Assessment of beach erosion/retreat at island scale
Models displayed differential behavior for almost all tested conditions, showing as expected significant ranges of results due to the varying initial conditions and forcing used i.e. different bed slopes, sediment sizes and wave conditions. The means (and the polynomial equations describing them) of the lowest and highest projections by all models of the two ensembles were estimated. Also the equations that describe beach temporary inundation/flooding due to wave run-up combined with SLR were estimated. The polynomial equations were then used for the estimation of potential ranges of beach retreat/erosion and temporary inundation/flooding under recent projections of (a) long-term MSLR (0.15, 0.5 and 0.7 m), (b) temporary SLR (0.2, 0.4, 0.5 and 0.6 m) (i.e. episodic storm-induced) and (c) combined MSLRs and short-term extreme levels (0.55, 0.9, 1.1 and 1.3 m). The reduction of the ‘dry’ beach width (carrying capacity) and the impacts on the infrastructures/assets fronted by the beaches were estimated through the comparison of the predicted beach retreat ranges and the beach width maxima.
Projections of beach retreat/erosion (a and b) and temporary inundation/flooding (c and d) due to short-term (a and c) and long-term (b and d) SLR. Projections are for different beach slopes, sediment sizes and wave conditions. The means of the highest and lowest projections of all models in the ensembles are shown as yellow stippled lines.
In the figure below, initial and final widths (projected according to the mean low and high predictions of the ensembles) of all beaches of the 6 six Aegean islands are depicted for 3 SLR scenarios (0.7 m-MSLR , 0.4 m-storm induced and 1.1 m-combined). Final widths with negative values show beaches that will be entirely lost (if backed by coastal cliffs and/or assets/infrastructure) or shifted landward and/or fully flooded to their entire maximum ‘dry’ width. Even under a moderate scenario (RCP4.5), the projected MSLR of 0.5 m (2100), will have severe impacts (4 – 64 % of the beaches will retreat more than their max. ‘dry’ width and 18 – 94 % will be temporary flooded). The worst impacts are projected from the combined mean and short-term SLRs. For example a combined SLR of 1.1 m will have devastating effects since 36 – 94 % of all beaches will be completely (at least temporarily) eroded and 59 – 98 % of the beaches will be occasionally overwhelmed by flooding. It appears that appropriate adaptation measures are required, which will ensure the long-term sustainability of the beaches of the 6 islands and their ecosystem services and uses.
Minimum and maximum retreat and flooding of all beaches of the six islands for different SLR. Scenarios on the basis of the low and high means respectively of the ensemble projections. a) and b) Minimum and maximum retreat and flooding under a MSLR of 0.7 m. c) and d) Minimum and maximum retreat and flooding under a storm-induced level of +0.4 m. e) and f) Minimum and maximum retreat and flooding under a combined long- and short-term SLR of 1.1. Initial and final (after SLR) maximum beach widths are shown. Bars show currently existing infrastructure/assets fronted by beaches.
Αssessment of beach erosion for the pilot beaches
Simulations using a state-of-the-art quasi 3-D hydro-morphodynamic model for some pilot beaches presented some interesting results regarding their projected hydrodynamic/morphodynamic response. The figure below shows model results for the Eresos beach (Lesvos) in case of SW winds and a MSLR scenario of 0.5 m. The results of the model revealed non-linear nearshore flows and strong offshore flows. The predicted morphological changes showed increased erosion/retreat of the coastline along the entire length of the beach of about 19 m.
Results of the hydro-morphodynamic model. Left: Significant wave height (m) and wave induced currents at Eresos beach in case of SW winds and a MSLR scenario of 0.5 m. Right: Shoreline evolution and erosion trends along the beach.
