Trees vs Tsunamis: Mangrove Forests as a Defence against Tsunamis

Tsunamis can cause large scale disasters, costing many lives and destroying property and livelihoods. The devastating 2004 Indian Ocean Tsunami highlighted the need for better preparation for extreme events, and rising sea levels mean we are likely to see increased risk from major tsunamis over the coming decades and beyond. Simulations examining the relationship between sea-level rise and tsunami impact find a doubling of inundation distance and a rapidly increasing risk to life. It is therefore critical and timely to devise suitable mitigation strategies, focusing on those that can track sea-level rise.

After the 2004 tsunami, benefits of using ‘greenbelts’ for protection from future events became prevalent in the literature. Mangroves are a type of coastal vegetation which occupy tropical intertidal zones, and have been used by coastal communities for protection against storms and flooding for centuries. After the 2004 tsunami, many locals stated that mangrove deforestation allowed the tsunami to travel further inland, and many lives could have been saved if these forests had been protected. It has been recommended that those in high-risk tsunami areas should live at least 1 km from the shoreline, and that dense mangrove forests should be planted between the villages and the ocean. Due to rising sea levels, mangrove forests are being forced to move landwards, however the distance mangroves can migrate is limited by the presence of infrastructure. The pressure being applied from both the land and sea narrows the area in which mangroves can survive; a process of coastal squeeze.

Field observations have found that complex root systems, such as mangrove aerial roots, provide a significant drag force, and are able to attenuate waves. However, to date, we cannot quantify the reduction to risk, which is required as part of an effective mitigation strategy. Flume and numerical experiments have also found them to act as an effective natural barrier against tsunamis, reducing the flow speed and inundation distance. Creating 3D numerical models of waves travelling through arrays of mangrove trunks, modelled as cylinders, allows dissipation of the tsunami’s energy to be calculated in a quantitative way at flume scale. Without the presence of any mangroves, solitary waves interact with the bed, leading to damping or shoaling of the wave, depending on the ratio of water depth and wave height. Waves which are damped in an empty flume tank are likely to experience more pronounced levels of damping when a cylinder array is in place. The damping resulting from interactions with the bed has not been considered in previous flume experiments, potentially leading to overestimations in the role that cylinders play in wave dissipation. Waves are generally more effectively damped in shallow water depths, however, due to a complex interplay between the initial wave height and the water depth, the initial wave height also has an impact on damping. Our results show that mangrove forests can be particularly effective as they are found in shallow water, but further work is needed to scale the results to better simulate the real world.