Abstract
In order to gain understanding of kinetic sieving-type segregation in bedload sediment transport, numerical experiments of two-size particle mixtures were carried out, using a validated coupled fluid-discrete element model developed at Irstea. A 3D 10% steep domain consisting at initial time of a given number of layers of 4 mm particles deposited on top of a coarser 6 mm particle bed, was submitted to a turbulent and supercritical fluid shear flow (Shields numbers of 0.1 and 0.3). The elevation of the centre of mass of the infiltrated fine particles is observed to follow the same logarithmic decrease with time, whatever the initial number of fine layers. This decrease is steeper for a higher Shields number. The main result is that this typical behaviour is related at first order to the shear rate depth profile.
Highlights
Bedload transport by opposition to suspension is defined as the part of sediment transport remaining close to the bed in stream channels
Both the fluid and particles starting from rest, the system follows a transient behaviour before reaching a characteristic steady state with a constant sediment transport rate
For Sh=0.3, the cloud of fine particles reached the bottom of the domain between 102 and 103s, whereas this duration was above 104s for the Sh=0.1 case
Summary
Bedload transport by opposition to suspension is defined as the part of sediment transport remaining close to the bed in stream channels It concerns the coarser grain material shear-driven by the turbulent water flow. In mountains, steep slopes drive an intense transport of a wide range of grain sizes leading to size segregation, named grain size sorting [3] This poorly understood phenomenon largely modifies fluxes and results in patterns that can be seen ubiquitously in nature. In this paper we present a study of vertical size segregation and kinetic sieving using the model which has been extended to simulate two grain sizes Following classical theories such as [8], segregation rates are analysed in term of shear rates
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