Compound flooding due to interaction of waves and river discharge at Breede Estuary, South Africa

Abstract

Recent studies on compound flooding have considered the interaction of storm-tide and fluvial or pluvial flood drivers, whereas the contribution of waves to compound flooding so far has been neglected. In this study, we investigate compound flooding from waves, tides and river discharge at Breede Estuary, South Africa, using a hydrodynamic model. We estimate the contribution of extreme waves to compound flooding by analysing the driver interaction and by quantifying vertical and horizontal differences of flood characteristics. We further consider the additional effect of waves on flood timing and compare results of compound flood scenarios to scenarios in which single drivers are omitted. We find that flood characteristics are more sensitive to river discharge than to waves, particularly when the latter only coincide with high spring tides. When interacting with river discharge however, the contribution of waves is high, causing larger flood extents and higher water depths. With high wave intensities the first flooding can begin up to 12 hours earlier. Our findings provide insights on the magnitude and timing of compound flooding in an open South African estuary and demonstrate the need to account for the effects of waves during compound flooding in future flood impact assessments of similar coastal settings.