The Effect of Stochasticity of Waves on Coastal Flood and Its Variations with Sea-level Rise

Abstract

Coastal floods are driven by many hydro-meteorological forcing factors, among which are mean sea levels, tides, atmospheric storm surges, and waves. Depending on these conditions, wave overtopping may occur and, in some cases, lead to a significant flood. In the present study, we investigate the effect of the stochastic character of waves on the flood itself using a phase-resolving wave model (SWASH). We focus on the macro-tidal site of Gâvres (France), consider two past flood events (both resulting from wave overtopping), and investigate how the effect of randomness of waves on the flood is changing with the forcing conditions and the time span (minutes to hours). We clearly show that the effect of waves’ stochasticity on the flood itself is far from being negligible and, especially on a short time scale (~15 min), generates an uncertainty comparable to that induced by the sea-level rise scenarios, as long as the still water level remains smaller than the critical level above which overflow occurs. This implies that lower confidence should be assigned to flood projection on sites where wave overtopping is the main process leading to flood.

Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site

“…13) whatever the time horizon. This result differs from the one of Idier et al (2020b), who showed the importance of this effect that was there comparable to the one of SLR as long as the still water level remains smaller than the critical level above which overflow occurs. The differences between both studies may be explained by the differences in the procedure.…”

“…A global flooding indicator is defined as the total water volume Y that has entered the territory at the end of the simulation. To account for the random character of waves, the modelling of the coastal flood induced by overtopping processes is combined with a random generation of wave characteristics in SWASH as described by Idier et al (2020b). For given offshore forcing conditions, the simulation is repeated 20 times, and the median value (denoted Q50) of Y is calculated as well as the quartiles (25 th and 75 th percentiles, respectively denoted Q25 and Q75).…”

“…The differences between both studies may be explained by the differences in the procedure. Idier et al (2020b) analysed this effect for two specific past storm events, whereas our study covers a large number of events by randomly sampling different offshore forcing conditions (Fig. 4).…”

A User-Oriented Local Coastal Flooding Early Warning System Using Metamodelling Techniques

“…In the framework of the present study, we follow the last approach mentioned above and use the non-hydrostatic phase-resolving model SWASH [4,36] to model floods at the study site (Figure 1b). The modelling chain was previously set up by [6,7]. We briefly summarise the key elements here.…”

“…Floods can be induced by overflow (when the total water level, including the wave setup, exceeds the coastal defence crests) and wave overtopping. Owing to the significant progress achieved in numerical hydrodynamic models (especially phase-resolving models, e.g., Simulating Waves till Shore (SWASH), [4]), it is now possible to precisely reproduce floods, even in the case of wave overtopping (see, e.g., [5][6][7]).…”