Page 238 - Soil and water contamination, 2nd edition

12

Sediment transport and deposition

12.1 INTRODUCTION

Suspended matter plays an important role in surface water quality. The amount of suspended
matter determines the turbidity of the water, and so the underwater light climate. Moreover,
suspended fine sediments are an important vector for contaminant transport in surface
waters, since many chemicals adsorb to the negatively charged surfaces of clay minerals and
organic matter (see Chapter 4). The sediment may already be contaminated as a result of
accidental spill s, atmospheric deposition , or the application of fertiliser or pesticides on soils
susceptible to erosion . Soil erosion and deposition (sedimentation ) may accordingly result
in an ongoing transfer of contaminants from terrestrial ecosystems to surface water, bed
sediments , and floodplains . Moreover, during transport the sediment may be contaminated
by discharges into surface water and the subsequent exchange between the soluble and
particulate phase. To be able to predict the fate of sediment-associated chemicals, it is
therefore not only necessary to understand the adsorption reactions between sediment
particles and the soluble phase, but also to understand the fate of the sediment itself.
The processes of erosion and sedimentation come about particularly in two different
environments, namely on hill slopes and in surface water bodies. The basic principles
of sediment transport are essentially the same in both environments, but the processes of
sediment detachment or erosion may differ. On slopes, a considerable part of the sediment
detachment is due to the impact of raindrops falling on the soil surface. As soon as overland
flow occurs, the detached sediment can be transported downhill; this water flowing over the
ground surface cushions the impact of the raindrops. The runoff water itself can bring about
sediment detachment, due to the flow shear stress es occurring at the soil–water interface. In
river channels, this is the main process of sediment detachment, though the collapse of river
banks and other forms of mass movement (e.g. debris flows) can supply significant inputs of
sediment into a river channel. In lakes , the sediment detachment occurs mainly under the
influence of bottom shear stresses induced by waves. Sediment detachment of bed sediments
in rivers and lakes is also often referred to as resuspension . If the shear stresses are sufficiently
small, the sediment settles, i.e. is deposited. The various processes involved are described in
detail in the following sections, considering single rainfall or flood events (time scales from
minutes to days). Subsequently, models for long-term erosion and deposition (time scales of
years to decades) are discussed.

12.2 TRANSPORT EQUATION
For modelling sediment transport in the short term (single hydrological events), the general
one-dimensional advection –dispersion equation for surface water, including the erosion and
sedimentation of suspended solids , reads:

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