Tide-induced air pressure fluctuation in coastal unsaturated zones

Abstract

Air pressure in coastal unsaturated aquifers fluctuates with sea tides, as does the groundwater level. Tide-induced air pressure fluctuations in coastal subsurface systems, however, are not well addressed in literature. Significant diurnal air pressure fluctuations are observed below the asphalt pavement in a coastal reclamation area of Hong Kong. A two-dimensional cross-sectional model is built to describe the air-water two-phase flow in the unsaturated zone of the area. TOUGH2, a general-purpose numerical simulation program for multi-dimensional fluid and heat flows of multiphase, multicomponent fluid mixtures in porous and fracture media, is used to obtain the numerical solutions. The numerical simulation reveals that the significant air pressure fluctuation results from a combination of various factors such as asphalt pavement of extremely low permeability, soil structures of the reclamation area, and rainfall intensity during spring tides. Numerical simulation also reveals that the air pressure is approximately proportional to the tidal rate. A great amplitude of the sea level fluctuation will not necessarily lead to high air pressure, if the sea level changes slowly. Coastal unsaturated zones inhale and exhale as the tidal level falls and rises. The paper goes on to discuss various engineering and environmental implications of air pressure fluctuations in coastal aquifers.