典型潮汐水道悬沙浓度的潮控机理模拟分析

Simulation and analysis of the tidal control mechanism of suspended sediment concentrations in a typical tidal channel

根据南京大学海岸与海岛开发教育部重点实验室2012年在南黄海辐射沙脊群西洋水道所获的全潮水文观测数据,分析了流速、悬沙浓度以及近底部切应力的时间变化序列.应用均衡态条件建立了悬沙浓度特征值模型,探讨了各个相关参数变化对悬沙浓度均衡态的影响.结果表明:(1)本研究区潮周期内的大部分时间近底部切应力大于沉积物临界起动切应力,再悬浮作用活跃;(2)给定初始悬沙浓度、沉降速率等参数,经过若干潮周期后悬沙浓度会达到均衡态,此时大小潮周期内侵蚀通量和沉降通量的总量相等,此后各个潮周期内悬沙浓度时间序列随潮相发生变化,但在相同潮相的情况下悬沙浓度显现出相同的时空分布;(3)若改变悬沙沉降速率大小而固定其余参数不变,则沉降速率越大,达到均衡态时的悬沙浓度特征值越低;(4)若改变再悬浮系数大小而固定其余参数不变,则再悬浮系数越大,达到均衡态时悬沙浓度特征值越高.因此,潮汐环境中的悬沙浓度特征值不仅受控于潮流的强弱,同时也受控于表层沉积物本身的性质(粒度和粘性,它们决定了沉降速率与再悬浮系数的大小).

Based on the measurements of water level,tidal currents and suspended sediment concentrations（SSCs）in the Xiyang sub-tidal channel,Jiangsu coast,within the radial sand ridges of the South Yellow Sea,which were carried out by the MOE Key Laboratory for Coast and Island Development in 2012,the time series of current velocities,nearbed shear stress and SSCs were analyzed.An equilibrium SSC model was established that includes the various feedback mechanisms and,on such a basis,the impact of different parameters on the characteristic SSC level was evaluated.The results indicate that：（1）in the study area,for most time during a tidal cycle,the near-bed shear stress exceeds the critical shear stress for initial sediment motion,with active resuspension of seabed sediment;（2）under the conditions associated with the local hydrodynamics and sediment dynamics,the SSC will reach equilibrium after several tidal cycles,at which the erosion and deposition fluxes will be balanced over a spring-neap tidal cycle;（3）a lowsettling velocity of sedimentary particles is associated with a low equilibrium SSC level;and（4）a high seabed erosion coefficient results in a high equilibrium SSC level.Therefore,the characteristic SSC is controlled not only by the flow intensity,but also by the sediment properties（i.e.,particle size and viscosity that influence the settling velocity and the erosion coefficient）.