Experimental study on wave attenuation and beach profile protected by rigid and flexible vegetation

A comprehensive understanding of the protective effects of coexisting rigid and flexible vegetation on coastlines is essential for coastal defense. This laboratory-scale comparative study experimentally examined wave attenuation by varying the relative proportion and spatial configuration of rigid and flexible vegetation under diverse wave conditions. The results demonstrate that, compared with the pure flexible vegetation model, the wave height behind the vegetation zone was slightly reduced after the introduction of rigid vegetation, but the pattern of wave height evolution was not significantly altered. Experimental observations showed that the overall wave attenuation on vegetated beaches did not change significantly with increasing proportion of rigid vegetation; under some wave conditions, the differences caused by vegetation configurations were within the measurement uncertainty. Analysis of the quasi-equilibrium beach profiles showed that the overall profile type remained unchanged with increasing rigid vegetation proportion. Maximum scour depth generally decreased, whereas scour pit length and quasi-equilibrium point location showed no consistent trends. For a given rigid-to-flexible ratio, differences in scour parameters among spatial configurations were not statistically significant. An empirical formula was developed to estimate maximum scour depth under combined rigid–flexible vegetation within the tested range. These findings inform future research on mixed vegetation coastal protection.