海岸带含水层咸淡水界面随潮汐波动的数值模拟

Numerical simulation of fresh-saline water interface interactive regularities in coastal areas due to the tidal fluctuation

在分析、研究了滨海地带含水层和不透水层的水文地质特性及含水层内的咸淡水渗流运动特征的基础上,运用数学推理的手法,建立了一种较为简单适用的模拟海岸带含水层咸淡水界面和天然地下水面变动规律的二维数学模型,通过该模型的计算值与实验值的比较,证明该模型的计算结果与实验结果有着非常好的拟合度,即该模型能较客观地揭示海水入侵引起的咸淡水界面的变化规律。其后,运用该模型系统的探讨了含水层以下为非平坦的不透水层的滨海地带天然地下水面、咸淡水界面伴随着潮汐的波动而变化的规律。即伴随着潮汐的波动,一方面天然地下水面和咸淡水界面与潮汐具有相似的振动波形,波动的幅度随离海岸距离的增加而减小。在海岸附近咸淡水界面的振幅大于天然地下水面的振幅,而在离开海岸一定距离后天然地下水面的振幅超过咸淡水界面的振幅。总体上天然地下水面的振幅呈负指数衰减,而咸淡水界面的振幅几乎呈直线衰减(其衰减直线的倾角在135～150°之间变动),且潮汐波动对天然地下水面影响的范围远大于其对咸淡水界面的影响;另一方面天然地下水面和咸淡水界面波动的振幅及它们之间的相位差的大小还与天然地下水面的水力坡度、含水层的渗透系数、有效孔隙率、不透水层形状及其变动的幅度有关,但它们之间的相位差的大小与潮汐波动的幅度无关,其振幅与潮汐的振幅成比例增减。当不透水层的形状一定时,不论天然地下水面的水力坡度、含水层的渗透系数、有效孔隙率及潮汐波动的幅度怎么变动,天然地下水面、咸淡水界面振动的相位差的变化趋势及其峰值出现的位置几乎不变,即不透水层的形状决定着天然地下水面、咸淡水界面振动的相位差的变化趋势,而其大小与含水层的水文地质参数(水力坡度、渗透系数及有效孔隙率)密切相关。此外,当天然地下水面和咸淡水界面的振动存在相位和振幅的较大差异时,可以断定含水层以下存在非平坦的不透水层,且这种差异越大,不透水层凸凹不平的程度越高。

This paper is inclined to present a simple but effective 2D mathematical model to simulate the interactive regularities of freshsaline water interface and natural groundwater surface in coastal aquifer. The above said model has been established on the basis of careful observation and investigation of the hydrologic and geologic characteristics of coastal aquifer and impervious layers and the features of seepage flow of fresh water and salt water in coastal aquifer. The model has also passed corresponding validity verification by comparing experimental data concerned. What is more important, the paper has carefuUy studied the interactive regularities of the interface change of the two kinds of water with the tidal fluctuation. That is to say, on the one hand, natural groundwater surface can be found similar to that of fresh-saline groundwater with the wave surge and the wave receding, with their fluctuation decrease with the increase of the distance from sea coast and increase with tiding surge. Moreover, the impact sphere of the tidal fluctuation on the natural groundwater surface can be much bigger than that on fresh-saline groundwater interface. On the other hand, the vibration impact of natural groundwater surface and fresh-saline groundwater interface on the oceans waters and the phase difference between them is closely related to the hydraulic gradient of natural groundwater surface, permeability coefficient and the active porosity of aquifer,, shape and changing extent of impervious layer. But the phase difference is independent of tidal fluctuation amplitude in proportion to that of the tidal fluctuation. In addition, if the impervious layer shape is known, no matter what changes take place with the hydraulic gradient of natural groundwater surface and the other factors, no change will take place with phase difference between natural groundwater surface and fresh-saline groundwater interface and the position of peak value, i.e., the changing trend of the phase difference is determined by impervious layer shape. Furthermore, if there are great differences between groundwater surface and fresh-saline groundwater interface in phase and vibration amplitude, it can be concluded that there would be un- even impervious layer under the aquifer. The greater this difference, the higher the convexo-concave degree of the impervious layer would be.