Groundwater Dynamics and Salinity in Coastal Barriers

Abstract

A qualitative description based on field measurements is given together with a simple modelling framework for watertable heights and salinity structure in coastal barriers. In coastal barriers of width less than one kilometre the shape of the watertable and the salinity structure are quite different from the classical scenarios. On this scale the extra watertable height caused by wave runup on the ocean side can drive a significant landward ground water velocity. A primary consequence of this is that any wastewater released into the aquifer, including oil spills on the beach, will travel towards the land rather than towards the ocean. Secondly, this landward flow of salty groundwater makes the freshwater lens much thinner than the "Ghyben-Herzberg thickness". A shallow aquifer model is presented for the watertable including quantitative boundary conditions that account for the effects of waves and tides. The salinity structure is modelled in terms of the thickness of the freshwater lens, accounting for freshwater recharge as well as convective and diffusive salt transport. The field measurements indicate that 1D modelling can be done with a simple uncoupled model except within a narrow (one or two metres) diffusive boundary layer on the landward side of the barrier within which the freshwater displacement thickness varies very rapidly.