Beach profile changes induced by surrogate Posidonia Oceanica: Laboratory experiments

Abstract

New flume experiments with surrogate seagrass meadows are presented. The experiments included the creation of a full-scale and realistic Posidonia Oceanica model to evaluate the effect over wave attenuation, sediment transport and shoreline erosion. A hydrodynamic and morphodynamic comparison between cases with seagrass and without seagrass for two wave energy conditions was performed. Meadow density and submergence ratio were constant for tests under irregular waves. The wave height reduction, bar crest location, total sediment transport and shoreline position were used to evaluate the coastal protection efficiency of the meadow. The measured wave heights suffered a reduction due to the presence of the seagrass. These reductions were persistent in the area located between the onshore edge of the meadow and the depth of closure, being of greater magnitude in the more energetic case. All tests showed the development of a bar and the migration of the crest offshore over time. However, the dissipation of the incoming wave energy on the meadow made the bar crest stay closer to the shoreline and consequently generated a lower freeboard. In addition, the bar migration rate was reduced by the simulated meadow effect, in particular in the lower energetic case. A sediment volume significantly smaller was transported offshore when the seagrass meadow was present in both wave conditions. Additionally, in the higher energy case, a smaller shoreline retreat was observed when the meadow was present. • Flume experiments were carried out with a realistic Posidonia Oceanica model to assess the effect on beach sediment transport. • Two wave condition were analysed comparing the test with and without seagrass meadows. • Wave heights were reduced due to the effect of the Posidonia Oceanica model in areas after the meadow. • The total volume of sediment transported in the seagrass meadows cases was considerably less than without seagrass meadows. • Bar crest morphological parameters and shoreline position were analysed to verify the meadow effect on the coastal protection.