The UK government currently spends Pound 2.6 billion on flood defences in England, and that amount is set to double by 2026. Flooding in February 2020 showed how that's likely to be a good investment, as climate change drives warmer and wetter weather each winter. But when it comes to managing rivers to prevent flooding in towns and cities downstream, we're often our own worst enemy.
After the second world war, Britain embarked on a mission to reconstruct its rivers. Workers cut ditches to drain moorland, making it suitable for livestock farming. Looping rivers which once wound lazily through floodplains - flooding these areas once every two years or so - were straightened into rigid channels. River beds were dredged to deepen them and banks excavated to make them steeper, an unnatural situation that takes routine management to maintain.
The idea behind all of this was to reduce flooding by increasing the speed at which water moves downstream. But this also increased the power of rivers to move sediment. Gravels and cobbles dash along these modified and heavily managed rivers, accumulating where the water slows down, as it moves through towns and cities. Here, the river bed swells as sediment piles up, increasing local flood risk.
Over 60% of the UK's watercourses have been transformed in this way, changing the fundamental character of many British rivers - and the natural processes that would usually govern them - over just a few generations. In a new study, we found that doing nothing is often a better course of action for reducing flooding than these heavy handed attempts to mechanically alter rivers.
Going with the flow
We studied the River Caldew in Cumbria, which has caused three major floods in nearby Carlisle since 2010. Satellite data showed that straightening, deepening and embanking was common along the river between 2005 and 2016. Very little sediment was spotted in the river and across the floodplain, suggesting that almost all of it was being funnelled downstream towards Carlisle.
During this time, the channel through the city was widened in the hope that this would cause flood water to spread out and lose energy. But this only increased the problem of sediment building up within the river, creating a shallower channel through Carlisle that's prone to overflowing.
Outside of the city, in parts where maintenance has been relaxed, the river has begun to return to a more natural state. Multiple "wandering" channels can now be seen alongside wide areas of deposited gravel. This is encouraging, as it suggests that the main river and its floodplain are reconnecting, allowing the sediment it transports to fall out of the channel and collect upstream.
We found that rivers which are allowed to behave more naturally are better at locking up sediment upstream, rather than letting it accumulate in unnaturally high quantities in flood-prone towns and cities. If more rivers are allowed to behave naturally and develop this way, it could help reduce future flooding.
This hands-off approach to managing rivers is also much cheaper than hard engineering and brings a wealth of environmental benefits with it. The wandering channel system that's evolving on the River Caldew has the greatest variety of features and habitats across the entire watercourse.
There are gravel bars, deep pools, floodplain wetlands, ponds and river cliffs. This diversity provides greater spawning habitat for fish, and cooler refuges for their fry. The open water habitats benefit amphibians, the trees and shrubs help kingfisher hunt and sand martins can nest in the river cliffs. Beetles and spiders scurry in the shingle, earning this wilder stretch of the Caldew a designation as a site of special scientific interest.
The last 75 years have seen many UK rivers change beyond recognition. The way we manage them in future must look very different. Relaxing our iron grip and allowing natural processes to flourish on rivers once more could be our best hope for reducing flooding, while reviving lost ecosystems rich in native wildlife.
Authors: Neil Entwistle - Reader in River Science, University of Salford | George Heritage - Honorary Research Fellow in Hydromorphology, University of Salford