Modelling compound flooding at the Breede Estuary, South Africa

Abstract

Extreme coastal flooding along the South African coast can be caused by the co-occurrence of waves, storm surges and tides. Importantly, in low-lying areas such as estuaries, the additional influence of river discharge can further exacerbate flooding intensity. The CASISAC* project aims to assess these compound flood impacts on a local scale by modelling flood extents with input parameters generated from ocean and atmospheric models. In this study we model the flood extent and flood depth of compound flooding events, induced by high river discharge, large waves, storm surge and tides at the Breede Estuary, South Africa. We simulate compound flooding using the open source hydrodynamic model Delft3D by coupling the two modules FLOW and SWAN. The model input parameters are river discharge time series, observed tides, and modelled extreme water levels and waves. We use a high-resolution elevation model in combination with freely available global elevation data (MERIT) and bathymetry created by combining the freely available GEBCO dataset with nearshore bathymetric contour lines. We further analyse the influence of the individual flood drivers, i.e. coastal and river flooding, on flood extent and depth. Preliminary results show non-linear increases in flood characteristics when accounting for all flood drivers. We also observe that the combination of moderate intensity individual events can lead to extreme flooding. These results highlight that not accounting for compound flooding in coastal impact assessments may lead to the underestimation of flood impacts.