Instabilities of submarine groundwater discharge under tidal forcing

We conducted laboratory experiments and numerical modeling to investigate the instability of the freshwater-seawater interface and associated submarine groundwater discharge locations under tidal forcing. Contrary to today’s common view of the hydraulic conditions in the intertidal region, i.e., the formation of a stable upper saline plume concurrent with the existence of a so-called “freshwater discharge tube” below, which pinches out at the beach surface close to the low tide mark, is actually extremely unstable under certain conditions. We found that for a gentle beach slope of 1: 12, a transient upper freshwater-seawater interface develops between the high and low water line characterized by several migrating saltwater fingers that intrude into the aquifer. Groundwater discharge occurs between these saltwater fingers at various locations in the intertidal region and is not limited to the low tide mark. Numerous scenarios are possible between the stable and the unstable cases depending on the ambient groundwater flux, tidal amplitude, beach slope, permeability of the sediment, heterogeneity, and anisotropy.