TIDALLY-INDUCED GROUNDWATER FLUCTUATIONS IN AN UNCONFINED AQUIFER STUDIED USING A HELE-SHAW CELL

As ground water moves through the subsurface it is frequently discharged into rivers, lakes, estuaries, and oceans; and it is well known that tidal fluctuations affect groundwater flow on the margins of these environments. To study the affect of the diurnal rise and fall of the tide within an unconfined groundwater system, and the extent to which tidal fluctuations affect rates and patterns of seaward fluid motion, several vertical Hele-Shaw experiments were conducted. The Hele-Shaw apparatus is ideal for modeling flow in a porous medium, particularly in relation to nearshore geometries, because the equations of motion are analogous to those for porous media due to the predominant effects of viscous forces, and to ensure that viscous forces dominated, glycerin was used as the experimental fluid.

The Hele-Shaw cell used was custom built with a realistic boundary condition (10º slope) and placed inside of a 125.0 cm by 30.0 cm by 55.4 cm acrylic plastic tank; and to create a tidal oscillation a plunger was raised and lowered within the tank. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques were used to visualize the flow within the unconfined model, and based on initial results it is clear that tidal forcing creates a distinct circulation within our model, restrained spatially by the amplitude of tidal fluctuations and temporally by the tidal period. Along a sloping surface, vertical infiltration during a flooding tide is greater than horizontal seepage during an ebbing tide resulting in a tendency for unconfined aquifers to fill faster then they drain. This phenomenon is believed to be responsible for the observed circulation, which occurs between the high tide mark and well below the low tide line. When scaled to the physical setting, our model indicates that tidally-induced fluctuations to groundwater flow extend vertically down to about 10 m in an unconfined aquifer. The maximum horizontal seaward flow and maximum vertical flow within the model, averaged over three tidal periods, were both about 0.3 mm/s, which also agree reasonably well when scaled to the natural environment. The standard deviation of flow velocities is largest in the zone of circulation, indicating that there is a substantial oscillatory component within the overall flow, which was confirmed by creating PTV animations.