Author(s): Roman Dunst; Mario Klosch; Thomas Gold; Arline Khunl-Brady; Rolf Rindler; Helmut Habersack
Linked Author(s): Rolf Rindler, Helmut Habersack
Keywords: Grain sorting sediment transport physical model moveable bed armor layer
Abstract: Anthropogenic alterations of river systems has led to disruptions of the sediment connectivity, while simultaneously increasing the sediment transport capacity, resulting in a sediment deficit and consequently in riverbed incision. An armor layer may develop, which mitigates the incision process and leads to an apparent equilibrium state. This results in a riverbed that is now steeper than in a dynamic equilibrium state without the presence of an armor layer. How these states differ is highly relevant, since high flows exceeding the stable range can break up the armor layer and cause abrupt shifts in the bed level. This study aims to assess differences in slope and surface grain size distribution of a riverbed that develops due to incision both with and without an existing static armor layer and compare them to a riverbed that developed via aggradation. For this, a 10 m long and 0.3 m wide physical flume model with a moveable bed was set up. Colored sediments ranging from 0.25 mm to 3.15 mm with a mean diameter of 1.10 mm were used to monitor changes in the grain size distribution of the bed surface. Sediment was supplied evenly over the entire cross section by an automatic sediment feeder; the output was continuously weighed by a load cell in the sediment trap and its GSD assessed. The bed geometry was periodically measured using a laser line profiler. A dynamic equilibrium at a slope of 0.17 % at a fixed supply sediment rate could repeatedly be established in phase 1 and was used as starting condition. During this phase, lateral grain sorting was observed, represented as a path of fine sediment forming at the centre, coinciding with where most of the bedload transport was also observed. When the sediment input was reduced to 50 %, bed incision occured, leading to a decrease in the overall mean slope. After equilibrium was re-established, the abovementioned sorting process became more pronounced. To understand this phenomenon, the flow was analysed via a 2D Laser Doppler Velocimetry (2D-LDV), which indicate the presence of secondary flow cells, that may be responsible for this particular pattern. The ongoing experiment further investigates the development of the bed slope and surface GSD (1) when first low discharges combined with no sediment supply are used to form a static armor layer before the supply is reduced and (2) when forming a riverbed in a dynamic equilibrium state by aggredation by suppling the reduced load already as initial load.
Year: 2025