Flooding of sandy beaches in a changing climate and the potential of nature based solution for impact mitigation.

Sandy beaches have a paramount importance as natural barrier for coastal protection (Vousdoukas et al., 2020) and a valued socioeconomical asset for touristic activities (Enríquez & Bujosa Bestard, 2020). The unprecedented population expansion along with the sea level rise put the coastal areas under a high risk for the coming decades. An accurate estimate of the future evolution of sandy beaches is a key step for the development of effective adaptation strategies.

In this work, we estimate the potential loss of sandy beaches due to climate change under different scenarios of greenhouse gas (GHG) emissions. We focus on the Balearic Islands beaches as a case study representative of the Mediterranean sandy beaches. We combine a high-resolution nearshore total water level (TWL) with the detailed characteristics (area, granulometry and slope) of the 869 beaches of the Islands. TWL is obtained combining the contribution of mean sea level, storm surge and wave runup. In particular, a cost-effective methodology that combines numerical modelling and statistical techniques has been developed to  explicitly obtain wave runup on beaches.

An additional important point to be considered is that, in the Mediterranean Sea, seagrass meadows are commonly present at shallow depths (Marbà et al., 1996; Ruíz, Boudouresque, & Enríquez, 2009). This presence of vegetation plays a relevant role in the dissipation of the incident wave energy against the coast (Infantes et al., 2012) and, consequently, induces a reduction in the coastal flooding during extreme events.  So, in this work we consider the wave attenuation due to bottom flow and seagrass interaction in the wave propagation to the coast using a specific parametrization (Mendez & Losada, 2004).

The results show that the beach surface in the Balearic Island (~310ha) would be significantly reduced at mid and long term under the most likely scenarios of future GHG emissions (RCP4.5 and RCP8.5). In particular, at mid-century, around 35-45% of the beach area would permanently disappear under mean conditions, mainly due to the contribution of the mean sea level rise. Under storm conditions, the beach loss would increase up to 75-78% due to the wave runup effect. At the end of the century, around 55-65% of the beach area will be permanently flooded and around 85% under storm conditions. These results could be safely extrapolated to most regions in the Mediterranean as they share similar beach and hydrodynamic characteristics. Finally, some results on the potential of climate-based solutions to reduce the impact of storms will be presented.