How waves are accelerating global coastal overtopping

Abstract

The world’s coastal areas are home to about 10% of the human population and support unique and dynamic ecosystems, offering € trillions worth of environmental and societal benefits. Climate change and anthropogenic pressures are however exacerbating devastating hazards such as episodic coastal flooding, the magnitudes of which remain highly uncertain to date. This study, for the first time, presents global scale coastal overtopping estimates, which account for not only the effects of sea level rise, storm surge and wave setup as traditionally done, but also that of wave runup and existing coastal protection measures. While the latter are widely recognized as important determinants of episodic coastal flooding, they have hitherto been ignored in assessments thereof. Our results show that the combination of tides and large wave runup events is the main contributor to episodic coastal overtopping. The Gulf of Mexico, northern Europe, Mediterranean region, east coast of Africa, south east Asia, and north western Australia emerge as hotspots of episodic coastal overtopping under the current climate. Future projections of overtopping with the the global mean sea level rise under “business-as-usual” scenario RCP 8.5 indicate that the globally integrated number of annual overtopping hours will increase at a rate faster than that of the global mean sea level rise itself. This study also shows that, under the RCP 8.5 sea level rise trajectory, the projected acceleration in coastal overtopping should be starting about now and will be clearly discernible by about 2050. Global overtopping has increased almost by 1.5 from 1993 by now and will reach values more than 50 times larger by the end of the 21st century. The global projections presented here are anticipated to lay a solid foundation for the development of effective climate adaptation measures at the identified hotspots, ideally through detailed local scale studies.

Influence of El Niño on the variability of global shoreline position

“…However, while wind-generated ocean surface waves are known to dominate the impact on beaches at short event to sub-annual scales (e.g 37 , 38 . ), recent studies have highlighted the contribution of waves to longer-term interannual coastal water levels 5 , 39 , 40 and erosion (e.g 21 , 41 , 42 .). In addition, the often neglected rainfall/river discharge variability has been shown to play a major role on shoreline evolution through sediment 43 – 45 but also changes in freshwater river discharge have also been reported to influence coastal sea level 46 – 49 .…”

Climate risk assessment of the Tangier-Tetouan-Al Hoceima coastal Region (Morocco)

“…The coastal defense structures built in the bay of Tangier to fight against beach erosion and protect the beachfront hotels will be exposed to more severe hydrodynamic conditions. Indeed, in shallow waters, with the rise in mean sea level and the potential intensification of offshore storms, the height of the waves will exceed the design values (Sergent et al, 2015;Almar et al, 2021;Bongarts Lebbe et al, 2021;Mohamed Rashidi et al, 2021). Most of the adaptation strategies proposed in the literature are based on coastal engineering and consist in: a) repairing the structures as they are b) reinforcing them c) demolishing and redesigning them d) accepting coastal realignment.…”

El Niño controls the evolution of shorelines worldwide

“…The global dataset used in this study was re-sampled equidistantly with transects spaced at 0.5 o intervals, following the same vector as in Almar et al, 2021. The initial coastline dataset used is Global Self-consistent Hierarchical High-resolution Geography (GSHHG version 2.3.6 August 17; Wessel and Smith, 1996) to define locations along the world shorelines.…”

“…However, recent studies have highlighted the contributions of wind-generated ocean surface waves in controlling the variability of interannual coastal water level (Melet et al, 2018;Kirezci et al 2020;Almar et al 2021) and erosion (Barnard et al, 2015;Mentaschi et al, 2018). In addition, the reduction of sediment supply due to river damming (and/or or land use change) has been shown to play a significant role in coastal erosion (Syvitski et al, 2003, Ranasinghe et al, 2013Anthony et al, 2015;Bamunawala, et al, 2021), highlighting the often neglected rainfall/river discharge variability as a driver of coastlines evolution (Darby, et al, 2016;Bamunawala, et al, 2020;Syvitski and Milliman, 2007).…”