Process-based modelling of the St Lucia Estuary

Abstract

The St Lucia Estuary is Africa’s largest estuarine system and part of the iSimangaliso Wetland Park, which was listed as South Africa’s first World Heritage Site in December 1999. The St Lucia Estuary is connected upstream to Lake St Lucia and downstream it flows into the Indian Ocean. The St Lucia Estuary is located at a wave dominated coast and experiences a micro-tidal regime. Along the coast of South Africa about 70% of the estuaries do not have a permanently open link to the sea, i.e. they are temporarily open/closed estuaries, which are mainly regulated by the amount of river flow entering the estuary. Anthropogenic impacts and fresh water reductions have altered the natural system. The mouth of the St Lucia Estuary is unstable and has been closed for longer periods in the last century. The drought period ranging from 2002 has drastically affected the St Lucia System as a whole with the desiccation of 90% of the lakes, reaching the all-time low. The management authorities had to take action and research supported by the Global Environment Facility (GEF) which started in 2009 showed re-linkage of the Mfolozi River is the preferred solution to bring fresh water to the St Lucia System. This MSc study was initiated to investigate the effects of re-linkage of the Mfolozi River and dredge spoil removal from the estuary on the tidal inlet. The main focus is on the closure times of the inlet, the transition between an open – and closed inlet, and the multi-year morphological development of the inlet. The process-based model Delft3D is used to simulate various scenarios. The model is calibrated quantitatively on hydrodynamics and qualitatively regarding morphodynamics. The results from this study show that the re-linkage of the Mfolozi River leads to an inlet that remains open for majority of the time leading to a connection between the St Lucia Estuary and the Indian Ocean. This can make the St Lucia Estuary and Lake System a breeding ground for many marine species as it was before. The removal of dredge spoil however does not seem to be very effective. The closure times of the inlet barely change and the inlet re-imports most of the sediments. It is advised to limit the amount of dredge spoil removal, because the estuary will return to its equilibrium state.