In 2007 River Don overtopped it banks, causing widespread flooding in Sheffield. More than 35,000 people were affected by flooding in Hull, with 700 people affected in Doncaster; much of Barnsley and Rotherham were also cut off.
In 2016, we spoke to author of Taming the Flood. Rivers, Wetlands and the Centuries-old Battle against Flooding, Jeremy Purseg’s love about whether climate change is to blame and what the next steps need to be.
Once ever 100 years?
The likelihood of a flood event occurring is usually expressed in terms of its predicted frequency of return. For example, a flood may be referred to as a 1-in-100-year event or as having a 1% probability of being equalled or exceeded in any one year.
It can sound counter-intuitive but low-probability floods, considered over a long period of time, have a significant likelihood of happening. For example, a 1-in-100-year flood has a 25% chance of occurring at least once in a 30-year period (a typical mortgage duration) and a 50% probability of happening at least once in a 70-year period (a typical human lifetime). The Somerset Levels suffered 1-in-100-year floods in 2012 and again in 2014.
“Just because it happens once, it doesn’t mean you are good for the next 100 years,” says David Demeritt, professor of geography at King’s College London.
How to mitigate the impact of heavy rain and flooding
Stop building on floodplains
Sheffield, Doncaster and Rotherham are all built on the River Don’s floodplain. Fishlake is located in the epicentre of the floodplain zone. “Planning has been supposed to steer away from floodplains but it hasn’t worked, partly because local authorities have requirements to build new housing,” says David Demeritt, professor of geography. “We have developed lots of floodplains – first draining them for agriculture and then, often, subdividing for housing.”
Many modern homes are built on floodplains (Alamy)
Manage the uplands to hold water back
The way water is managed upriver – before it reaches floodplains – is also key. This applies to upland areas, such as the water catchment areas of the Pennines, and lowland regions, such as around the Thames, according to Martin Evans, professor of geomorphology at Manchester University.
In the Pennines, the Making Space for Water Project has shown how restoring degraded moorland can reduce the volume and speed of water run-off. Measures included blocking gullies and returning vegetation where there was only bare peat. As a result, water tables have risen by 35mm, peak storm discharge is reduced by 35%, and lag times increased by an average of 28 minutes.
Flooded river (Getty)
“It’s one measure that can help to make flood peaks longer and lower, rather than having flash floods,” says Evans. “If it means the peak flow is 5cm below your doorstep in Doncaster rather than 5cm above, it has made
a huge difference.”
However, Demeritt says such measures have limited impact. “Land management is a good thing for biodiversity. We should be doing it anyway. But it only really works for moderate rainfall. When you get crazy rain falling for weeks, it doesn’t make
Allow farmland to flood
Farmers may be paid to allow their land to flood. “It’s easier and cheaper to store water in fields than in your kitchen,” says Demeritt. “It’s an idea that seems worthy of subsidy.”
In lowland areas, livestock grazing can compact soil, raising water run-off. Changes in grazing times and farmers ploughing land along contours can reduce water flow during heavy rain in lowland areas. “That comes at a cost to farmers, so there is a discussion about how you pay farmers for this service,” says Evans.
What about dredging?
“There has been an awful lot of garbage, if not factual untruths about dredging,” says Demeritt. “Dredging gives a bit of extra capacity in the drains, so a bit more rainfall will drain away faster. But it won’t make any difference with really heavy rain. If you dredge, the water ends up in someone else’s kitchen downstream, more quickly.”
What is dredging?
Dredging removes silt from the riverbed, allowing rivers to hold more water – and to remove floodwater more quickly.
What are the drawbacks?
The Environment Agency has warned that in order to stop rivers bursting their banks they would need to be dredged several metres wider and deeper.
It has also been claimed that the greater volumes of water carried by dredged rivers could lead to serious flooding in towns downriver.
Conservations have expressed strong concern that dredging destroys delicate wildlife habitats such as fish spawning areas and freshwater mussel beds.
What are the alternatives?
Other suggestions focus turning upland pastures into bogs, effectively soaking up rain like a giant sponge and releasing it more slowly. Extensive tree planting in the catchment area may help to soak up excess water before it reaches low-lying areas. De-canalising the rivers, allowing them to meander, may also slow the force of floodwater.
“People have to take some responsibility when they buy,” says Demeritt. “Rather than blame the Environment Agency, they need to look at a floodplain map. It’s nonsense to think it’ll be okay because the area has never flooded before.”