Winnowing: Sub-surface entrainment of particles

Abstract

This research focusses on the behaviour of fine particles which pass through the armour layer of bed material of the upper and middle reaches of a river. When the fine particles are entrained in the armour layer, it is described as ‘winnowing’ or vertical entrainment. A river bed generally consists of a coarse surface layer, which is the exterior or upper boundary of the river bed from where particles can be picked up instantaneously by the flow. Below this a sub-surface layer is situated, which consisting of more fine sediments. The surface or armour layer, consists of particles that can be picked up instantaneously by the flow (depending on their size and the flow conditions). Entrainment of sub-surface sediment by the flow is hindered by the surface layer above it. If the entrained fines are picked up by the main flow, it may result in bed level subsidence which could lead to instability of constructions in the river bed. Winnowing can occur when the pores of the coarse surface layer are large enough and the hydraulic load is strong enough for the fines to be picked up and leave the surface layer. A better understanding of the winnowing process helps to determine if it is necessary to take this process into account in the morphodynamics. The objective of this research is to examine under which conditions sub-surface entrainment (winnowing) of particles occurs and to estimate which parameters are governing the winnowing process. The investigation is executed with laboratory experiments. Laboratory tests has been performed with specific material types, namely M32 sand (D50 = 370 im) and Carara gravel (D90 = 23 mm). Model tests performed by Detert et al (2010), show that the lift intensity decreases exponentially vertically through the porous bed. According to Diplas and Parker (1992), the surface layer thickness can be estimated as 2-3 times of the surface material size D90. Applying this assumption, two values of the surface layer thickness are investigated, a surface layer thickness of 6.4 cm (2.8 D90) and of 4.6 cm (2.0 D90). The study of Cui (2007) shows that an increase of entrained fines can be achieved by generating a higher surface layer mobility. For this experiment, the layer thickness and surface layer mobility are varied to investigate their influence on the winnowing process.