Influence of Canopy Fragmentation and Spacing on Seagrass-Induced Wave Attenuation

As traditional 'hard' coastal defences become increasingly economically and ecologically unsustainable, seagrass meadows are gaining attention as a Nature-based Solution (NbS) capable of delivering both ecological enhancement and wave energy dissipation. However, while numerous studies have quantified wave attenuation in continuous canopies, the hydrodynamic effects of spatial heterogeneity, a pervasive feature of natural meadows, remain largely unexplored. To address this gap, this study conducted comprehensive laboratory experiments using dynamically scaled artificial mimics of Zostera marina subjected to irregular waves spanning the range from swell to storm sea states. The investigations quantified the influences of two key parameters: canopy fragmentation (reduction in biomass) and longitudinal gap spacing. The results reveal that fragmentation can significantly compromise wave attenuation, particularly under low submergence-ratio conditions. Notably, the measured wave attenuation coefficients were highly sensitive to canopy fragmentation: a 12.5% level of fragmentation resulted in an average 78% reduction in attenuation. In contrast, the longitudinal arrangement of patchy canopies exhibited a complex, non-monotonic influence. While significant gaps reduced attenuation by 3.2%–42.0%, small gaps unexpectedly enhanced performance by 9.8%–15.0%. This divergence is likely to be attributed to the interplay between the 'edge effect' and 'flow recovery' mechanisms characteristic of discontinuous seagrass canopies. These findings extend the physical understanding of canopy hydrodynamics beyond idealised continuous fields, providing critical datasets for developing resource-efficient seagrass restoration strategies for sustainable coastal protection. • Quantifies effects of fragmentation and longitudinal spacing on wave attenuation. • A 12.5% fragmentation leads to an average reduction of 78% in wave attenuation. • Longitudinal gap space exhibits a non-monotonic influence on the wave attenuation. • Longitudinal gaps exhibit a less significant influence than canopy fragmentation.