PREDICTING THE COMPOUND EFFECT

Flooding is a constant threat in parts of Southeast Asia. The combination of tropical cyclones, heavy monsoon rainfall and rising sea levels adds up to potential disaster for people living on low-lying coastlines and deltas.

A group of Southampton researchers is behind a project to better understand the flood hazard from tropical cyclone-induced storm surges for coastlines around the South China Sea, and how projected future climate change will alter this risk.

The project is called Comp-Flood (Compound flooding in coastal Vietnam). It began in 2019 under the leadership of Dr Ivan Haigh, Associate Professor in Coastal Oceanography, with Robert Marsh, Professor in Oceanography and Climate, Steve Darby, Professor in Physical Geography, and Robert Nicholls, Professor in Coastal Engineering.

Ivan explained: “The aim of Comp-Flood is to predict the impact of compound flooding, when combinations of flooding happen at the same time or in quick succession, which is always pretty disastrous. It’s a real risk along the Vietnam coastline and the Mekong Delta, where they endure typhoon weather and see lots of rain coming at the same time as storm surges. We have used a statistical approach to try and generate likely scenarios from actual activity.”

The group has used a huge dataset of potential future tropical cyclone activity in this region to simulate how the coast could be inundated over the next 30 years. The dataset was created from a record of historic global tropical cyclone statistics and simulates 10,000 years of possible cyclones, including events that are much bigger than we have seen in the past but are still physically plausible.

Building on this, the researchers are now exploring compound effects – how the storm surges from the sea might interact with river flooding in major deltas, such as Vietnam’s Mekong Delta.

Dr Melissa Wood, Research Fellow in the School of Earth and Ocean Sciences who is also working on Comp-Flood, explained: “We’ve got this amazing new database of potential future tropical cyclone behaviour for the region, based on climate assumptions. We can plug this information into ocean hydrodynamic models to simulate the possible future flood risk to coastal communities.”

Millions of people have livelihood activities centred on the Vietnamese coast.

Steve said: “In Vietnam, seasonal river flooding is common in the south, while coastal flooding due to tropical cyclone driven storm surges is frequent in the north. However, the possibility of such events coinciding, creating heightened flooding, has not been considered before.

“Our study has shown that the frequency of these combined events will increase dramatically in the future as a result of climate change, especially in places like the Mekong Delta. This is worrying because the risks posed by these events have not really been considered in disaster management plans until now. We hope that our study will enable flood management agencies to update their risk planning accordingly.”

Compound flooding has caused some of the world’s worst flooding disasters.

“Compound flooding is not very widely discussed, even though when it happens it can be catastrophic,” said Melissa. “Hurricane Harvey in 2017 in the USA was a classic example of compound flood hazard. This was a category 4 hurricane that caused catastrophic conditions in Houston Texas, as heavy rainfall caused flash flooding in rivers at the same time as the strong winds were forcing storm surges along the coastline.

“For an entirely different part of the world, but one that is very familiar with heavy rainfall and passing typhoons, we have been able to model what might happen by the year 2050.

“It’s a game changer for them as they are going to have to now look at how they will defend against storm surges – these are something they are not currently thinking about. People living on low-lying coastlines and deltas here understand there is a chance of flooding from the sea on any given day, and know that sea level rise is coming. But they are not really prepared for typhoons to add to the hazard.”

Preparations could include building higher and stronger sea and river defences or incorporating more green infrastructure such as coastal mangrove habitats to soften the energy of storm surges.

Melissa concluded: “One recommendation I would like to make is about flood planning in a sustainable way, and good communication about what inhabitants can do when a typhoon warning occurs to protect themselves against flood hazard.”

Farm land under water in Cambodia

A map of the ground elevations for the Mekong region