Integrating mangroves in dike designs: effect of tree growth and failure

Abstract

Mangrove forests reduce wave attack along tropical coastlines, decreasing the design requirements of coastal embankments. Wave reduction by mangroves depends on their surface area and stability against storms, but both aspects are often oversimplified or neglected in coastal protection designs. Here we present a framework to evaluate how mangrove belts influence embankment designs, including mangrove growth and failure by overturning and trunk breakage. This methodology is applied to Sonneratia apetala mangroves seawards from embankments in Bangladesh, considering forest widths between 10-1000 m (cross-shore). For design water levels of 5 m, wave reduction by mangrove forests shorter than 1 km mostly affects the slope protection and the bank erodibility, whereas the embankment height is less influenced by mangroves. Sonneratia apetala trees induce a relative maximum in wave attenuation by 10 years old, due to their large submerged canopy area. Once trees grow older than 20 years old, their canopy is emergent, and most wave attenuation is caused by trunk and roots. Canopy emergence exposes mangroves to wind loads, which are much larger than wave loads, and can cause tree failure during cyclones. These results stress the importance of including tree surface area and stability models when predicting coastal protection by mangroves.