IC9_I

State of the art
Sediment erosion is understood as the initiation of motion of single grains by a hydrodynamic driving force. Most classical studies have attempted to predict sediment erosion as a function of single grain size at a defined flow force, e.g., Shields (1936) and Hjulstroem (1935). However, at present, it is understood that the erosion behavior of sediments is also controlled by additional factors that shift the initial erosion conditions towards higher regimes. This increase in the erosion resistance of a sediment bed is termed as ‘sediment bed stabilization’ and is controlled by: (1) biostabilization, (2) cohesion, and (3) textural caging structures. (1) Biostabilization, i.e., biological activity of the micro and macro fauna, can increase the stability of sediment by binding sediment particles together (Paterson, 1994; Young and Southard, 1978). (2) Small clay minerals in a sediment bed have the ability to bind each other forming larger aggregates due to electrostatic forces (cohesive forces) that can increase the erosion resistance significantly (Alvarez-Hernandez, 1990; Dyer, 1989; Hir et al., 2008; Hir et al., 2011; Kamphuis, 1990; Murray, 1977; Panagiotopoulos et al., 1997; van Ledden et al., 2004). (3) Erosion resistance can also be increased by texture related caging structures. In such a case, fine particles encompassing coarse particles, like a cage, prevent erosion successively (Hir et al., 2008; Torfs, 1997; van Ledden et al., 2004; Whitehouse et al., 2000).
An additional factor controlling the erosion behavior in sediment beds is related to the differing densities of the mineral grains forming the bed (Li and Komar, 1992; Middleton, 2003). Due to their higher density, magnetite particles are heavier and are therefore, more resistant to erosion than the light quartz particles (Komar, 1987; Komar, 2007). Hence, the density distribution of particles in sediment beds also plays a major role in erosion characteristics.

Therefore, the following overall research questions were addressed:

Traditionally, for the investigation of sediment erosion and transport processes, analogue laboratory based flume or in situ field investigations are used (Amos et al., 1992; Black and Paterson, 1997; Komar, 2007). However, analogue approaches are limited in their ability, e.g., due to the low resolution of the sensors, to resolve and quantify the physical processes at the sediment surface and in the interior of the bed (Komar, 1987; Komar, 2007). In order to quantify the processes controlling sediment transport and bed stabilization, such as inflow rates, particle transport rates, and porosity changes, in the sediment beds, numerical models are required. Thus, numerical modeling approaches can quantify previous assumptions. and conceptual approaches shown by flume tank experiments and further explain transport behavior and textural changes in sediment beds during fluid flow.

Project description
With a focus on the physical parameters, i.e., cage like structures and density related effects, the overall goal of this project was to identify and quantify their role of erosion resistance of sediment beds and transport behavior. In order to identify these effects influencing sediment movement at a bed laboratory-based annular flume tank experiments were undertaken. These experiments focus on the effects caused by non-cohesive silt and sand grains on erosion resistance and the transport behavior of layered - and mixed sediment beds consisting of a simplified two-grain fraction (silt and sand). To mimic layered and mixed sediment beds, two suites of experiments were designed: (1) “The Layering Experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness; and (2) “The Mixing Experiment” where sand beds were homogenously mixed with increasing amounts of silt. To initiate erosion and to detect bed stabilization effects within both set-ups, the flow speeds were increased in increments of 2.5 cm/s up to 30 cm/s.
The results showed that the sediment bed (or the underlying sand bed in the case of the layered experiments) stabilized with increasing silt composition. Both cases showed that small amounts of fine sediment fractions can completely change the erosion characteristics of a seabed. By this approach it was possible to formulate the hypothesis that textural caging structures made of fine silt particles encompass the coarse sand particles like a cage (i.e., pore space plugging) and prevent erosion successively. However, as the caging effects occur within sediment beds, direct observation and quantification of the textural in-homogeneities are limited using only analogue techniques, and, hence, previous studies on this topic are not comprehensive.
For the quantification of the processes occurring between individual particles in the direct vicinity of the sediment-water interface and in the interior of sediment bed, a 3D high resolution numerical model was developed and simulations were undertaken. The 3D numerical model was designed for the simulation of sediment transport by a fluid by coupling two numerical simulation techniques. The two coupled techniques were the Finite Difference Method (FDM) and the Distinct Element Method (DEM). This 3D model was given settings that correspond generally to previous flume tank studies. In particular the model domain focused on a small-scale ‘cut-out’ of a sediment bed. These experiments are able to validate the most fundamental questions on sediment transport; for example: