Author(s): Johannes Schobesberger; Thomas Gold; Kevin Reiterer; Dominik Worf; Petr Lichtneger; Christoph Hauer; Helmut Habersack; Christine Sindelar

Linked Author(s): Johannes Schobesberger, Dominik Worf, petr lichtneger, Helmut Habersack

Keywords: Sediment entrainment; Coherent structures; Tomographic particle tracking velocimetry; Shake the box

Abstract: Shields (1936) provided the basis for describing sediment transport and sediment entrainment based on mean parameters such as the shear stress. The Shields diagram was later supplemented with occurring bed forms or morphodynamics, such as riffles, dunes or flat bedforms. These conditions are well observed in field surveys and in experiments conducted in hydraulic flumes and are fitting quite well to the Shields diagram. Therefore, it might be possible to argue that these processes can be adequately described by the mean shear stress. In contrast to the processes above the critical shields number θcrit it is still not yet clear which processes are responsible or adequate for describing sediment entrainment below the critical values proposed by Shields. Based on field measurements as well as in experiments, conducted in hydraulic laboratories, sediment entrainment was reported below critical Shields values. Most of the conducted studies dealing with sediment entrainment below critical Shields numbers are focusing on turbulence and coherent structures. This seems logical as turbulent fluctuations or coherent structures are leading to peak events regarding streamwise velocities, pressure, impulses and drag forces which are all important parameters for describing sediment entrainment. Here coherent structures and their influence on sediment entrainment at varying Shields numbers below threshold conditions of particle motion are studied. Therefore, experiments in a hydraulic flume, with a length of 12 m and a width of 0.3 m were conducted. The flume bed was covered with natural sediment particles, with a size of d = 5.6 – 6.0 mm and a density of 2650 kg/m³. The hydraulic conditions were sub critical and fully turbulent and were adjusted to achieve three different Shields numbers below the critical values proposed by Shields, namely, (i) 0.027 (ii) 0.022 and (iii) 0.019. For all cases sediment entrainment was observed and the flow field during sediment entrainment was measured with a tomographic particle tracking velocimetry (TOMO-PTV) system. “Shake the box”, a Lagrangian particle tracking algorithm was applied for calculating time resolved velocities (all three velocity components) in a volume. Based on the conducted experiments and the used measurement system it becomes possible to analyze the decisive coherent structures in a three-dimensional context during sediment entrainment and to classify these motions based on different Shields numbers.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022793

Year: 2022