Deposition and erosion of silt-rich sediment-water mixtures

From a granulometric point of view, sediment can be classified as sand, silt and clay. Silt is thereby defined as sediment with particle sizes equal to, or larger than 2 μm and smaller than 63 μm, with quartz or feldspar as base mineral. Note that quartz and feldspar particles can be smaller than 2 μm. To date, our knowledge on deposition and erosion processes of silt and siltrich sediment-water mixtures is small compared to their sandy or clayey counterparts, hampering our understanding of large-scale morphological behaviour of silt-dominated systems. The most important difference between clay and silt is that clay consists of clay minerals which have cohesive properties, and as a result, the erosion and deposition of clay beds is influenced by cohesion. The behaviour of cohesive sediment in suspension and in the bed is influenced by flocculation, permeability, effective stress and rheological properties which are related to electro-chemical properties of the base minerals. Silt particles do not have cohesive properties, but there are indications that their erosion behaviour can be apparently cohesive (Roberts et. al., 1998, Van Maren et al., 2009a). Permeability effects are likely to play a role in the behaviour of silt-water mixtures due to the small particles sizes of silt. Such effects result from a difference in timescales between the forcing and the response of the bed and result in apparently cohesive behaviour. Examples are the development of the bed strength with increasing hydrodynamic forcing or the dissipation of overpressures within a compacting silt bed. Such behaviour is characteristic for cohesive material and not for granular material such as silt, and is referred to as apparently cohesive behaviour. In existing literature, the physical processes that control the behaviour of silt are mostly described qualitatively, and silt-specific formulations for hindered settling, compaction and erosion do not exist. Through this thesis, quantitative insights into the physical processes controlling the behaviour of silt-water mixtures have been derived. The overall objectives of this thesis were to i) determine the hindered settling behaviour of silt-water mixtures, ii) determine the deposition and compaction behaviour of silt beds, and iii) determine the erosion behaviour of silt beds.