Quantifying the Geomorphologic and Urbanization Influence on Coastal Retreat under Sea Level Rise

In response to increasing greenhouse gases emissions, the global climate is undoubtedly changing. As a consequence of rising temperatures, mean sea level also shows an increasing tendency globally, still, uncertainties in relation to its regional specific trends can be identified. Besides that, uncertainties also remain regarding regional and local coastal response to sea level rise. Coastal geomorphology (topography, bathymetry, and sediment texture) plays a relevant role, especially in defining how sediment exchanges occur in the active zone, thus inducing different morphodynamic readjustments. In this context, this study is focused on projecting the future coastline position for the years 2040 and 2100 along three sectors at Hennenegildo Beach, and on investigating the influence of site-specific geomorphological characteristics, urbanization and the presence of hard coastal protection structures on the coastal response under accelerated rates of sea level rise using a stochastic morphokinematic model, the Random Shoreface Translation Model. Model outputs as coastal recession distances were submitted to a Kruskal-Wallis test to verify if there were significant differences in coastal recession 1) amongst the three sectors (standard own topography and bathymetry); 2) due to changes in dune topography only; and 3) due to the presence or absence of hard coastal protection structures at the urbanized sector. Overall, the results indicate that the urbanized area presented the highest recession distance amongst the sectors. Differences in dune heights between the northern and southern dune field sectors at Hermenegildo Beach do not significantly influence the mean coastal retreat. On analyzing mean coastal recession results for the urbanized sector, with and without hard coastal protection structures, we conclude that the presence of urbanization and hard structures on the active dune and beach contributed to a maximum increase of 13.52% in mean coastal recession distance and that it significantly (P < 0.01) affects coastline recession in comparison to that in the case of a non-structured dune field for both the time horizons considered (2040, 2100). The results presented here provide a basis for future planning and management at the area, pointing out to the increased erosion risk caused by the existence of an artificially structured shoreline.