Experimental study on the morphodynamic evolution of sandbar-lagoon system with emergent vegetation

Abstract

A mobile-bed sandbar-lagoon model was employed in a series of flume experiments to investigate the influence of vegetation with different densities over the sandbar on wave attenuation and bed profile evolution. Nineteen flume tests were carried out for the same sandbar-lagoon model under different wave conditions. Extensive analyses were conducted on the profile evolution and the wave parameters along the profile of the sandbar-lagoon system. Based on the weighted vegetation density by the canopy width, a new empirical relationship was established between wave attenuation and vegetation density, water depth and sandbar freeboard. It was found that wave attenuation increases with the sandbar freeboard and the weighted vegetation density, and decreases with incident wave height. The collision (s-shaped scarp) and overwash (upward concave scarp) types of fore-dune morphological evolution were observed in the experiments. The maximum erosion thickness of dune follows a quadratic polynomial relationship with vegetation density, indicating that too sparse vegetation cover may lead to more erosion. It follows that an optimal vegetation density may lead to the least wave reflection due to the quadratic polynomial relationship. However, when the maximum erosion thickness of fore-dune is small, the quadratic polynomial relationship become linear. Furthermore, the experimental results indicate that wave height, water depth, sandbar freeboard and vegetation density are all important contributing factors to the morphodynamic evolution of sandbar-lagoon system with emergent vegetation. These relationships promote the development of wave-vegetation-sediment mechanics and provide a scientific guide to carry out coastal ecological restoration projects especially in the sandbar-lagoon system.