On the use of large-scale biodegradable artificial reefs for intertidal foreshore stabilization

Combining foreshore ecosystems like saltmarshes and mangroves with traditional hard engineering structures may offer a more sustainable solution to coastal protection than engineering structures alone. However, foreshore ecosystems, are rapidly degrading on a global scale due to human activities and climate change. Marsh-edges could be protected by using connected ecosystems, such as shellfish reefs and seagrass beds, which can trap and stabilize sediments, thereby reducing hydrodynamics loads on the saltmarsh edge. In our study, we aimed to test the effect of large-scale biodegradable artificial reefs on tidal flat accretion and/or stabilization. We hypothesized that the structures would attenuate waves and trap sediment. For this, a large-scale experiment was conducted on the tidal flats of the Dutch Wadden Sea, by installing biodegradable artificial reefs along 630 m. Waves, sediment dynamics and sediment properties around the structures were monitored over three years. Our results demonstrate that intact structures attenuated circa 30% of the wave height with water levels below 0.5 m. Variability in wave-attenuation increased when the wind direction was parallel to the structures/foreshore. Sediment dynamics were variable due to the exposed nature of the location and environmental heterogeneity because of the landscape-scale set-up. We observed local sediment accretion up to 11 cm, however the effect did not expand beyond 10 m from the landward edge of the structures and up to 10 cm scouring was also found. Additionally, near sediment properties were not affected by the presence of the artificial reefs. Long-term effects could not be assessed due to the degradation of the structures during the experimental period. In general, we conclude that artificial reefs have the potential to attenuate waves and trap sediment on tidal flats. However, to benefit connected foreshore ecosystems like salt marshes, an even larger implementation scale and the use of more resistant structures in exposed sites is needed to affect long-term tidal flat morphology.