波流作用下黄河三角洲硬壳层液化渗流形成机制研究

MECHANISM OF LIQUEFACTION SEEPAGE OF UPPER SEABED LAYER IN THE YELLOW RIVER DELTA UNDER WAVE-CURRENT VIA NUMERI-CAL SIMULATION

波流作用于海床产生动态孔隙水压力,如不能及时消除会在其内部产生累积孔隙水压力,相邻两点间的孔隙水压力差值造成的水力梯度产生渗流力,渗流力引起水流动,海床表面为排水界面,从而会在海床内部形成向上的渗流力作用于泥沙颗粒上,使泥沙发生启动向海床表层运移,从而形成一定范围的粗颗粒层。本文采用数值模拟对不同流速下的海床累积孔隙水压力进行了研究,同时分析了硬壳层的存在对海床累积孔隙水压力的影响规律,根据取得的不同流速下海床内部的累积孔隙水压力值,计算海床任意位置处的渗流压力梯度,采用王虎等(2014)推导建立的海床临界冲刷深度的计算方法,分析不同流速下的硬壳层形成深度。结果表明:海流流向与波浪行进方向一致时,对累积孔隙水压力起促进作用,流速越大累积孔隙水压力越大,反之对累积孔隙水压力有抑制作用。表面硬壳层的存在会显著促进累积孔压的消散,累积孔隙水压力沿深度分布的极值均出现在下层原始海床中,流速U 0=0m·s^(-1)时硬壳层厚度由1m增加到3m,极值点深度下降了1.38m。累积孔隙水压力引起的渗流力对于海床泥沙启动影响显著,在流速U 0=0m·s^(-1),U 0=1m·s^(-1)时泥沙启动深度均为海床1.5m深度处,并且海流流向与波浪行进方向一致时,会产生较大ΔP/ΔL值带动较粗的泥沙颗粒至海床表层,但对泥沙启动的最大深度影响不大。

Dynamic pore water pressure would occur due to the wave-current acts on the seabed.If it cannot be eliminated in time,cumulative pore water pressure would grow in the seabed.The hydraulic gradient caused by the difference of pore water pressure between two adjacent points can produce seepage force,and then cause water flow.The seabed surface is a drainage interface,so an upward seepage force would form in the seabed and act on the sediment particles.It leads to the sediment transport and movement to the seabed surface,thus forming a certain range of coarse-grained layer.In this paper,numerical simulation is used to study the cumulative pore water pressure under different velocity.The influence of the hard shell layer on the cumulative pore water pressure is analyzed.The calculation method of critical scour depth of seabed established by Wang et al.(2014)is used to analyze the final depth of hard shell layer under different velocity.The results show that when the direction of wave and current is the same,it would promote the cumulative pore water pressure.The greater the velocity is,the greater the cumulative pore water pressure is.The opposite direction would inhibit the cumulative pore water pressure.The surface hard shell layer can significantly promote the dissipation of cumulative pore pressure.When the velocity U 0=0m·s^(-1),the thickness of the hard shell layer increases from 1m to 3m,and the depth of the extreme point decreases by 1.38m.The seepage force caused by the cumulative pore water pressure has a significant effect on the seabed sediment movement.When the velocity at both U 0=0m·s^(-1) and U 0=1m·s^(-1),the depth of the sediment incipient motion is 1.5m.If the direction of wave and current is the same,it would produce a largerΔP/ΔL value to drive the coarse sediment particles to the surface of the seabed,but has little effect on the maximum depth of the sediment incipient motion.