The influence of the Maputo and Incomati rivers on the mixing and outflow of freshwater from Maputo Bay (Mozambique)

Abstract

Markull, K., Lencart e Silva, J.D., Simpson, J.H., Dias, J.M., 2014. The influence of the Maputo and Incomati rivers on the mixing and outflow of freshwater from Maputo Bay (Mozambique). In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13 th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 580-585, ISSN 0749-0208. Maputo Bay is a tidally-energetic embayment, influenced by strong rainfall and associated river runoff during the wet season. Literature shows that salinity can regulate the nutrient cycle in mangrove estuarine ecosystems affecting the early life stages in these habitats which sustain the economically important shrimp stocks. The freshwater flow into Maputo Bay is for a part controlled by dam systems on its main rivers. In this work we investigate how varying flows from the Incomati and Maputo rivers interact with the tide to influence the evolution of the bay’s salinity field. A 3dimensional hydrodynamic model was applied to Maputo Bay, improving a previously published model through vertical and temporal refinement and recalibration, leading to a more accurate representation of the semidiurnal and fortnightly stratification-mixing cycles occurring during the wet season. However, the model still predicts salinities lower than those found in observations. An analysis showed that uncertainties in the salinity field increased towards the mouth of the Maputo River indicating the uncertainty of the modeled flow at the catchment as a possible cause of the underestimation of salinity in the bay. A set of experiments of varying Maputo and Incomati river flows show that for a flow as frequent as 5 times per year, the buoyancy input and the associated density driven flow contributed less than the tide in forcing the bay-shelf exchange. The inverse is observed for results where a flow with a 5-year return period leads to a more efficient bay-shelf exchange during neap tides than during spring tides. For this scenario, the results suggest that the estuarine plume was arrested inside the bay by tidal stirring during spring tide and released during neap tide when tidal stirring was subdued. The analysis of the separate and joint effects of the two rivers in setting the salinity field in Maputo Bay show a difference in extent of influence area between the rivers. These results suggest that the management of the freshwater inflow from the Maputo River can be crucial for maintaining lower salinities over parts of the bay to sustain the economically important marine resources.