Observations of cohesive sediment behaviors in the muddy area of the northern Taiwan Strait, China

摘要

To study cohesive sediment behaviors and their time-variability in the near-bottom layer in the muddy area of the northern Taiwan Strait (TS), a short-term bottom-mounted tripod, which was equipped with an acoustic Doppler velocimeter (ADV), an upward acoustic Doppler current profiler (ADCP) and a CTD with a Seapoint turbidity sensor, was deployed at the northern TS over seven days between July 22 and 29, 2012. The near-bed current velocity, wave orbital velocity, bottom shear stress induced by wave and current, near-bed suspended sediment concentration (SSC) and variations in the bed elevation were resolved. The observation results indicate that the most important semidiurnal constituent at the northwest TS is M-2 and that the near-bed (0.7 mab) current speed showed spring-neap cyclic characteristics, with a maximum value of 0.453 m/s. Additionally, the water flowed in acounter-clockwise rotary current pattern. The weather was fair during the observation, and the significant wave height ranged from 0.60 m to 1.81 m, which generated an extremely low wave orbital velocity (the maximum value was 0.128 m/s). The near-bed SSC estimated from the signal-noise ratio recorded by the ADV ranged from 1.48 mg/L to 74.95 mg/L. The relation between the bottom shear stress and the SSC suggested that the local sediment resuspension is primarily controlled by a tidal current and that the wave action was practically negligible during the observation. The resulting net near-bed horizontal transport flux was 1387.59 kg/m(2) during the observations, dominated by the along-shelf flux (approximately 3.74 times than cross-shelf flux) and directed toward the NNE. The bed elevation revealed that the mean value of the erosion and deposition depth during a tidal cycle could reach 14.8 mm in an intermediate tide, with a maximum value of 17.9 mm; the net effect during the observation was erosion, with a depth of 0.4 mm. (C) 2014 Elsevier Ltd. All rights reserved.