海潮引起的滨海地区包气带气压周期性变化的数值模拟

滨海地区的地下水位由于海潮而上下波动, 这早已是为人熟知的事实.但是, 滨海地区包气带中气压也会随海潮的起伏而作周期性变化, 这一点却很少为人注意.如果地表由渗透性差的材料如水泥或沥青路面覆盖(这种情况在香港高度城市化的近海地区很常见), 海潮的起伏会产生幅度异常高的气压波动.在某些情形下, 足够高的气压会引起如路面拱起等工程问题.因此, 研究海潮引起的包气带气流, 不仅具有理论意义, 还对滨海地区土木工程有实际指导作用.以香港某滨海地区为例, 建立了该地区一剖面上水气两相流的二维数学模型, 并用TOUGH2程序进行了数值模拟.通过数值敏感度分析, 探讨了影响沥青路面下气压的主要水文地质因素.在根据该地区实际水文地质情况选取了适当的模型边界条件和模型参数后, 沥青路面下气压的数值模拟结果和观测数据吻合程度良好. 

Numerical Simulation of Tide-Induced Periodic Air Pressure Fluctuations in Coastal Vadose Zone

While it is well known that the groundwater level in coastal areas fluctuates frequently due to tidal seawater, it is not so well-recognized that air pressure in the subsurface soil near the coast may also fluctuate in response to sea tides. If the soil is well capped by less permeable materials such as concrete or asphalt, which is common in the extensively urbanized coastal areas in Hong Kong, significant high pressure may be produced by the watertable fluctuation and causes geotechnical problems such as surface heave. Therefore, understanding air flow induced by tidal fluctuation is of not only theoretical significance but also engineering implications. In this paper, a case study is conducted to explore the air pressure fluctuations observed below the asphalt pavement in a coastal reclamation area of Hong Kong. An isothermal, two-dimensional, cross-sectional model was built to describe the water-air two-phase flow in the unsaturated zone of the area. TOUGH2 simulator was used to obtain the numerical solutions. Sensitivity analysis was carried out to investigate the effects of model parameters on the air pressure fluctuations. After the boundary conditions and the model parameters were derived from the hydrogeological background in the area, the simulation reproduced the observed air pressure below the asphalt pavement with a satisfactorily small error.