Field assessment of wave attenuation by BESE-elements in a mangrove environment

Abstract

Coastal ecosystems provide critical ecosystem services, including carbon sequestration and coastal protection, yet face continuing global decline. In areas where natural revegetation is impeded (e.g., altered hydrodynamics, substrate instability, erosion), active restoration techniques using artificial structures like Biodegradable Elements for Starting Ecosystems (BESE-Elements), may facilitate ecosystem recovery. While laboratory studies have demonstrated promise, field evidence quantifying the hydrodynamic benefits of these structures in mangrove areas remains limited. This study evaluated BESE-Elements' wave attenuation performance in a tide-dominated mangrove (Avicennia marina) embayment in Western Port, Australia through two field experiments over one-month periods, using an array of six pressure sensors to measure wave attenuation. In Experiment 1, we compared a single 4-cm high BESE-Element (0.414 m²) to existing mangrove vegetation and bare sediment. Experiment 2 tested single, double (0.828 m²), and quadruple (1.656 m²) 4-cm high BESE-Element configurations. Results demonstrated that existing mangrove vegetation achieved the highest wave attenuation (15.2 % reduction relative to bare sediment), while BESE-Elements showed statistically significant but modest wave height reductions ranging from 2.1 % for single element to 3.7 % for quadruple configurations (p < 0.01) in water depths less than 1 m. Wave attenuation efficiency decreased with increasing water depth across all configurations. Long-term monitoring over a 20-month period revealed significant sediment accumulation within BESE-Element plots (mean 6.61 mm/year). While this study demonstrates modest wave attenuation effects of 4-cm high BESE configurations, these structures may influence other hydrodynamic processes relevant to mangrove establishment, such as flow velocity reduction, sediment retention, or propagule stabilisation. Future research should evaluate these potential mechanisms through field experiments with transplanted seedlings and continued monitoring of sediment dynamics.