Theophanis V. Karambas

Intercomparison of wave-driven current models

Abstract Seven numerical models which simulate waves and currents in the surf-zone are tested for the case of a reduced-scale detached breakwater subjected to the action of regular waves with normal incidence. The computed wave heights, water levels and velocities are compared with measurements collected in an experimental wave basin. The wave height decay in the surf-zone is predicted reasonably well. Set-up and currents appear to be less well predicted. This intercomparison exercise shows that radiation stresses are systematically overestimated by formulations used in the models, mean bottom shear stresses are not always co-linear with the mean bottom velocity vector in shallow water, and turbulence modelling in the surf-zone requires a sophisticated

Assessment of island beach erosion due to sea level rise: the case of the Aegean archipelago (Eastern Mediterranean)

The present contribution constitutes the first comprehensive attempt to (a) record the spatial characteristics of the beaches of the Aegean archipelago (Greece), a critical resource for both the local and national economy, and (b) provide a rapid assessment of the impacts of the longterm and episodic sea level rise (SLR) under different scenarios. Spatial information and other attributes (e.g., presence of coastal protection works and backshore development) of the beaches of the 58 largest islands of the archipelago were obtained on the basis of remote-sensed images available on the web. Ranges of SLR-induced beach retreats under different morphological, sedimentological and hydrodynamic forcing, and SLR scenarios were estimated using suitable ensembles of cross-shore (1-D) morphodynamic models. These ranges, combined with empirically derived estimations of wave runup induced flooding, were then compared with the recorded maximum beach widths to provide ranges of retreat/erosion and flooding at the archipelago scale. The spatial information shows that the Aegean “pocket” beaches may be particularly vulnerable to mean sea level rise (MSLR) and episodic SLRs due to (i) their narrow widths (about 59 % of the beaches have maximum widths < 20 m), (ii) their limited terrestrial sediment supply, (iii) the substantial coastal development and (iv) the limited existing coastal protection. Modeling results indeed project severe impacts under mean and episodic SLRs, which by 2100 could be devastating. For example, under MSLR of 0.5 m – representative concentration pathway (RCP) 4.5 of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate change (IPCC) – a storm-induced sea level rise of 0.6 m is projected to result in a complete erosion of between 31 and 88 % of all beaches (29–87 % of beaches are currently fronting coastal infrastructure and assets), at least temporarily. Our results suggest a very considerable risk which will require significant effort, financial resources and policies/regulation in order to protect/maintain the critical economic resource of the Aegean archipelago. Published by Copernicus Publications on behalf of the European Geosciences Union. 450 I. N. Monioudi et al.: Assessment of island beach erosion due to sea level rise

SPECTRAL EVOLUTION OF BREAKING-NON BREAKING WAVES IN THE SURF ZONE

In the present study the spectral evolution of breaking-non breaking, regular and irregular waves in the surf zone is studied experimentally in a large scale facility. A great number of numerical and experimental studies exists in the literature concerning surf zone dynamics. A part of them is focused on the evolution of variance spectra (Becq-Girard et al, 1999, Beji & Nadaoka K, 1997, Ozanne et al, 1997). The experimental study was conducted in the CIEM flume of the Catalonia University of Technology, Barcelona (fig. 1). Regular and irregular waves of various heights and periods were generated for breaking and non breaking conditions in order to study the spectral variances and the energy decay especially in the surf zone. The irregular waves were generated according to JONSWAP spectrum with a shape parameter γ equal to 3.3. Approximately 1200 waves were generated in order to obtain the appropriate statistical information for the reconstruction of the energy spectrum. It is revealed that for breaking conditions (plunging breaker) the spectrums undergo significant changes under the combined effects on non-linear energy transfer and dissipation (fig.2 & fig.3). The phenomena of higher frequency harmonics generation is more evident in the case of regular waves (fig.3B). Spectra in the outer and in the inner region of breaking for regular and irregular waves verify the intense energy decay (fig.2A-fig.2B & fig.3Afig.3B). The experimental results are compared with an existing Boussinesq type model which incorporates higher-order non-linear terms and additional dispersion terms (according to Madsen & Sorensen, 1992). In the model a new method is used to simulate wave breaking, proposed by Karambas and Tozer (2003). Both turbulent stresses and non-uniform vertical distribution of the horizontal velocity due to the presence of roller are incorporated in the momentum equation. The eddy viscosity coefficient is calculated using a k-model, while the velocity distribution of the breaker is derived adopting an analytical solution of the vorticity transport equation for the turbulent flow, according to Veeramony and Svendsen (2000). The numerical scheme proposed by Wei and Kirby (1995) is used. Wave generation is implemented inside the computational domain using the source function method as described by Wei et al. (1999). This method employs a mass source term in the continuity equation that acts on a limited ‘source region’ while it is combined with wave damping sponge layers at the boundaries. The above method is adapted to be consistent with the Madsen & Sorensen (1992) equations, used in the present work, instead of the Nwogu type of equations, used by Wei et al. (1999). The comparisons in the outer and inner breaking region are shown in Figure 3.