A state of equilibrium
Despite the riverbank erosion, the isthmus has maintained its shape over the last 25 years. We found that overwash events transported sediment from the beach to the riverbank, compensating for the erosion. Although one might expect beach erosion due to this loss of sediment, our analysis of the aerial images indicates that this was not the case.
Using MIKE 21 Spectral Waves (SW), we assessed nearshore wave conditions along the Macaneta Peninsula. We ran the model for an eight-year period (2005-2013) corresponding to the period for which wave data was available. We found that waves approach the Macaneta Peninsula at a right angle due to the presence of Inhaca Island and a 35 km long shallow area off the coast.
Utilising the LITDRIFT module of LITPACK, we analysed sediment samples and calculated littoral drift (the transportation of sediments along a coast at an angle to the shoreline). We evaluated the direction of the sediment transport. We also determined the equilibrium orientation of the coast – when the beach faces straight towards the mean wave direction, resulting in zero net yearly sediment transport. We found that the Macaneta coast is stable as it is aligned with the equilibrium orientation determined from the wave conditions.
Based on our analysis, the dynamics of the river and the sea seem to have reached a balanced state that maintains the isthmus. This means that the river is unlikely to break through the isthmus. If a breakthrough were to occur, it would likely be due to an extreme wave and water level event, such as a storm with surges and large waves.
With this information, Mozambique now has a better understanding of the:
- general processes acting on the Macaneta coast
- processes that could cause the Incomati River to break through the isthmus
- possible consequences of such a breakthrough
This will enable them to weigh and prioritise protection efforts in the future.