Fingering Flow in the Subterranean Estuary under Tidal Influence

In a coastal aquifer under tidal influence, seawater recirculates beneath the intertidal zone along the classical saltwater wedge (SW). The recirculating seawater on top of the saltwater wedge is called upper saline plume (USP). In between the USP and SW, a freshwater discharge tube (FDT) prevails. Both fresh and saline water components leave the aquifer and flow into the sea as submarine groundwater discharge (SGD). Due the density-gradient across the saline/fresh interface, the USP is prone to instability, resulting in the fingering flow. Whether the USP becomes instable or not depends on several factors, for instance, hydrological (tidal amplitudes, storm surges, precipitation, freshwater flux), morphological (beach slope, aquifer depth), and physical-chemical (temperature, pressure, and dissolved solids of the fluid) boundary conditions as well as aquifer physical properties (porosity, permeability). Unstable USP, which tends to sink to the bottom of the aquifer generating salt fingers, has been described in the context of numerical studies and physical experiments. Yet, flow and transport patterns and the effects of the boundary conditions and parameters described above are not well understood. USP alters the travel path and time of terrestrial nutrients, metals, and contaminants from the coastal aquifers to the marine environment, necessitating a thorough investigation to reveal its critical role in the hydrological cycle. 

For the present study, laboratory sand tank experiments have been carried out to evaluate the effect of homogeneous/heterogeneous conditions, beach slope, fresh groundwater influx and tidal amplitude on USP instability. The results have been used to delineate the conditions that either promote or suppress fingering flow in a tide affected aquifer.  The results define beach slope as the foremost parameter, along with tidal frequencies and beach morphology for the instability of USP. The tank experiments also support the general idea that the presence of low permeable layers disrupts USP formation. Regardless of the aquifer medium, the 3D effect of the salt fingers was observed during the experiments. Furthermore, the laboratory results are found to be consistent with results from previously undertaken generic simulations for field scale conditions. It appears that both laboratory and field scale behavior can be predicted in previously developed non-dimensional stability diagram that separates unstable from stable conditions.