Assessment of flood hazard during extreme sea levels in a tidally dominated lagoon

Abstract

The inundation of coastal regions is recognized as a major threat to people, livelihoods, and the ecosystem health. The assessment of the magnitude of flooding drivers and the flood extension mapping are essential to avoid and reduce the adverse impacts of floods. Attending these issues, the present study aims to assess marine-induced inundation in Ria de Aveiro coastal lagoon under extreme sea levels induced by astronomic tide and storm surge events. The approach followed integrates joint probability analysis of residual and astronomical levels from the lagoon inlet with application of the ELCIRC hydrodynamic model, which was validated for tidal and storm surge conditions. The model is applied under extreme sea levels corresponding to 2- and 100-year return periods of storm surges combined with tidal elevation for the present mean sea level and also considering a mean sea level rise estimate of 0.42 m for both return periods. A mean spring tide was also simulated as the reference case. The maximum levels, the lagoon flooded area and the tidal prism across the lagoon main channels were analysed for all simulations. The application of joint probability analysis of residual and astronomical levels resulted in extreme sea levels between 3.85 and 4.56 m, relative to the local chart datum. The validation results evidence that model reproduces accurately both tidal and storm surge propagation. The lagoon flooded area increased between 22 and 79 % for the most optimistic and pessimistic scenarios, respectively, relatively to the reference tide. The morphological lagoon features (depth of channels and topography of margins) determine the tidal prism and consequently the marginal inundation patterns found. Consequently, the more exposed regions present low altitude and are located at the margins of deeper channels.