Forecasting cyclic coastal erosion on a multi-annual to multi-decadal scale: Southeast African coast

Abstract Coastal erosion on the southeast African coastline shows an apparent 18 year cycle which last peaked in 2006. It is in phase with the longshore sediment transport cycle. Both these cycles appear to be in phase with the Lunar Nodal Cycle (LNC). However, the dominant tidal erosion driver on this coast appears to be the 4.4 year Lunar Perigean Subharmonic (LPS). We suggest that the apparent 18 year coastal erosion and longshore sediment budget cycle is a response to the 18 year Mean Annual Precipitation Cycle. This cycle is 180° out of phase with the apparent coastal erosion- and longshore sediment transport- cycles. The summer rainfall areas, of southeastern Africa show an 18 year MAP cyclicity, which drives river runoff and hence controls sediment input to the coast and nearshore environment. The MAP cycle dominates the coastal sediment budget during the LNC trough and suppresses the LPS coastal erosion cycle during this time. This explains why LPS coastal erosion occurs close to the LNC peak. Thus although the LPS cycle dominates the coastline, it is masked during the wet portion of the 18 year MAP cycle. It seems very likely that the LNC drives the MAP cycle in some way but this process is not known. Nevertheless, these relationships can be used to predict, in a general way, both cyclic coastal erosion and the longshore sediment volume fluctuation. This can be translated into a vital coastal planning tool which has the potential to forecast cyclic coastal erosion and hence significantly reduce the sea-defense expenditure bill. Based on this, severe cyclic coastal erosion is anticipated in 2023 and 2024.