IC9_II

Knowledge about sediment transport dynamics and sediment stability is a key component to understand the geomorphology of fluvial and coastal systems. However, many small-scale processes and factors influencing the transport dynamics of mixed sediment remain obscure. The research objectives in this dissertation were inspired by studies investigating the influence of fine sediment on the mobility of a mixed sediment bed on different grain scales and in different flow environments. While studies investigating fluvial sediment transport have concluded that the addition of fine material mobilizes the riverbed, other studies investigating mainly estuarine and marine sediment transport have found that the addition of fine material can lead to bed stabilization. In both cases, the changes in bed stability are attributed to small-scale processes at the bed surface.

Based on this contrast, two series of laboratory flume experiments and a numerical model were used to analyse the influences of the sediment texture and the particle shape on the near-bed flow field and the mobility of a mixed bed, and to find a possible transition between the different modes of behaviour. The sediment texture was characterized by the fine-grained fraction and the grain-size ratio RD = D_coarse/D_fine between the diameters of the coarse and the fine particles. Laboratory experiments in an annular flume were carried out at the University of Waikato in Hamilton, New Zealand. In these experiments with spherical glass beads (D₅₀ ≤ 367 μm) and various grain-size ratios (RD = 3.9; 5.8; 9.4) and fine fractions (10; 20; 40 % dry weight) the mobility of the bed and the near-bed flow velocities were investigated. One unimodal bed and three glass-bead mixtures were subjected to increasing flow velocities (U = 0.01–0.19 m s^-1). The bed “mobility” was derived from changes in suspended particulate matter, as well as from changes of the bed level over time, using a new approach to analyse the data collected by an acoustic Doppler velocimeter. A transition between mobilizing and stabilizing behaviour was found at 3.9 < RD_cr < 5.8: Relative to a unimodal reference bed, the bimodal beds with a low grain-size ratio (RD = 3.9) became more mobile with an increase in fine content (from 10 to 40 %), whereas beds with a high grain-size ratio (RD = 5.8; 9.4) became more stable. With the addition of fine material, the bed roughness decreased, as the fine particles filled the surface gaps between the coarser particles, and the near-bed flow accelerated. In the mixed experiments the flow velocities at the bed surface increased with a decrease of the grain-size ratio. It is hypothesized that due to differences in particle packing (which are induced by the different grain-size ratios) the inflow into the bed is higher if RD is low. The high inflow can subsequently lead to more particle entrainment.

Based on these findings a numerical micro-scale model was used to investigate the differences in the 3D flow field at the sediment-fluid interface. Different combinations of spherical particles (D ≤ 600 μm, one unimodal reference model, three mixed beds with RD = 4; 4.8; 6, and 14–18 % fines) were generated and laminar flow (U = 0.08–0.31 m s^-1) was simulated above and through the particle matrix. The model showed that the flow velocities in the upper layers of the sediment bed increase with a decrease of the grain-size ratio. The velocities were highest in the bed with RD = 4 and lowest in the bed with RD = 6. In addition, the flow direction changed to cross-stream and vertical flow, as the streamwise flow through the bed was deflected around the particles. The results suggest that high cross-stream and vertical flow velocities inside a bed with a low grain-size ratio could facilitate particle entrainment at the bed surface.

In a second series of laboratory flume experiments (U = 0.02–0.23 m s^-1) the findings from the glass-bead experiments and the numerical model could be validated with natural sediment. For sand-sand and sand-silt mixtures (D₅₀ ≤ 410 μm) with various grain-size ratios (RD = 2; 3.5; 7.7) and large fine fractions (40 % dry weight) a transition between the mobilizing and stabilizing behaviour was found at 3.5 < RDcr < 7.7. The mixed bed with a low grain-size ratio of RD = 2 behaved similar to the unimodal reference bed. The flow velocities at the surface of the different beds could be related to the erosion and mobility during different stages of the experiment. In direct comparison with the glass-bead experiments the natural sediment was more stable, implying the stabilizing effect of the particle complexity. In addition, the near-bed flow profiles above the natural beds were slightly different from the flow profiles above the glass beads, indicating dissimilarities in bed roughness resulting from the different particle shapes. The presented studies show that in the tested sand-silt range (D₅₀ ≤ 600 μm) the grain-size ratio is of greater importance for the mobility of a mixed bed than the amount of fines. As observed in gravel-bedded rivers, the addition of fines to a unimodal sand bed can lead to bed mobilization if RD of the bed is about 2–4. At a higher grain-size ratio however, the bed will stabilize with the addition of small amounts of fines. The presence of the fine particles influences micro-scale flow processes at the bed surface that subsequently control particle entrainment and bed mobility.

The dissertation “The influence of sediment texture on the mobility of mixed beds – Annular flume experiments and numerical modelling” is available
online and in print at the State and University Library Bremen.