Abstract of the proposal

The tide-influenced coastal margin is an environmentally and ecologically sensitive zone. Coastal groundwater regime has a significant impact on the coastal environmental and ecological conditions. Air pressure in coastal unsaturated zones fluctuates with sea tides, as does the groundwater level in the aquifers. Although tide-induced groundwater flow in saturated aquifers has been widely investigated, air pressure fluctuations in unsaturated zones has not been well addressed in the literature. We propose to investigate systematically and quantitatively the phenomenon of air flow driven by water table fluctuation induced by sea tides by combining laboratory and field experiments with analytical and numerical modelling. We will also examine physical processes including enhanced rainfall infiltration and contaminant movement in unsaturated zones caused by cyclic air pressures. Some engineering and environmental implications of tide-driven air flow will also be explored. Successful completion of this study will provide a much better understanding of the interactions among the sea tide, groundwater table, air and water flow, barometric pressure, and rainfall. We hypothesize that tide-driven subsoil air flow may have important environmental and engineering implications such as the stability of ground surface of coastal structures of great areal extent and the respiration rate for aquifer bioventing. Thus, the findings from this project will not only have theoretical interest but will be of practical benefit for coastal communities such as Hong Kong and elsewhere..

Left: Simulated air pressure (color) and flow velocity (arrows) distributions when water table falls (a) and rises (b). Negative pressure is created and the ground inhales when the water table falls. High pressure is created and the ground exhales when the water table rises. Right: Simulated degree of water saturation (color) and flow velocity (arrows) distributions when the water table falls (a) and rises (b). Perched water is formed above the geotextile. Soil below the surfaces covered by pavement is relatively dry and the thickness of the perched water on the geotextile below the pavement is relatively small. The areas below the pavement become the main pathway and storage of air when the water table rises.

(from Jiao JJ & HL Li, 2004, Breathing of coastal vadose zone induced by sea level fluctuations, Geophysical Research Letters, Vol. 31, No. 13 L11502 10.1029/2004GL019572. [Full paper] )

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