Aerial view of devastated fishing village after Cyclone Kenneth in northern Mozambique, 1 May 2019.
Predicting when and where a flood will hit saves people's lives and livelihoods, but why is forecasting floods so hard?
In November 2019 and February 2020, severe floods wreaked havoc across much of the UK. Given the huge costs to people’s lives and livelihoods caused by floods, one may wonder why more cannot be done to prevent flooding, or at least to warn people when a flood is expected.
Professor Hannah Cloke OBE is a natural hazards researcher and hydrologist working on exactly this problem. We catch up with her during lockdown to learn about her work predicting floods in the UK and around the world, as well as advising governments and humanitarian agencies.
“I’m interested in early warning and early action, as it’s so important to get people out of the way if we can tell a flood is coming,” says Cloke.
“Predicting when and where a flood will hit can literally save people's lives, as well as their livelihoods, however it’s also an incredibly cost effective way of limiting the damage from floods; if you can do it it’s an easy win.”
So how can scientists predict when and where a flood is likely to occur? The first thing you need to get right is the weather forecast – you need to know how much rain is going to fall and in what areas. However, whilst weather predictions have come a long way in the past few decades – today’s three-day forecast is as accurate as a 24-hour forecast was in the 1990s – they are never perfect.
In addition to a weather forecast, you also need to know a great deal about the landscape of local areas, so that you can predict the path that rainfall will take.
“For every single raindrop that falls you need to track it and know where it goes,” says Cloke. “There are billions upon billions of different routes through the landscape that a single raindrop might take, for example it could soak into the ground, or runoff into a lake or a river. Based on our knowledge of the landscape and environmental conditions, we have to model all these possibilities to predict how much rain will flow into rivers from above and below ground.”
This complexity goes some way to explaining why, despite the advent of supercomputers and the hard work of some of the world’s brightest scientists, we are still some way from being able to predict exactly when and where a flood will hit.
“You’ve got all of these uncertainties, from the weather forecast to how you represent the landscape,” says Hannah. “We don't have the best information, so we have to make a lot of assumptions and this means there are big uncertainties.”
However while predicting floods at a local level is still notoriously difficult, researchers are making strides in global flood forecasting systems. For example, on 15 March 2019, tropical cyclone Idai made landfall in Mozambique, causing around a thousand fatalities.
Professor Cloke and her Reading University team used global earth system models to predict where the floods would hit, how bad they would be and when they would happen. Working with European partners such as ECMWF, the European Centre for Medium-range Weather Forecasts, and the University of Bristol – the team provided emergency flood bulletins for the Department for International Development and humanitarian agencies, allowing them to guide their aid efforts on the ground.
Global early warning systems could become increasingly important in the future, as climate change leads to increased flooding events around the globe.
“We know that many different types of floods are going to get worse in different parts of the world,” says Hannah. “Coastal flooding and flash flooding from intense rainfall in particular will increase as our climate warms.”
However, while warning systems have a great role to play, more measures may be necessary to save lives and livelihoods. As part of her work, Professor Cloke advises the Environment Agency about the relative costs and benefits of these decisions.
“We have a huge number of people living in coastal cities which are at great risk of flooding in the future,” says Cloke. “Unless we make the radical decision to move these cities, which is unlikely, we will have to look at hard engineering and costly solutions such as coastal barriers and defences.”
“In the future we may be faced with some really difficult decisions about whether it is best to defend an area, or to retreat and give the land back to the water. One natural solution we are looking at is soil management, which means making sure we are looking after our soil so that it can soak up water. This means working with farmers and landowners to make sure that it's in their interest as well to provide a resource to stop us being flooded.”
However, Cloke believes that one of the most important things we can do to avoid the most severe outcomes is to make sure everyone is singing from the same hymn sheet when it comes to climate change and its impacts.
“There is a lot we can do now such as going round schools making sure that everyone is clear on what the science is saying, and making sure that everyone is taking climate change seriously – that has to be one of the most effective ways of dealing with what is going on.”
Minimising the effects of flooding
For over 50 years, NERC-funded scientists have researched UK sea levels, river basins, storm surges, aquifers and surface flows, saving lives, homes and businesses. For example:
- Research led by NERC’s Centre for Ecology & Hydrology (CEH) has resulted in annual savings of between £76 million and £127 million due to a new model that enables accurate flood warnings five days rather than two days in advance.
- Accurate tide and storm-surge information for the Thames Barrier provided by NERC’s National Oceanography Centre (NOC) helps protect 1.25 million people on the London floodplain and £200 billion of property. NERC data and understanding also showed that the existing Thames Barrier can continue protecting London until 2070, avoiding billions of pounds in premature replacement costs.
- Improved forecasting systems and advice provided by NERC’s British Geological Survey (BGS) mean that over 50,000 fewer UK homes have been flooded. The UK floods in 2013-14 affected 6,000 homes, whereas the 2007 flood affected more than ten times that number, even though the more recent storms were more severe.