Exploring the Phenomena of Fingering Flow in the Subterranean Estuary upon Oceanic Oscillations

Submarine intertidal zones are dynamic regarding flow and transport, complex biogeochemical processes, and variable volumes of freshwater discharge. In the nearshore area, the land-derived groundwater associates with recirculated seawater and forms an active mixing zone. Thus, submarine groundwater discharge (SGD) has a critical role in the hydrological cycle by delivering terrestrial nutrients and metals from the coastal aquifers to the marine environment. The density difference between freshwater and seawater causes buoyancy effects which are also responsible for the formation of a saltwater wedge (SW) in coastal aquifers. Under the influence of oceanic oscillations (tides and waves), seawater recirculates beneath the intertidal zone and is commonly called upper saline plume (USP). This USP lies over the terrestrial groundwater, which discharges from the beach aquifer to the proximity of the low watermark (ebb tide) as a tube. The location, dimension, and stability of the USP, along with the freshwater discharge zone (FDZ) are driven by 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 conditions causing the USP to sink down to the bottom of the aquifer generating fingering flow have 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. The present study focuses on the physical properties of three homogeneous unconfined aquifers with two different beach slopes, and tidal oscillations as driving forces for the USP instability. The flow patterns and transport processes will be examined by both laboratory experiments and numerical simulations. The study aims to amplify the current understanding of the generation of the salt fingering flow and its significance on SGD patterns. Moreover, the results will be used to further assess the stability diagram of fingering flow prediction in both field and laboratory scaled conditions. The findings of this research will help to better understand the distribution and extent of the salinisation in subterranean estuaries under tidal influence.