Determination of threshold parameter in quadrant splitting for identifying coherent motions in Lake Taihu, China
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Coherent motion plays a significant role in sediment resuspension, which is an important cause of eutrophication in Lake Taihu. The most popular approach in detecting coherent motion in wall turbulence is quadrant splitting. However, an important threshold parameter (H) is set by empiricism during inspection above which burst event occurs. This study is aimed at eliminating the erroneous detection caused by this empirical parameter in identifying coherent motions, giving an objective range of H for Lake Taihu and then illustrating the interactions between coherent motion and sediment resuspension.
Materials and methods
Based on in situ monitoring with Acoustic Doppler Velocimetry (ADV) and Optical Backscatter Sensor (OBS), high-frequency data of three-dimensional velocities and suspended sediment concentration at the bottom of Lake Taihu were obtained. Autocorrelation function and quadrant splitting were jointly adopted to determine parameter H without any empiricism.
Results and discussion
The value of parameter H varied from 0.72 to 1.28 in Lake Taihu, and an increase of 0.56 in H value led to decreases of 10.7–24.1% in the stress fraction and 14.7–26.5% in the sediment flux fraction. At the Central Zone site, the dominant motions were ejection and sweep, while inward and outward interactions dominated the exchange processes at the East Bay site. Only 16.8, 14.3, and 17.0% of the time fractions of coherent motions contributed up to 58.3, 74.5, and 55.0% to the sediment flux fractions at different observation sites, and the larger time fractions of burst events contributed less to sediment flux fractions due to the less effect from wind waves and the lack of upwelling currents.
Burst events with large amplitude and short duration contributed significantly to the processes of sediment exchanges. Calculation results of momentum and sediment resuspension fluxes are very sensitive to parameter H, taking one fixed empirical value for H is unsuitable for Lake Taihu. These findings will provide a useful reference for identifying coherent motions precisely and understanding the interactions between coherent motion and sediment resuspension in Lake Taihu.
KeywordsCoherent motion Lake Taihu Quadrant splitting Sediment resuspension Threshold parameter
The research was supported by National Science Funds for Creative Research Groups of China (No. 51421006); National Key Research and Development Plan (2016YFC0401703, 2017YFC0405203), and Chinese National Science Foundation (51779072, 51579071, 41323001, 51539003); the program of Dual Innovative Talents Plan and Innovative Research Team in Jiangsu Province; Postgraduate Research & Practice Innovation Program of Jiangsu Province; the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities.
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