Cross-shore profile evolution induced by real emergent vegetation in a sandbar-lagoon coast: Laboratory experiments

Abstract

Pressures from anthropogenic activities and climate change in lagoon ecosystems have increased rapidly in recent years. Coastal vegetation serves as a natural sea defense mechanism and is pivotal in maintaining a robust ecological balance. However, our understanding of how vegetation influences the evolution of typical sandbar-lagoon coasts is limited. To address this gap, a series of mobile-bed flume experiments were conducted within a typical sandbar-lagoon cross-section to investigate the impact of sparse Phragmites spp. planted on the sandbar on wave propagation and the evolution of the cross-shore profile under varying water depths and irregular wave conditions. These findings revealed that vegetation attenuates infragravity and sea-swell waves. Notably, infragravity waves are associated with the formation of a right- or obtuse-angled foredune scarp, whereas sea-swell waves tend to create an acute-angled scarp. Vegetation-induced changes in the local amplitudes of short waves within wave groups and the local mean water level are instrumental in reshaping foredune scarps. Additionally, the cross-shore width of the vegetation had a more pronounced influence on sediment transport within the lagoon than that of the solid volume fraction. Sparse vegetation not only reduces sediment deposition in the lagoon but also enhances offshore sediment transport. These insights significantly advance our understanding of wave-vegetation-sediment dynamics and provide essential scientific support for the implementation of coastal ecological restoration initiatives, particularly in the context of sandbar-lagoon coasts.