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

Abstract

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 M2 and that the near-bed (0.7mab) current speed showed spring-neap cyclic characteristics, with a maximum value of 0.453m/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 0.60m to 1.81m, which generated an extremely low wave orbital velocity (the maximum value was 0.128m/s). The near-bed SSC estimated from the signal-noise ratio recorded by the ADV ranged from 1.48mg/L to 74.95mg/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 1387.59kg/m2 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 14.8mm in an intermediate tide, with a maximum value of 17.9mm; the net effect during the observation was erosion, with a depth of 0.4mm.