Abstract
Abstract. Recent studies have drawn special attention to the significant dependencies between flood drivers and the occurrence of compound flood events in coastal areas. This study investigates compound flooding from tides, river discharge (Q), and specifically waves using a hydrodynamic model at the Breede Estuary, South Africa. We quantify vertical and horizontal differences in flood characteristics caused by driver interaction and assess the contribution of waves. Therefore, we compare flood characteristics resulting from compound flood scenarios to those in which single drivers are omitted. We find that flood characteristics are more sensitive to Q than to waves, particularly when the latter only coincides with high spring tides. When interacting with Q, however, the contribution of waves is high, causing 10 %–12 % larger flood extents and 45–85 cm higher water depths, as waves caused backwater effects and raised water levels inside the lower reaches of the estuary. With higher wave intensity, the first flooding began up to 12 h earlier. Our findings provide insights on compound flooding in terms of flood magnitude and timing at a South African estuary and demonstrate the need to account for the effects of compound events, including waves, in future flood impact assessments of open South African estuaries.
Highlights
Floods, regardless of fluvial or oceanic origin, are among the world’s most devastating coastal hazards, causing numerous deaths and large economic losses on an annual basis (Kirezci et al, 2020)
As we focus on the additional contribution of waves during compound flooding, the model domain is restricted to the lower estuary (Fig. 2)
As flooding is usually caused by peak water levels (WLs) and simulated peaks showed an root mean square error (RMSE) of 0.15 m compared to observations for all validation runs, we considered the model performance as fit for purpose
Summary
Regardless of fluvial or oceanic origin, are among the world’s most devastating coastal hazards, causing numerous deaths and large economic losses on an annual basis (Kirezci et al, 2020). The interactions of oceanographic, hydrological, and meteorological phenomena can lead to extensive flooding. In estuaries, such floods can result from combined spring tides and extreme wave or storm surge conditions occurring simultaneously with high river discharge (Kumbier et al, 2018; Olbert et al, 2017; Ward et al, 2018). These events are commonly referred to as compound flood events. Neither of the drivers needs to be extreme in order to cause severe impacts as drivers that occur simultaneously or successively can result in extreme events which contribute to societal or environmental risk (Leonard et al, 2014; Seneviratne et al, 2012; Zscheischler et al, 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
