The coastal lagoon adjacent to the River Murray mouth has been filling with sediment brought in from the ocean by tidal currents for at least the last 170 years. Recently, reduced river flows have led to an increase in the rate of infilling, and in 2002 a program of dredging commenced in the lagoon to remove the millions of cubic metres of sediment that had accumulated there. Elsewhere, researchers have argued that it should be possible to manipulate the natural tidal asymmetry and, in doing so, alter the balance between ebb and flood dominance in a coastal lagoon. If this is possible in practice, it should be feasible to move a lagoon towards a self-flushing mode through a program of strategic dredging.
Geographic Information Systems (GISs) were used to interpret bathymetric survey data in the lagoon so that channel characterisation, changes to sediment volume, and basin hypsometry could be analysed. Analysis of water-level data in the lagoon showed that flood durations were usually shorter than ebb durations, which demonstrated the occurrence of flood dominance. It was also found that the degree of flood dominance has reduced since dredging. The dredging has led to fewer, deeper channels and hence a reduction in the degree of flood dominance. The main conclusion is that it has been possible to manipulate the tidal asymmetry through the use of dredging and move the lagoon system towards ebb dominance and, therefore, natural flushing.