Impact of wave-water level non-linear interactions for the projections of mean and extreme wave conditions along the coasts of western Europe

Abstract

Abstract. Wind-waves are a main driver of coastal environment changes. Wave setup and runup contribute to coastal hazards such as coastal flooding during extreme water level (EWL) events. Wave characteristics used to estimate wave setup are sensitive to changes in water depth in shallow waters. However, wind-waves models used for historical simulations and projections typically do not account for water level changes whether from tides, storm surges, or long-term sea level rise. In this study, the sensitivity of projected changes in wind-wave characteristics to the non-linear interactions between wind-waves and water level changes is investigated along the Atlantic European coastline. For this purpose, a global wave model is dynamically downscaled over the northeastern Atlantic for the 1950–2100 period and for two climate change scenarios (SSP1-2.6 and SSP5-8.5). Twin experiments are performed by accounting (or not) for hourly variations of water level from regional ocean simulations in the regional wave model. The largest impacts of wave-water level interactions are found in the Bay of Mont-Saint-Michel in France, due to a large tidal range of 10 m. At this location and during an historical extreme event, significant wave height was found to be up to 1 m higher (or +25 %) when considering water level variations, leading to an increase in wave setup by between +8.4 cm and +14.7 cm, depending on the value of the beach slope used. At the end of the 21st century under SSP5-8.5 scenario, the wave simulation including water level variations exhibits an increase in extreme significant wave heights and wave setup values by up to +20 % and +10 % respectively. These results are found for many coastal points of the large continental shelf where shallow-water dynamics prevail, and especially so in macro-tidal areas.