Water Industry News

Spotlight On: The Ouse Washes and Flood Risk Management

Back in 1630, dutch engineer Cornelius Vermuyden set about creating the Old Bedford and New Bedford rivers (now the New Bedford River and the River Delph), with the idea being to improve drainage of the Great Ouse. The area between the two rivers is 20 miles long and nearly a mile wide, serving as a flood storage reservoir throughout the year.


This area is known as the Ouse Washes and it’s the UK’s largest area of frequently flooded grazing marsh, making it an attractive habitat for various bird species – so much so, in fact, that it is now a designated Special Protection Area.


However, over the last 75 years or so, river bed levels in the Great Ouse Tidal River have risen, with silt drawn from the sea during periods of drought settling in the river. While the majority of this silt is taken back out to sea after drought periods come to an end, not all of it is removed and, over time, it builds up.


The Great Ouse Flood Protection Scheme has also led to higher silt levels because of reduced flushing flows in the river, all of which are now having an impact on drainage in the Ouse Washes – and this, in turn, has resulted in increased flooding in the area.


As the Environment Agency explains, higher bed levels and more flooding is problematic for various reasons, including harming habitats for wildlife, impeding navigation, affecting landowners using the Washes for grazing marsh and potentially reducing the effectiveness of the South Level Barrier Bank.


In addition, future sea level rises are also expected to make it more difficult to manage silt levels in the Great Ouse Tidal River.


As such, the agency has now devised a 100-year management plan for the region, covering parts of the Cambridgeshire and Norfolk Fens, to help protect approximately 2,400 properties and 26,000 hectares of agricultural land, which are currently at risk of flooding.


It’s possible, however, that over the next 100 years the number of properties at risk of flooding in the region could climb to around 5,000 if the defences that are currently in place fail, with the situation exacerbated by climate change impacts and rises in sea level.


The Environment Agency’s preferred strategy for this part of the country involves maximising flows through the Denver Sluice to flush out silt, implementing new erosion protection, changing how Earith Sluice is operated, replacing crest walls on embankments, carrying out bank raising work and monitoring river flows and bed levels in the river to gauge the effectiveness of the plan.


Just this week (May 24th), it was announced that a new temporary flood barrier will be installed to help prevent water from spilling into Welney village from the Ouse Washes flood storage reservoir.


The reservoir itself projects 2,000 properties, railways and roads from winter flooding, as well as over 67,000 hectares of farmland. When full, it stores around 90 million cubic metres of water, which is enough to fill Wembley Stadium 22 times!


Ouse Washes project lead Nicola Oldfield said: “We know the impact that flooding can have, which is why protecting people and communities is our priority. This is one of the final parts of a vital long-term investment and refurbishment programme to maintain the dam of the Ouse Washes flood storage reservoir.”


River floodplains themselves represent an important part of flood risk management plans, as well as conservation and climate change adaptation strategies.


They are able to absorb and store carbon, so can help address the climate crisis in this way, as well as storing silt and preventing it from clogging local rivers. And, of course, they can help reduce flooding risks by soaking up excess water as and when necessary.


They are able to filter and store this water, helping to promote healthy functioning of river ecosystems and working to sustain and support local biodiversity and wildlife.


However, estimates from the European Environment Agency (EEA) are that between 70 and 90 per cent of Europe’s floodplain area is now ecologically degraded as a result of human activity, particularly that which has taken place since the 1950s.


Climate change is also expected to hit floodplains hard, with some regions experiencing more frequent high intensity rainfall and others experiencing more frequent periods of drought, which will impact floodplains even more.


Changes to floodplains over the last 80 years or so have resulted in making floods more damaging, with waves now higher and able to travel faster because rivers have been straightened to drive improvements in navigation and transport. Larger amounts of fine sediment is also carried down rivers as a result, creating larger deposits than would happen naturally.


Other changes, including flood and water flow control, hydroelectricity and water supply, may have supported flood protection and economic growth, but the toll has also been taken on the natural environment.


Rivers are now disconnected from their floodplains, which has made them less effective for flood and drought mitigation, as well as for wildlife habitats and protection of water quality.


The EEA observes: “There is a link between the amount of natural floodplains and achieving the key objectives of European policies, in particular in the context of the Water Framework Directive, the Floods Directive and the Habitats and Birds Directives.


“Floodplain management or protection is encouraged but only indirectly required under European environmental policies, but floodplain health is important for achieving multiple European policy objectives. Many of these policies have not reached their objectives to date.”


Naturally and regularly flooded floodplains can provide a range of ecosystem services, including erosion control and prevention, water purification and denitrification, replenishment of groundwater reservoirs, flood management and habitat and species support.


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