TR(BR)-16/99-2000 : Modelling of sea water intrusion

Abstract

Coastal aquifers that are in hydraulic continuity with the sea are vulnerable to seawater intrusion. Besides the lateral intrusion of seawater, the coastal aquifer may also be subject to a vertical recharge of saline water due to submergence of land under saline water during cyclonic storms and infiltration of saline water from aquaculture tanks. This is a familiar scenario along the low-lying eastern coast of India. Under such conditions, the dynamic equilibrium existing between freshwater and seawater in a coastal aquifer may get disturbed. For a sustainable development of these coastal aquifers, it is thus essential to know the response of the freshwater-saltwater interface for a given hydrologic condition.

In this study, a two-dimensional numerical model in the vertical plane for simulating miscible transport of saltwater in a coastal aquifer has been developed. In order to simulate the variable density flow, the governing partial differential equations of flow and solute transport have been written in terms of pressure and concentration, respectively. The Iterative Alternating Direction Implicit (IADI) scheme of finite differences has been used to solve these non-linear equations. Density coupling of these equations is accounted for and handled using a Picard sequential solution algorithm with provision for automatic reduction of time step values when the stipulated number of iterations is exceeded. The model has been validated using the semi-analytical solution of the benchmark Henry's Problem for saltwater intrusion in a coastal aquifer as well as the SUTRA numerical solution for this test problem. Different hypothetical scenarios have been simulated using the model, to evaluate the short-term and long-term effect of vertical saline water recharge on the fresh water seawater mixing zone.