Laboratory investigation of turbulence effect on particles settling and field exploration of sediment movement features

Abstract

Sediments are present in most natural water bodies. In the view of topographical aspect, deposition usually happens after a long-time movement of sediments in the flows. In the view of water quality aspect, existence of sediments can affect local ecological environment, which primarily refers to farmland irrigation, aquatic animals and plant livings. Undoubtedly, all relevant sediment problems are dependent on the concentration and retention time of sediments in the flow. In order to explore how the movement of sediments leads to water quality problems, this thesis adopts laboratory and field experimental techniques for studying sediment behavior in a turbulent flow and sediment influence on the local environment. The laboratory experiments investigate sediments settling patterns in response to ambient turbulent fluid flows in the microscopic scale, and the field measurements interrogate sediment contributions to the catchment rivers in the macroscopic scale. The field study also focuses on impacts of hydrological dynamics on sediment spatiotemporal variations. A two-camera PIV-PTV measurement technique is adopted in the laboratory experiments. Distinction of the two phases (solid and fluid phase) is obtained through optical separation method and the velocity field information of each phase is acquired by Eulerian and Lagrangian algorithms, respectively. Detailed interactions between the fine particles with sub-millimeter size and the turbulent flow are explored by conducting experiments in ambient turbulence with and without mean flow. The results suggest that the large-scale vortex array appears when horizontal turbulent velocity arrives at peak value with a lower TKE dissipation rate, which could cause the acceleration of settling velocity of the particles. Meanwhile, the turbulence characteristics would be affected by the occurrence of the falling particles when the vertical turbulent velocity is lower than 0.005 m/s. To illuminate the global behavior of sediment particles in a practical engineering case, full-scale fieldwork was carried out in the Lai Chi Wo catchment for a monitoring period of around two years (year 2017 and 2018). The sediment conditions in the stream are monitored by turbidity and suspended solids concentration sensors installed at five observation stations from upstream to downstream. Analysis of the recorded field data shows that rainfall intensity with a 15-min or 20-min time interval has the most considerable potential influence on sediment conditions in rivers in the study area. Moreover, occurrence of farmlands would cause the rivers in downstream areas to have higher suspended solids concentration when the 15-min rainfall intensity is larger than 35 mm/hr, thus leading to poor water quality in the downstream areas. Finally, a study of the relationship between turbidity and suspended solids concentration provides us an approach for estimating the suspended load of sediments at ungauged locations in the river. Different rainfall types are classified with the corresponding estimation methods. Overall, this study provides a consolidated comprehension of sediment transport and motions through multiple aspects. The findings can be beneficial to environmental management in the future.