CSDMS 2016 annual meeting poster JilianXiong
Effect of waves on shear stress over the offshore shoal in the southern Yellow Sea
Ya Ping Wang, Nanjing Univesity Nanjing , China. ypwang@nju.edu.cn
Xiao Hua Wang, Sino-Australian Research Centre for Coastal Management, UNSW Canberra at the Australian Defence Force Academy Canberra , Australia. X.Wang@adfa.edu.au
Abstract:
To obtain the accurate estimation of critical shear stress (τcr) based on in situ observations and evaluate effect of waves over the tide-dominated offshore shoal, water depth, near-bed current velocity (0.3m above the seabed), suspended sediment concentrations (SSCs), wave parameters and bottom sediment compositions were measured in the southern Yellow Sea. Based on these data, we calculated bottom shear stresses generated by current (τc), wave (τw), and wave-current interaction (τcw). Values of τcw were calculated according to the Grant-Madsen model and the Soulsby model, both giving a quite close tendency. It showed that during the observation period, sediment movements were influenced by current, wave or wave-current action for different tidal cycle. However, due to the mild weather condition and sheltering of the huge sand ridge, waves can only influence sediments when winds were strong or during slack shallow water when there was easily wave penetration onto seabed at the lowest water depth. SSCs in present research were larger than particular values for most of the time, suggesting the existence of background SSCs no matter whether tidal currents were strong or not. Through SSC time series, the background SSCs at stations d1, s1, d2 and s2 were estimated as 0.17, 0.20, 0.13 and 0.05 kg/m3, respectively. Combing the Rouse profile, background SSCs were also estimated. Giving reasonable estimations of τcr and settling velocity (ws), the results turned out that background SSCs obtained from the Rouse profile were the same as the method of SSC time series. Meanwhile, the harmonic analysis method was then used in order to solve semi- (representing advection) and fourth-diurnal (representing local resuspension) constituents of SSC time series. By comparing the amplitude of semi-diurnal and fourth-diurnal constituents, results showed that except for station s2, SSCs were mainly controlled by local resuspension. At station s2, the SSC was dominated by advection, may due to the specific geomorphology. Obvious resuspensions were observed in the flood stage or ebb stage or both. Therefore, combining synchronous variations between SSCs and τcw, the temporal-changed critical shear stress (τcr) at each station was identified and the average τcr were estimated as 0.13, 0 .08, 0.08, 0.07 N/m2 for stations d1, s1, d2, s2. According to the classical method based on bottom grain size, the critical shear stress (τ*) was 0.18, 0.15, 0.14, 0.15 N/m2 for stations d1, s1, d2, s2. Since the classical method only produced one constant value, it suggested the necessity of in situ estimation of τcr. This study also emphasized the importance of combining wave and current into sediment dynamics of coastal environment.
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