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One-dimensional airflow in unsaturated zone induced by periodic water table fluctuation

机译：地下水位周期性波动引起的非饱和区一维气流

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This paper investigates the vertical airflow driven by fluctuating water table within the lower layer of a coastal two-layered system. The upper layer is unsaturated and semipermeable, while the lower is permeable. An analytical solution of the subsurface air pressure fluctuation is derived on the basis of model simplification assumptions, the reasonability of which was examined by numerical solutions of the original nonlinear model. The airflow in the upper layer is controlled from the top by the constant atmospheric pressure and from the bottom by a temporally fluctuating air pressure P_0(t), which is spatially constant in the unsaturated zone of the lower layer. For a sinusoidal head the amplitude of P_0(t) increases with the frequency of the head fluctuation, the upper layer's thickness, and the unconfined aquifer's air-filled porosity and decreases with the upper layer's permeability. The phase shift of P_0(t) ranges from 0 to π/2, indicating a "time advance." Particularly, P_0(t) is approximately proportional to the temporal derivative of the head for sufficiently thin or permeable upper layer and to the head itself for sufficiently thick or less permeable one. The fluctuation amplitude of the water table is always less than that of the head and can be only one tenth of the latter if the upper layer is sufficiently thick or less permeable, which may slow significantly the landward attenuation speed of the tide-induced head fluctuation in a coastal "air-confined" aquifer. The analytical solution was used to estimate the value range of the air permeability of the marine sand fill at a coastal reclamation area of Hong Kong.
机译：本文研究了沿海两层系统下层内地下水位波动引起的垂直气流。上层是不饱和和半渗透性的，而下层是可渗透的。在模型简化假设的基础上，得出了地下气压波动的解析解，并通过原始非线性模型的数值解来检验了其合理性。上层中的气流从顶部通过恒定的大气压控制，从下层通过随时间变化的气压P_0（t）控制，该气压在下层的不饱和区域中在空间上是恒定的。对于正弦波的水头，P_0（t）的幅度随水头起伏的频率，上层的厚度和无侧限含水层的充气孔隙度而增加，并随上层的渗透性而降低。 P_0（t）的相移范围从0到π/ 2，指示“时间提前”。特别地，对于足够薄或可渗透的上层，P_0（t）大约与杆头的时间导数成正比，对于足够厚或不可渗透的上层，P_0（t）与杆头本身成正比。地下水位的波动幅度总是小于水头的波动幅度，并且如果上层足够厚或渗透性较低，则只能是后者的十分之一，这可能会大大降低潮汐引起的水头波动的向陆衰减速度在沿海的“气密”含水层中。该分析解决方案用于估算香港沿海填海区的海沙填充物的透气度值范围。

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