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.@en

Braided rivers are known as highly dynamic systems characterized by several channels (Williams, et al., 2013) divided by unsteady bars (Egozi and Ashmore, 2009). Training is often a necessary action in order to either control their lateral expansion (Carolina Rogeliz, et al., 2006) and undesirable bank erosion, or for land reclamation (Jurina, 2017). The latter is usually associated with local channel narrowing which leads to vast changes in river morphology. According to the equilibrium theory developed by Jansen et al. (1979), narrowing leads to channel incision, local decrease in longitudinal bed slope, upstream erosion, opposite bank pushing and decrease in the braided degree of the river (Duro et.al, 2016). Moreover, during high flow condition, local channel narrowing creates backwaters that increase the upstream water level and risk of flooding in the areas adjacent to the river. Thus, land reclamation might deeply change braided river systems and cause undesirable hydraulic and morphodynamic alterations. Therefore, efficiency of different river training alternatives regarding to land reclamation should be examined. Fracassi and Di Pietro (2018) presented some lateral land reclamation schemes that are floodable during high flows, which minimizes their hydraulic and morphological impacts. These interventions proved to be successful for farming along a few braided rivers of Bolivia. Agricultural land reclamation was obtained by constructing Gabions wall with openings to protect the land up to a specific flood and inundation level. In addition, the area was subdivided by screens perpendicular to the water flow to decrease the flow velocity during flood events and enhance deposition of fine material. Therefore, this type of interventions proved successful regrading to fertility rise in the new agricultural fields. However, there is no precise description of the river morphdynamic responses. This work aims at evaluating the degree of success of this land reclamation technique considering the morphological adaptation of the river. The Mao River is chosen as case study (Fig. 1). This is a gravel-bed braided tributary of the Brahmaputra. The study area is located in Bhutan, where the river channel has high potential of agricultural land reclamation. The degree of success is here considered in terms of water level raise during floods which depends on the morphological changes induced by the land reclamation scheme. @en