Sea-level rise has been recognised as a major global threat to low-lying coastal areas since the 1980s. Global mean sea level has risen by 16-21 cm since 1900, partly due to human-made climate change caused by increases in the greenhouse gasses. It affects the sustainable development of society and economy of the coastal areas, affecting about 2.4 billion people worldwide living within 60 miles of an oceanic coast. In particular, coastal erosion is a major threat to tourism and productive ecosystems. Found in the inter-tidal zone, mangroves naturally protect shorelines, shelter coastal ecosystem habitats and provide water filtration. In many cases, these roots trap sediments flowing down rivers and off the land, helping to stabilize the coastline.
Certain mangrove root systems even have the ability to dissipate tidal energy through unique hydrological flows and divert the energy of water in different directions reducing risk of coastal damage. Because of their strong structures, mangroves have survived for more than 8000 years.
What is truly amazing about mangroves is that they can adjust to changes in rising sea levels by forming upward structures through a natural process of accumulating layers of mud carried by tides and other sources. It’s their root system in particular that contributes to this complex hydrodynamics. Although many low-lying areas have storm surge protection such as seawalls, these structures are expensive to build, cause their own set of environmental concerns, and obstruct the natural landscape.
A study published in the American Physical Society’s journal, Physical Review Fluids, researchers have picked up red mangrove tree (Rhizophora mangle) from more than 80 different species of mangroves, because of its robust network of roots that can withstand extreme environmental conditions. The red mangrove provided the researchers with an ideal model for bio-inspired shoreline applications. They modeled the complex mangrove roots as a network of circular cylinders called patch. They performed a series of experiments varying key parameters such as length scale and porosity or flexibility, and studied the mangrove roots under different flow conditions to quantify how the flow structure would interact with the mangrove.
Results from the study show that for rigid roots, the drag force varied linearly with patch diameter and spacing between the roots. For flexible roots, the researchers discovered that a decrease in stiffness increased both the patch drag and the wake deficit behind the patch in a similar fashion as increasing the blockage of the patch. They have introduced a new length-scale (effective diameter) based on the wake signature to characterize the drag coefficient exerted on the patch for different porosities. The effective diameter incorporates the patch porosity, arrangement and individual root diameter in the patch. The results have proven that the effective diameter of the patch decreases as the porosity increases.
Information from this study has the potential to help environmental engineers develop methods to design resilient bio-inspired coastline structures.
