Erosion of fine sediments from a rough bed

Michele Trevisson,Olivier Eiff

doi:10.1051/e3sconf/20184005058

Abstract

Gravel beds in river systems represent important aquatic habitats, which may be endangered by the introduction of large amounts of fine sediments. In order to better understand the interaction between fine sediments and coarse immobile beds in sediment supply-limited systems, a series of flume experiments was conducted. The main goal was to determine under what conditions erosion stops. The experiments were performed over a bed of regularly arranged spheres. Plastic particles were taken as sediment and the erosion was investigated under uniform flow conditions for variable bed shear stress conditions just above critical conditions. The system was observed to behave in two different ways: with higher bed shear stress fine sediments were completely washed out, whilst with lower stress the sediment bed reached a stable level just above the equator of the spheres.

Highlights

Sediment supply-limited environments characterize many river systems, where the availability of sediments is reduced due to land use practices or dam regimentations
Since sediment transport is strongly influenced by the grain sizes involved, classical theories for the prediction of sediment transport for beds composed of uniform sediments, such as [4,5,6], cannot directly be applied any more
This work aims at analysing the erosion of fine sediments in a coarse immobile substrate, made of a regular array of spheres, under sediment supply-limited conditions

Summary

Introduction

Sediment supply-limited environments characterize many river systems, where the availability of sediments is reduced due to land use practices or dam regimentations. Models for sand/gravel systems have started to be developed [7,8,9,10], but the interaction between fine sediments over a rough immobile substrate is still not well understood. In the field of aeolian sand transport over rough surfaces, [14, 15] have shown that the erosion rates of sand under sediment supply-limited conditions reduce in time as the roughness elements protrusion increases, in general accordance with shear stress partitioning, an initial enhancement of transport, when the elements first appear, has been observed, suggesting the influence of turbulence induced vortices [14]. Under sediment-supply limited conditions, two different types of bed-forms can develop: barchans and ridges [16], caused by secondary currents [17] The presence of these bed-forms can greatly influence the fine sediment coverage of the bed and interacts with the shear stress distribution over the bed [18]. The experiments were conceived so as to analyse the morphodynamics of the bed, by identifying trends in the erosive behaviour

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Results

Conclusion