How to protect coastal areas from the impacts of climate change

The last 10 to 20 years have seen a significant increase in coastal hazards such as storms, tsunamis, typhoons, flooding… Their impact is more serious now than it would have been 50 years ago, not only because the events are bigger than before, but also because there are more people living in the coastal zones. According to the United Nations, about 40% of the world's population lives within 100 kilometers of a coast.

Image: The Ganges Delta, the largest and most populous in the world with over 143 million inhabitants. © Nasa

The AXA Chair on Climate Change impacts and Coastal Risk at the UNESCO Institute for Water Education is an ambitious 10-year research programme which aims at better understanding the physics that govern coastline recession, to understand how exactly climate change may affect the look of our coastlines. This new knowledge allows for simplified, faster-to-calculate models to quantify climate change impacts, which in turn can be applied to create innovative quantitative coastal risk assessment methods. The model produces concrete risk information that can be directly used by coastal zone managers and planners for risk informed decision-making.

One of the major outputs of Prof. Ranasinghe’s programme so far has been a reduced complexity, physics-based, multi-scale, probabilistic model of climate-change-induced coastal change. «This is basically a simple and fast modeling tool to predict coastline recession, in other words, how much the coastline might move backward, or forward, in a 100-year (or longer) time period, » he explains.

What concrete impacts on the ground?

Roshanka’s work has allowed the Coast Conservation Department of Sri Lanka to model the impact of climate change on Sri Lanka’s East coastline. Using his approach, the Department’s reasearchers were able to determine new set back lines* for existing and new coastal developments and infrastructure along the east coast of the country.

With around 2M inhabitants, the eastern coast of Sri Lanka is not the most populated of the country, but is very exposed to the effects of climate change. Here in 2013, we can see Tropical Cyclone Mahasen is moving north through the Indian Ocean. © Nasa

Setback lines prescribe distance to the coast line; within which developments are prohibited. Setbacks provide protection to properties against coastal flooding and erosion by ensuring that buildings are not located in an area susceptible to these hazards.

“In some areas, they remained the same as our previous setbacks. In others, they moved quite a lot, with setback lines going from 100 meters to 25 meters from the sea. These refined results help us greatly in our engagement with stakeholders from coastal communities and other local groups that work in urban development, agriculture or nature protection », said Mangala Wickranayake, Chief Engineer in Research and Design at the Coastal Conservation and Coastal Resource Management Department, Sri Lanka.

This data is particularly valuable as global warming and rising sea levels become  very real and measurable facts. According to the Intergovernmental Panel on Climate Change (GIEC), over the 1901-2010 period, the global mean sea level rose by approximately 20 cm (8 inches). The rate of sea-level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.

True innovation in risk modeling

The model developed by Roshanka is a major innovation. Previously, the only model available for predicting coastline evolution as a function of sea level was Bruun's rule, developed in 1962 and in which there is limited trust today.

« Before prof. Rosh Ranasinghe developed his model, we were using only one method for assessing the response of the shoreline to changes in sea level: the Bruun rule. It’s an old model – its formulation dates back to 1962 –, so there’s limited trust in it today. Rosh’s modelling approach uses different, more advanced physics assumptions than the Bruun rule’s, which means that they also give different results. For instance, Rosh’s model predicts that one meter of sea level rise would cause the sandy shoreline of Aquitaine to retreat by about 10 meters. With the Bruun rule, the projections are around 100 meters of coastal setback. While current models are not perfect, and scenarios can differ, having more models allows for a smaller margin of error. Models like Rosh’s help refine guidelines for coastline management, and we recommended that risk prevention practical guides for coastal erosion issued by the French Ministry of Environment (MTES) take it into account », said Goneri Le Cozannet, Risk and Prevention Department, Coastal Risks and Climate Change Unit, French Geological Survey (BRGM), France.

Roshanka has also been working with Deltares, a world-renowned Dutch institute for applied research in the field of water and subsurface.

« The probabilistic coastal recession tool he has developed is a high added value to our existing model on coastal evolution and sea level rise. Our role as advice givers demands very solid scientific foundations. In this sense, Rosh’s contributions are priceless. His model helps us advise coastal managers and spatial planners on where they should allow the constructions of buildings, what the level of risk for each plot of land is, etc. For instance, we are currently applying Rosh’s modelling approach to a multi-million project, the European Climate Prediction system project, which aims to quantify present and future coastal flood and erosion risk in the European Union. With the probabilistic results this model provides, we’ll be able to determine tailored “setback lines”, which establish flood risk according to the typology and the distance from the sea », said Dirk-Jan Welstra, Head of the Department of Applied Morphodynamics at Deltares.