load upstream (a process known as downstream fining). This is due in part to attrition and abrasion which results from the stones colliding with each other and against the river channel, thus removing the rough texture (rounding) and reducing the size of the particles.
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