Turbulent Suspension of Sediments in Shallow Shelf Seas
|Other Titles:||Turbulente Suspension von Sedimenten in flachen Schelfmeeren||Authors:||Amirshahi, Seyed Mohammad||Supervisor:||Winter, Christian||1. Expert:||Winter, Christian||2. Expert:||Schüttrumpf, Holger||Abstract:||
The interaction between near-bed flow and sediment transport is one of the most attractive research areas for both hydraulic engineers and geomorphologists. Still, current knowledge on the suspension of sediment and near-bed sediment transport, are far from being resolved. This is due to the lack of understanding of near-bed hydrodynamics and the critical conditions at which the sediment starts to move. This thesis explores the near-bed flow-sediment interaction in highly dynamic coastal environments where the near-bed flow is governed by rotating tidal currents. During field campaigns in the German Bight in winter 2015, spring 2015 and summer 2016, data on the near-bed hydro- and sediment dynamics were collected using Acoustic Doppler Velocimeters and an Acoustic Doppler Current Profiler in different water depths, with different sediment characteristics and tidal phases. State of the art methods were used to analyze the data on large (tens of minutes) and process (few seconds) time scales. To portray the hydrodynamic conditions in large time scales, bed shear stress, i.e. the force exerted from the water flow against the seabed, and turbulent kinetic energy (TKE) and Reynolds stress, i.e. the two most common turbulent statistics characterizing turbulent conditions, are studied. In addition, in process time scale, turbulent events, i.e. short energetic velocity fluctuations describing the turbulent flow, are investigated. On large time scales, thresholding of suspended sediment concentration variations was used to estimate critical suspension and deposition stresses. Comparison of the computed suspension stresses with empirical equations predicting the critical condition, illustrates that the movability-based predictors yielded higher accuracy in tidal environments. As the bed shear stresses exceed the critical suspension stress, both bed shear stress and suspended sediment concentration closely tracked each other until bed shear stress fell below the deposition threshold. Results explicitly showed that the bed shear stresses calculated from the turbulent statistics, in comparison to those calculated from the average velocity profile, correlate better with the suspended sediment concentrations. Next, the near-bed turbulence was scrutinized on process time scales to investigate its role in the vertical mixing of suspended sediment. Turbulent events are defined as a sequence of strong velocity fluctuations classified in a quadrant plane (u', w') contributing to 90% of the Reynolds stress. It is shown that, turbulent events only occur in 25% of the time, with the rest of the time being occupied by small background fluctuations. These events are able to induce higher stresses than the average bed shear stress, resulting in suspension of sediments below the critical conditions obtain from mean velocity. Apart from the significance of ejection and sweep events in near-bed sediment dynamics, this thesis further shows the importance of outward interactions in moving sediments away from the bed. This is achieved using turbulent events characterization by their type, strength, duration, length and concentration of associated suspended sediments. Finally, this thesis allows inference from process time scales to large time scale considerations of flow-sediment interaction. Being detected in only 25% of the time, the turbulent events are responsible for more than 60% of the total suspended sediment movements with the rest likely governed by settling velocity. Even though small background fluctuations cannot move sediment in the water column, they impose a large effect on turbulent statistics and reduce bulk TKE and Reynolds stress estimates up to 1.6 and 3 times, respectively. Therefore, this thesis suggests that for the study of large scale sediment dynamics, TKE is a more appropriate descriptor of turbulence. However, for accurate prediction of sediment transport, one needs to consider turbulent events on process time scales.
|Keywords:||Tidal currents; Turbulent event characteristics; Turbulent Kinetic Energy (TKE); Reynolds stress; Bed shear stress; Suspended Sediment Concentration (SSC); Vertical turbulent sediment flux; Suspension and deposition threshold; Shields parameter; Movability number; Acoustic Doppler Velocimeter (ADV); Acoustic Doppler Current Profiler (ADCP)||Issue Date:||12-Aug-2019||DOI:||10.26092/elib/58||URN:||urn:nbn:de:gbv:46-00108621-18||Institution:||Universität Bremen||Faculty:||FB5 Geowissenschaften|
|Appears in Collections:||Dissertationen|
checked on Sep 30, 2020
checked on Sep 30, 2020
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