Image Credits: Pxhere

The following article was originally written by Ms. Divyapriya Chandrasekaran and published here.

A sustainable and resilient method to reduce wave forces and debris impact during extreme tsunami and coastal flood events

It’s been two decades since the devastating tsunami struck the Indian Ocean, which left an indelible mark on the minds and lives of thousands of people. The destruction and devastation could have been more if not for the myriad natural barriers along India’s coastline – its mangroves. This natural catastrophic event highlighted the need for an effective measure to reduce the tsunami wave force and trapping debris. Several ‘storm surge’ events – cyclone-induced flooding events as the sea rises – occur every year. Coastal mangroves act as bio-shields against such disasters. The traditional method of constructing sea walls is possible, but they are expensive and may disrupt natural processes.

Researchers from the Indian Institute of Technology Bombay (IIT Bombay) focused on evaluating how emergent coastal vegetation acts as a natural barrier against tsunami impacts. In their recent study, the researchers used both experimental and numerical methods to investigate the effectiveness of mangroves in reducing tsunami-induced debris impacts on buildings and bridges. They created a smoothed particle hydrodynamics (SPH) model, a computational method used to simulate the flow/behavior of fluids, to observe complex interactions between water, vegetation, and debris.

“We need to understand that Nature is supreme and we must align and work with nature and not against. In this case, waves, coastal currents and coastal sediment transport are the predominant natural processes. It is advisable that any coastal defence system must not adversely interfere with the natural processes,” remarked Prof. Behera, from the Department of Civil Engineering at IIT Bombay, about the need for natural barriers.

Among the varied vegetation types found in the coastal regions, researchers have chosen the emergent vegetation type for the study. Emergent vegetation is aquatic plants rooted in the soil, while their stems, leaves, and flowers emerge above the water surface. Mangroves are emergent trees with sturdy submerged roots, stiff stems, and trunks to reduce wave forces. “Mangroves are the best examples of natural bio-shields against extreme ocean disasters. The mangroves present at Bhitarkanika, Odisha have safeguarded the coastal regions against cyclones that attack almost every year,” says Prof. Behera. On the contrary, the study found that floating and submerged vegetation types are either swept away by tsunami waves or not strong enough to dissipate the wave energy.

The experimental set-up involved a replica of a coastal region using a large water tank (dam-break flume) containing a scaled-down column and an aluminium debris model. The column structure mimicked a coastal building, and the debris model was a replica of a shipping container. A vertical sliding gate was opened to release high-speed water mimicking tsunami-like conditions in the tank. Upon releasing the water, the sensor in the column measured the impact force of the debris hitting the structure. Similarly, the accelerometer in the debris model recorded its speed and movement before impact. The study found that heavier debris causes more impact forces on the column structure.

Simulated Coastal Defence System
Image Credits: Dr Aditya Gupta’s PhD thesis at IITB-Monash Academy, IIT Bombay (supervised by Prof Behera).

The numerical method involved computer simulations to measure the performance of the vegetation. The SPH modelling was used to simulate the debris impact on the column structure and the effectiveness of the vegetation in decreasing the wave forces. This simulation studied the wave interaction on the models of two types of emergent vegetation—rigid staggered vegetation (RSV) and tilting staggered vegetation (TSV). RSV stays upright, which represents the rigid mangroves or stiff emergent vegetation in real scenarios, while TSV symbolizes a natural bend in vegetation due to forceful waves.

The SPH simulation tested the performance of vegetation in reducing the wave force, slowing down the debris movement, and lowering the wave height, using three indices—Reduced Fluid Force Index (RFI), Reduced Momentum Index (RMI), and Transmission Coefficient (CT), respectively. RFI and RMI are higher for rigid staggered vegetation than tilting staggered vegetation. The rigid vegetation efficiently resisted enormous volumes of water and reduced the wave energy. Compared to the 89% reduction of debris impact with tilted vegetation, the rigid vegetation reduced the debris impact on the column structure by 96%.

“The rigid emergent vegetation can be planted along the coastal zones to reduce erosion, provide protection against storm surges and coastal floodings. Vegetations also known as bio-shields are the eco-friendly protection that will act as carbon sinks and help to achieve net zero target of India,” adds Prof. Behera.

The study shows that the emergent type of vegetation is an effective defence system that significantly reduces the damage caused by tsunami waves to coastal infrastructure. Further study is required to replicate it in natural conditions with varied vegetation types, patterns of wave movement, and different types of debris materials. Researchers believe that future studies can focus on advanced simulations for more accurate findings.

The findings of this research provide clues to coastal planners on how to select and use vegetation types in designing a better disaster mitigation strategy. This encourages policymakers and engineers to adopt a resilient, cost-effective, and sustainable defence system, fostering the coastal ecosystem using a nature-based solution.

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The original academic article titled Effectiveness of Emergent Coastal Vegetation as a Defense System to Mitigate Debris Load on a Structure During Extreme Events was written by IIT Bombay’s researchers and published in the Ocean Science Journal.

The original paper can be found here.