Author(s): Yiping Li, Chunyan Tang, Kumud Acharya, Wei Du, Jin Wei
Linked Author(s): Yiping Li
Keywords: acoustic inversion, sediment resuspension, suspended sediment concentration, turbulent structure, Lake Taihu
Abstract:
Sediment resuspended by wind-driven currents and waves releases nutrients that play a critical role in the nutrient cycling of large shallow lakes. Previous studies explain sediment resuspension either in a timeaveraged manner based on the laboratory/field experiments or as an instantaneous small-scale turbulence process related to coherent structures in the coastal or estuarial areas. However, few studies have been conducted in large shallow lakes because it is difficult to measure the instantaneous turbulence fluctuation and high-frequency sediment concentration near the lake bottom at the same time, especially because of the complex characteristics of wind-induced wave and current interactions. This study examines the effects of intermittent turbulent bursts on sediment resuspension at the bottom boundary layer of a large, shallow lake by taking high-frequency, synchronous in-situ measurements to monitor fluctuations in velocities and sediment concentrations in Lake Taihu, the third largest freshwater lake in China. To identify the relevant sediment resuspension processes, the near-bed, three-dimensional velocity; Reynolds shear stress; the instantaneous sediment flux (c'w'); and turbulent kinetic energy (TKE) are calculated based on the data collected from Acoustic Doppler Velocimeter (ADV) and Optical Backscatter Sensor (OBS) instruments placed close to the lakebed. The results suggest that the sediment resuspension process is closely connected with the coherent structure in the bottom boundary layers of the lake. The intermittent bursts of coherent structures (dominated by ejection and sweep) are the main energy sources and driving forces for sediment resuspension and nutrient release in large, shallow lakes. The bulk of the sediment flux is accomplished by ejection (57.52%), sweep events (31.49%), and outward interactions (11.17%), whereas the contribution coming from the inward interactions is negligible (-0.18%). Large-amplitude events (6.84%) contribute greatly to sediment resuspension and vertical transportation (40%), which affect internal nutrient release. The results demonstrate the mechanism of sediment resuspension, which are critical for evaluating internal nutrient release. (2600, 64, 346)
Year: 2017