Modes of water renewal and flushing in a small intermittently closed estuary

Abstract

Renewal and flushing of the waters of small, bar-blocked or intermittently closed estuaries is of importance for the maintenance of acceptable ecosystem functioning and socio-economic benefits. Three events in the Great Brak Estuary in South Africa are examined and the efficacy of water renewal and flushing of the water body is elucidated using empirical observations and simple, predictive models. Results indicate that a major controlled freshwater release accompanied by mechanical mouth breaching, mimicking a major flood, is effective in flushing the entire estuary. In contrast, a minor controlled freshwater release purges the bulk of the surface waters in the upper reaches, but the flushing of old, estuarine water in the lower and lower middle reaches is only achieved by the tidal intrusion of new seawater following mouth breaching. The combined effect of a minor controlled freshwater release, and secondary tidal flushing, is not effective in flushing the plug of old saline water in the upper middle reaches. This observation accords with predictions from the hydraulic model developed by Coates and Guo (2003) for calculating the longitudinal displacement of saline estuarine water by freshwater flows. Finally, we highlight the efficacy of an episodic overwash event in renewing the bottom water of the bulk of the estuary through density-induced intrusion of seawater. To our knowledge this mode of water renewal (accompanied by mechanical mouth breaching) has not received attention as a possible alternative flushing option in the sustainable management of estuaries which suffer from severe freshwater deprivation. This deeper understanding of the efficacy of water renewal and flushing modes in small, bar-blocked estuaries can improve the design of environmental flow and mouth breaching policies.