Long term water balance and estuarine mechanism simulation of the Mfolozi River floodplain, South Africa.
The St Lucia estuarine lake is a World Heritage Site due to its size and its unique ecology. In the past, the mouths of the Mfolozi River and the St Lucia Lake were combined. During drought periods, the combined mouth would close and the Mfolozi River would provide fresh water to the estuary, thus preventing low water levels, hypersaline conditions and prolonged closed inlets to the sea from occurring – factors which are detrimental to the functioning of the estuary’s ecology. In the 1950s, the mouths were forcibly separated. This separation deprives the lake of a much needed source of fresh water, which during drought periods results in extremely low water levels and high salinity – as seen in 2006 when approximately 90 % of the lake bed was exposed and hyper-saline conditions were prevalent. Numerous solutions regarding the lack of freshwater entering the lake have been debated. One school of thought is that the fresh water from the Mfolozi River, diverted through an existing Backchannel which connects the two water bodies, could have a positive impact on the St Lucia Lake. However, as in the 1950s, the same concerns regarding the inundation of farmlands in the lower Mfolozi floodplain, as well as excessive sediments entering the lake through the Backchannel, were still relevant. The aim of this study is to determine whether the use of the Backchannel is a feasible solution. To do so, a simplified model (consistent with the limited available data) of the river floodplain, inclusive of a functioning connection to the lake, is created. Trends are then evaluated using a long-term simulation of approximately 90 years – focussing not on flood and drought conditions (of which most water flows out to sea) but rather to study a combination of wet and dry periods, sometimes spanning decades. Optimum conditions, taking into consideration the potential concerns are recommended. The possible effects of the freshwater diversion on the St Lucia Lake are also quantified. The optimum simulation performed, as part of this study, indicates whilst the diversion of freshwater through the Backchannel alleviates the problem of low water levels and high salinities in the St Lucia Lake, the inlet to the lake remains closed for prolonged periods. This prevents the recruitment of marine species through the inlet and is detrimental to the sustainability of the estuary’s ecology.