Why do towns and cities keep flooding?

Places all over the world continue to be hit by devastating flooding.

In the UK, 2025 kicked off with a major one in Greater Manchester. Since then, we’ve seen deadly floods hit Australia, the USA and Botswana.

Though the amount of damage varies from year to year, the upward trend over recent decades is clear.

Why are we having more floods?

Flood risk is rising because of a combination of factors.

More people and assets are located in flood plains, which means that exposure is going up.

Flood plains have always been convenient places to live – near to water for domestic and agricultural purposes, and to rivers and ports for navigation.

Does rain play a part in floods getting worse?

The story is more complex when it comes to rainfall.

A warmer atmosphere can hold more moisture, so the Intergovernmental Panel on Climate Change (IPCC) is confident that climate change is leading to more intense rain.

That doesn’t inevitably lead to more floods, because rain varies so much depending on when and where it falls. There isn’t a direct relationship between the severity of rainfall and of flooding.

In Britain, for example, there’s evidence of floods becoming more powerful in northern England and Scotland, but not really in the south and east.

Overall, the message is that the future of flooding is very uncertain, but we can’t simply expect it to be like what we’ve experienced in the past – it could be a lot worse.

What can engineers do about flooding?

So the question for engineers is: what are we going to do about it?

The toolbox of options for reducing flood risk is pretty well established, and I don’t think we’re going to see any game-changing solutions.

Where the game is – or should be – changing is how we decide what to do, where and when.

Engineers can help reduce flooding at every step

Options to reduce the impacts of flooding exist at every step in the chain.

Think first about ways in which water can be held back in catchments or when it hits rooftops or the ground in towns and cities.

That means changing land management in the uplands, which in the past may have been deforested, drained and overgrazed.

If they can be restored to a more natural state, we can hold back water and create better places for nature and people to enjoy.

The role of natural flood management

I’m a big fan of so-called natural flood management (NFM), but I’m also worried about over-selling it.

NFM isn’t going to make much difference in the most prolonged and heavy rainfall, when all the ground will be completely saturated anyway.

And we don’t yet have enough measurements of the difference that NFM can make. We need to be able to compare the before and after to calculate their real impact.

Further downstream, there’s the opportunity to reduce water levels by ‘making room for the river’. In other words, increasing the amount of water that can be held in the floodplain and ‘slowing the flow’.

Sustainable drainage systems

Paving over towns and cities and building piped drainage increases the speed of runoff and makes surface water flooding worse.

We know how to mitigate that through the construction of sustainable drainage systems (SuDS) that can absorb and store water.

There are some great examples of these features – with the National Infrastructure Commission, I visited Mansfield, where Severn-Trent Water has taken a town-wide approach.

Through retrofitting vegetation and rain gardens into the city centre, as well as adding some below-ground storage, Mansfield has become a nicer place to be, and flooding problems have been tackled.

Less people at risk of flooding

Reducing flood risk also means taking steps to stem the increase in the number of people exposed to flooding – through wise land use planning.

Where flooding is quite likely and community-scale flood protection isn’t viable, property-level measures like flood-proofing can cut the impacts for householders.

One area where technology is making a big difference is through improved flood forecasting using the power of AI, and flood warning using communication technologies.

Deciding how to prioritise flood protection

In many places, especially large urban areas in floodplains, there are few alternatives to engineered flood defences – embankments, walls, detention basins, and channel modifications.

The big questions then become: what is affordable and how to prioritise?

The main benefit of a flood protection is the fact that it will reduce the impacts of flooding – but not to zero.

So even if we do something, there will always be a ‘residual risk’ of flooding.

The benefit is the difference between the risk of doing nothing, and the residual risk for a range of ‘do something’ alternatives.

We need a long-term, evidence-based strategy

If we look across the whole country, one can come up with a ranked list of the ratios between benefits and costs. The Environment Agency’s long-term investment scenarios (LTIS) analysis does this in England, for example.

But that prioritisation will rest on some strong assumptions: the modelled flood risk, and whether the schemes can actually be delivered for the assumed costs.

It’s really important to kick the tyres on this process of prioritisation and identify the interventions that are robust to uncertainty.

It’s also crucial to involve communities in this process of understanding flood risk and exploring solutions, as whatever happens, managing this issue will impact people’s lives.

But flood risk can’t be eliminated, and we can’t seek to fix every problem everywhere at the same time.

That’s why we need a long-term, evidence-based strategy. How we do that is what I’m focussing upon as 160th president of the ICE.

Watch Professor Hall's Presidential Address