Floods and dredging - a reality check

When major floods occur, like this winter, there is a commonly heard call to “dredge the rivers!” as a solution. Here’s a quick run through why it’s not the solution many want it to be, with reference to a report we published following the winter 2013/14 floods which impacted the Somerset Levels particularly badly, amongst other areas, and led to extensive calls for more dredging as a solution.

Dredging is most commonly used as a method to enable land drainage in low-lying landscapes which would otherwise be too wet to use for farming. Drainage channels, and rivers, allied to pumping, drain water from the land. This helps the land stay dry and helps it avoid frequent flooding. Dredging can also be used to enable navigation.

These rivers and drainage channels are often slow flowing and dredging the channel to expand its capacity helps to convey the water away more quickly. As the volumes of water flowing in these channels are not particularly great, especially compared to the amount of water seen when we experience serious flooding, this approach is effective at keeping the land in a non-flooded state.

Dredging the Somerset Levels

In the winter of 2013-14, and the preceding winter, the Somerset Levels experienced serious flooding as a result of successive winter storms bringing extreme levels of rainfall to the region time and again.

Large areas of the Levels were inundated as river embankments overtopped. It took weeks, stretching into months for the water to drain away, leading to calls for far more extensive pumping and dredging to get rid of the water more quickly.

Ultimately the Environment Agency dredged the rivers Parrett and Tone and the Somerset Rivers Authority undertakes frequent maintenance dredging to maintain larger channels. The Levels have not experienced such serious flooding since.

Yet the situation is far from black and white; there is considerable nuance. The region hasn’t been hit by a similar, relentless succession of winter storms since, either. Some parts of the Country are particularly flood prone and flood frequently. Other locations badly hit this winter hadn’t been affected for decades.

Storms can be many hundreds of miles wide; Storm Desmond was around 1200 miles across. Small movements in the track of such storms and their fronts can make big a difference in where associated rainfall comes.

And research at the time indicated that given that succession of storms, dredged rivers in Somerset wouldn’t have prevented the floods. Any additional capacity and conveyance created would have been far outweighed by the amount of water that fell and flowed down the catchment. What dredged channels would have done – and will do in future – is reduce the duration that flood water sat on farmland causing considerable damage to crops and homes.

So, in Somerset, a successful case was made for more dredging. It should be noted that many remain critical of this outcome and consider that growing certain crops such as maize extensively in the upper catchment of these rivers contributed to excessive runoff and mobilisation of sediment into rivers, and have concern about the impact on wildlife of removing bottom and bank sediments.

So, is it the answer elsewhere?

That is not the focus of this article. The question is could (and should) dredging be an effective intervention to manage flooding of the sort we’ve seen this winter, in areas elsewhere.

The answer is that it almost always wouldn’t be.

Most flooding has occurred in areas where rivers flow considerably faster than in low-lying areas like the Levels. Here such rivers suffer far less from sedimentation and often self-scour with the energy of the water mobilising sediment and preventing excessive siltation.

Expanding the channel in such places would be referred to more as channel engineering and would commonly be associated with the construction of hard defences through towns or cities.

Conveying water away more quickly in this manner is far from risk-free. Faster flows place greater strain on structures such as bridges or defences downstream and can be highly dangerous as we’ve tragically seen this winter. It speeds more water towards downstream communities even faster, potentially putting them at greater flood risk.

Much of how we’ve managed our landscape over recent decades – centuries even – has contributed to this rapid water conveyance and flood risk.

Allied to more frequent and intense rain events associated with climate change, more intensive agriculture characterised by compacted soils and fewer hedgerows, growing villages, towns and cities with their associated hard surfaces – which rainfall runs off rather than soaking into – are all collectively ushering rainwater into river systems more quickly.

There it flows, quickly downstream, in a flood peak. The faster water arrives in the main river, the steeper, and potentially higher that peak.

Slow the flow

That’s why flood risk managers are now looking to take every opportunity to ‘slow the flow’. This essentially means undoing the effects of the land management actions taken in the past.

It can be done through engineering artificial storage areas with quite hard structures, or working with nature to make maximum use of the highly effective ways in which it can hold back water.

In urban areas, sustainable drainage systems store water, usually in surface features such as ponds, channels, raingardens, green roofs or tree pits, or promote infiltration into the soil via soakaways. This is instead of directing rainwater straight into surface drains which lead straight on to rivers.

In more rural areas, more extensive tree cover can slow the rate at which rain hits the ground, then once it reaches the ground their roots help it to soak in – perhaps 60 times more effectively than on compacted grazed hillsides.

As water flows down hillsides, fallen trees in streams or woody dams can store water in ponded areas, backed up by these obstructions. Wildlife such as beavers can help create these features.

Increasing tree cover was recently prioritised as an action to help meet our climate change net zero emissions targets, as trees help to absorb carbon dioxide from the atmosphere. We should look to maximise this in upper catchments where there are downstream communities at flood risk.

Other habitats, such as upland peat bogs are also excellent at locking up carbon and similarly can store large amounts of rainwater in their sponge-like form high up in the catchment, out of harm’s way. They need to be restored and protected.

Obviously managing land in this different way will need land managers to deliver. These are most likely to be farmers and having left the EU, the UK is now free to set its own agricultural support systems and should look to actively support farmers to deliver this land management change.

Even with these interventions deployed extensively, it is likely that we’ll still experience more serious and frequent storms which will result in risk of flooding. More investment in new hard defences, alongside maintenance of existing ones, will always be part of the solution.

Yet still there will be ‘residual’ flood risk. Improving the flood resilience of more individual properties, using flood barriers and doors and more resilient construction techniques is something that must also be enabled more concertedly.

The exact combination of which of these sorts of interventions is most appropriate in any given location will vary. In limited cases, dredging could be part of that picture, but it is a long, long way from the silver bullet that some suggest.

Article

Discussion on BBC Sounds, CIWEM Diretor of Policy, Alastair Chisholm