Slowing the Flow and Carbon Reduction through Floodwater Storage

Climate change is already impacting the British climate, with more heavy and frequent rainfall resulting in local and regional flooding on an unprecedented scale. The Calder Valley has recently suffered its fourth major flood in a five year period.  

Traditional ways of dealing with flooding are no longer adequate, and only deal with main channel riverine flooding involving civil engineering construction of river channel walls, culverts, modified bridges etc.  These solutions are disruptive, expensive in monetary terms, and carbon intensive; involving materials such as steel, reinforced concrete and masonry.  

UK infrastructure emissions amount to some 515 MtCO2e/year (2010), some 53% of the UK carbon footprint(1).  By 2025 this will be 80%, rising again to 90% by 2050(1).

Green Infrastructure needed?

Following HM Treasury’s Infrastructure Carbon Review(1) the BSI developed PAS 2080:2016 Carbon management in infrastructure schemes,which encourages the infrastructure community to cooperate in the development of low carbon infrastructure projects.  The recommendations in the review have the potential to reduce up to 24 million tonnes of carbon and save the UK £1.46 billion a year by 2050(1).

An example of effective NFM

Other techniques are needed, to deal with the climate emergency we find ourselves now living through, and the intense flooding this entails. For example, slowing run off using less expensive and carbon intensive methods, by constructing storage areas in farmland above the valleys, away from traffic disruption and the towns where people live.  These storage areas, known by several different names (attenuation ponds, detention basins, run-off attenuation features and washlands) intercept streams or over land flow paths and store water temporarily during a storm event.  They have many advantages, as follows(2).

  • Water quality. Improved by capturing sediments and cycling nutrients and pollutants.
  • Habitat provision. Where pond outlets are raised slightly, attenuation ponds can have some element of permanent storage and provide habitat for aquatic life and invertebrates.
  • Carbon storage. Similarly, where pond outlets are raised slightly, attenuation ponds or washlands can store carbon.
  • Surface water flooding. Attenuation ponds can reduce surface water or pluvial flooding.
  • Fluvial flooding. Attenuation ponds can reduce main river flood peaks.  Monitored evidence from Belford shows the impact of individual features during a range of storm events. Belford flooded 7 times between 1997 and 2007. Since the project reached 35 constructed run-off attenuation features (~8000 m3 storage), only one property has been impacted by flooding.  There are now 45 features at Belford, amounting to ~12,000 m3 storage.
  • Carbon reduction. Attenuation ponds can reduce the scale and intensity of civil engineered traditional main river Flood Alleviation Schemes (FAS), by a reduction in the height of the flood peak. 
  • Carbon reduction. An attenuation pond constructed at Oldroyd, Todmorden in West Yorkshire in 2016 following the Boxing Day flood of 2015 has prevented surface water flooding of five terraced houses on two occasions since.  A conservative cost estimate of the savings afforded by these prevented floods is of the order £50,000 with the associated carbon savings from elimination of new building materials and the disposal of damaged furnishings(3).  
Attenuation pond under construction, Calder Valley, March 2020

Compare carbon to traditional methods

A comparison is made using the FAS at Mytholmroyd in the Calder Valley, West Yorkshire.  Mytholmroyd has suffered from frequent riverine flooding and pluvial surface water flooding from surrounding hillside cloughs and culverted streams.  A traditional FAS involving new and higher river channel walls, a new widened bridge and associated new culverts is currently (March 2020) under construction at a cost of £33 million.  Construction is 4 months behind schedule and the scheme disrupts traffic flows through the village, with queues of idling cars, buses and trucks emitting significant amounts of carbon.  

An estimate of the volume of reinforced concrete used in this project has been developed which amounts to 1500 m3, this results in the production of some 500 tonnes equivalent of CO2(4).  The same storage volume provided by the Mytholmroyd FAS could be provided with around 50 attenuation ponds of 520m3 capacity, the excavation of these ponds would entail the consumption of around 7500 litres of diesel, which at 2.61 kg CO2 e per litre(5) equals some 20 tonnes equivalent of CO2, 1/25th of the 500 tonnes emitted by the construction of the traditional FAS (which does not include additional effects e.g. of idling traffic, construction site vehicles, only of the concrete itself).

To be clear, the provision of equivalent volumes is not a directly comparable approach because ponds are filling throughout the flood event, when ideally they would only fill when the river system is approaching bank full.  This would entail the provision of a much larger volume of attenuation storage than that represented by the river channel from the example FAS.  However, even if this volume were 10 times the figure above, CO2 would be around 200 tonnes compared with the 500 tonnes plus CO2 from traffic disruption, construction plant and disposal of demolished structures.  

Multiple benefits

In addition to construction carbon savings, carefully located attenuation ponds, as demonstrated at Oldroyd, can:

  • significantly reduce localised pluvial flooding
  • protect areas that do not benefit from a main channel FAS
  • reduce carbon by eliminating replacement furnishings and disposal of destroyed items, and,
  • by sacrificing a small amount of storage and holding a small amount of water permanently, they can store carbon, enhance habitat and encourage biodiversity.

Rolled out in other areas around the country, for example the Don catchment above Sheffield, significant savings economically and environmentally are possible, compared with expensive channel improvements.  An old maxim in civil engineering goes…. “steel and concrete expensive, earthworks cheap”…updating that today…. reducing carbon reduces costs(1).

What are we waiting for?


  2. DEFRA, Working with Natural Processes – Evidence Directory, SC150005, p47-55
  4. Whole-Life Carbon and Buildings, The Concrete Centre, London, 2016.
  5. Conversion factors: Energy and carbon conversions 2016 update, The Carbon Trust, 2016.