How does a Catchment Based Approach help to manage flood risk?
For many years, moorland was drained to remove water and create areas for grazing livestock. In the 1960s and 70s landowners were actually paid subsidies to dig grips and drain the moorland. Drainage ditches were dug in fields so crops could grow, and forests were cut down to open up more land for farming. Instead of the natural cover of rough land, vegetation and geographical features which slow, store and infiltrate water, intensive land use has created an environment where water is channelled off the land and into rivers more quickly. The natural flow of rivers has been altered, for example by straightening them, or impeded by the addition of in-stream structures such as bridges and weirs.
In addition, there has been evermore development on floodplains and towns and cities have sprung up around rivers. This has reduced the amount of space for water to flow naturally when rivers overtop. The standard approach to drainage has seen surface water funnelled into pipes, to be taken away as quickly as possible. Now, climate change is bringing more frequent and intense storms and heavy downpours, placing greater demand on sewer networks which may already be at capacity. As well as this, warmer summers can cause a decrease in infiltration rates where soils have been baked hard, this increases the amount of surface water runoff entering watercourses or flowing onto developed land.
Large flood risk management schemes are built to try and keep floodwater out of vulnerable areas. However, shouldn’t we try and address the movement and accumulation of surface water throughout the catchment before its impacts become a problem?
Rather than a disjointed, piecemeal approach to tackling the impacts of surface water and flooding by different organisations, the catchment-based approach looks at the catchment holistically. This approach can better manage the flow of water within it and reduce flooding with a range of different interventions, not just at the local level.
To achieve this, government, landowners, water companies, risk management authorities, voluntary organisations, communities and businesses should all collaborate on an integrated approach which addresses how land can be restored, better managed, or left undeveloped to break up and slow the accumulation of surface water in a variety of ways.
There are many interconnected pieces/processes which make up the ‘puzzle’ of flood risk management within a catchment:
In the upper catchment where head waters form, peatland restoration, blocking drainage channels, and the revegetation of areas with mosses and grasses can restore their natural sponge like qualities. Not only does this store and slow water, but it also helps to remove CO2 and pollutants from the atmosphere, improving habitat, biodiversity and air quality.
Leaky dams can slow the flow of surface water and trap silt, stopping it from accumulating further downstream. They can also slow and disperse flow away from, and out of the channel, providing the opportunity for it to infiltrate into the surrounding land.
Tree planting and the implementation of buffer strips of vegetation around agricultural land or next to rivers can help to break up the flow of runoff and subsequent silt loss, while providing habitat for wildlife, stabilising riverbanks and reducing erosion.
Reconnecting rivers with the natural floodplain by restoring meanders or removing embankments can allow space for water storage away from property and infrastructure:
- Online storage – water is temporarily stored in the river channel or floodplain.
- Offline storage – water is diverted away from the river channel to be released back when peak levels have passed.
More space can be made for water by creating wetlands and storage basins or removing gravel and sediment from rivers where appropriate. Sustainable Drainage Systems (SuDS) on existing and new developments mimic natural drainage processes and increase opportunities for water storage and infiltration, reducing the demand on sewers.
Improved weather forecasting and flood warnings have allowed longer lead times for organisations and communities to respond to adverse weather and potential flooding. Flood risk maps can be used to make more informed choices when considering flood planning or the installation of property flood resilience (PFR) measures.
Slowing and making space for water doesn’t eliminate the need for hard engineered flood defences in vulnerable areas, but flood walls can’t be built higher and higher indefinitely. Large scale flood defences will always be necessary to protect communities from extreme weather events such as storms Desmond and Eva in December 2015. But integrating them with a range of other measures throughout the catchment provides the best approach, because what happens in one area affects what happens in another.
As such, a whole catchment approach to flood management looks at flooding like a puzzle made up of lots of interconnected pieces and attempts to solve the puzzle by implementing multiple interventions which fit those pieces together.
The Flood Hub have created a ‘Catchment Management’ resource which can be downloaded here.