Saltwater intrusion in Southern Eyre Peninsula, December 2009

4.2 Application of an Analytical Model
The following sections outline an analytical method for rapid and simplified
assessment of vulnerability to seawater intrusion, especially under scenarios of
climate change. Analytical modelling can be considered a “bare minimum” approach
to the characterization of coastal aquifers for seawater intrusion studies, representing
a useful starting point before one undertakes more detailed analyses such as
numerical modelling. Furthermore, the rapid assessment of seawater intrusion using
analytical approaches allows one to prioritise data collection and modelling efforts.
This work draws heavily on journal paper content from Ward et al. (In Prep.), which
has been produced under the Fellowship. Full derivations of the analytical solutions
are covered in the journal paper.
4.2.1 Basic Model
Following Werner and Simmons (2009), we adopt a steady-state, sharp-interface
analytical solution. Figure 11 shows a cross-sectional representation of flow to the
coast, as adopted by previous studies (e.g. Strack, 1976, 1989; Naji et al., 1998;
Mantoglou, 2003; Werner and Simmons, 2009). The situation is an unconfined
coastal aquifer, bounded below by an impervious horizontal layer at a depth B [L]
below sea level. The inland distance x i [L] represents the coastal fringe, with any
pumping activities presumed to occur inland of x i . x i is expected to be on the order of
several hundred to a few thousand metres and would be chosen such that it is of a
scale relevant to planning – e.g. the maximum allowable seawater intrusion extent.
The aquifer is assumed to receive uniform recharge R [L/T] in the coastal fringe. Note
that R is net recharge, after considering losses due to evapotranspiration from the
water table. It is assumed that R ≥ 0. The system receives a lateral inflow q i [L 2 /T]
through the inland boundary. q i may be considered to be the result of net recharge,
accounting for losses due to pumping inland from x i . The discharge q o [L 2 /T] of
freshwater to the ocean is equal to the sum of the inflows (Rx i + q i ).
As a result of the density contrast between freshwater and seawater, a wedge of
dense seawater penetrates the coastal region of the aquifer, held in place by the
discharge q o . In this model, the wedge is assumed to be defined by a sharp interface.
The depth of the interface below sea-level is denoted d [L] and the wedge penetrates
inland to a maximum distance x T [L].
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