Managing flood risk naturally
What is Natural Flood Management?
Natural Flood Management (NFM) aims to mimic and restore natural river and floodplain processes to store water in the catchment and reduce runoff into rivers. This can reduce the risk of flooding in downstream communities by reducing the volume and delaying the arrival of the flood peak, the highest flow rate reached in a flood event, giving people more time to prepare. NFM is also known as working with natural processes, and it can be used in rural and urban areas.
NFM can work in four key ways:
- Increasing storage space for water that will fill up during a flood, slowly releasing water afterwards.
- Increasing the roughness of the channel and catchment surfaces to increase resistance and therefore slow the flow of water.
- Increasing the volume of water lost to infiltration into the ground or through evaporation.
- De-synchronising peak flows from tributaries. Where two tributaries flow into a larger river, the risk of flooding is greater if they both discharge a large volume of water into the river at the same time. Slowing down one compared to the other reduces this risk.
NFM can be carried out on various scales, from vast agricultural fields to your own street or local stream. Think about how you could store water and reduce runoff from your garden, or get together with your neighbours to explore what you could do to reduce flood risk naturally in your community.
Why Natural Flood Management?
As flood risk increases in the future, conventional engineered schemes would need to get larger and larger. NFM uses a catchment-based approach, combining a range of measures from the river’s source in upland areas, down to the lowland areas where flooding is common. For example, flood risk in a lowland area might be caused by local land use and runoff upstream. NFM helps to tackle this by storing water safely at identified points in the river’s catchment to reduce the flood risk elsewhere.
NFM is an approach for the future, making it a vital part of longer-term flood risk management solutions. Engineered defences can contribute to managing the immediate flood risk, whilst Property Flood Resilience (PFR) has an important role in reducing flood damage to your property both now and in the future. The benefits of NFM, on the other hand, increase over time. As trees grow, they provide greater resistance to runoff and can capture more water in their canopies. Although their effects may start small, they therefore have a major role in the sustainable long-term reduction of flood risk over decades. As extreme rainfall may increase in the future, so too does the ability of naturally-managed catchments to accommodate it.
NFM also brings wider benefits beyond the reduction of flood risk. Green spaces provide calm, tranquil environment for relaxation and recreation, improving quality of life and bringing communities together. They also create and enhance habitats such as woodland and wetland to increase your area’s biodiversity, and can help to improve water quality by reducing the volume of polluted runoff from roads and agricultural fields reaching rivers. Reducing soil erosion from surface runoff also helps to reduce the loss of carbon from the soil.
Find out more about the benefits of NFM at The Flood Hub: https://thefloodhub.co.uk/wp-content/uploads/2018/09/Multiple-benefits-of-NFM.pdf
What can you do?
Planting hedges and trees helps to increase the roughness of the catchment’s surface, delaying runoff into rivers. Leaves also capture rainfall, increasing the amount of water lost to evaporation. Planting trees or shrubs in your garden can help to reduce runoff from your property, and you could consider getting together as a community to plant more trees in your area. Check out The Flood Hub’s Tree-Planting guide for more information. https://thefloodhub.co.uk/wp-content/uploads/2019/03/Tree-Planting-Natural-Flood-Management.pdf
Sustainable Drainage Systems (SuDS) are an example of natural flood management approaches in housing and industrial development. These areas are often covered by impermeable surfaces such as concrete and tarmac, preventing water soaking into the ground and increasing surface water ponding and rapid runoff into river channels. However, SuDS aim to replicate natural processes by making surfaces more permeable to soak up water, and by planting trees and vegetation to slow the flow of water over the surface. Many new developments are now planned with SuDS in mind. However, you can take steps to reduce the runoff of surface water from your property to help reduce the flood risk in your area and downstream.
You can increase the storage of rainwater as it falls onto your property by installing a green roof. On average, a green roof can store the first 5mm of rain in a rainfall event as it infiltrates into the soil, and provides an opportunity to incorporate green space in your property even if you have limited space. Find out more at: https://www.susdrain.org/delivering-suds/using-suds/suds-components/source-control/green-roofs.html
Rain gardens are shallow depressions with free-draining soil that downpipes from the roof of a building are directed into. You can build a rain garden in your garden itself, or in a planter that you connect your downpipe to. This increases the infiltration of water into the soil and therefore reduces the volume of surface water and runoff.
If you have large paved surfaces in your garden or driveway, you could consider permeable paving. The paving surface contains many gaps to allow water infiltration into the soil below, reducing surface runoff. Further details about permeable and pervious surfaces, and their benefits, can be found on the Susdrain website: https://www.susdrain.org/delivering-suds/using-suds/suds-components/source-control/pervious-surfaces/pervious-surfaces-overview.html
Although heavy rainfall can be a nuisance and sometimes even a threat at the time, rainwater harvesting allows this water to be useful at other times of the year. You can harvest rainwater that falls on your roof in a water butt, a small tank that is connected to your drainpipe. This helps to reduce the runoff from your property, and can also help to reduce your water bills if you use it to water your garden or wash your car later! Find out more about different approaches to using water butts at the Slow the Flow website: http://slowtheflow.net/retrofitting-water-butts/
For more information about SuDS measures for your property, and for new developments, check out the Flood Hub’s Introduction to SuDS guide.
To find out more about the key ideas behind slowing the flow, and more tips for what you can do, check out the Slow the Flow website:
A community approach…
NFM can also be carried out on larger scales on rivers and streams in your area through organised schemes and projects approved by your Lead Local Flood Authority. You can get involved with NFM in your community by looking for local groups that undertake NFM work.
Leaky dams are trees or logs that fall or are placed in a river channel to slow the flow of water. This reduced flow rate delays the arrival of floodwater downstream, reducing the flood peak. Sediment and debris can be trapped behind leaky dams, which helps to improve the water quality downstream. Leaky dams cam also help to restore more natural flow patterns by creating riffles and pools in the water, which can provide valuable habitats for fish and insects. If you are interested in installing leaky dams in your local stream or river, contact your local authority. If a main river flows through your area, contact the Environment Agency. You can find out more in the Flood Hub’s Leaky Woody Dams guide. https://thefloodhub.co.uk/wp-content/uploads/2019/03/Leaky-Woody-Dams-Natural-Flood-Management.pdf
Further details can be found in the Yorkshire Dales Rivers Trust’s Leaky Dams booklet.http://www.yorkshiredalesriverstrust.com/wp-content/uploads/2018/01/Natural-Flood-Management-Techniques-Leaky-Dams.pdf
Another simple way that you could help to reduce your area’s risk of flooding is by helping to maintain your local river channels. There may be organised volunteer groups in your local area that you could get involved in if you are keen to assist with this. Removing debris and obstructions where appropriate, such as from trash screens, can help to reduce the risk of floodwater backing up the channel and flooding the surrounding area if it becomes blocked. However, bear in mind that some vegetation and natural woody debris in channels is beneficial in slowing the flow of water to reduce the flood risk downstream; slowing the flow is helpful, but it should not be blocked completely.
Planting riparian (riverside) vegetation can help to retain water outside the river channel and slow down rapid runoff into rivers to reduce the risk of flooding. Roots from vegetation can also help to stabilise the riverbanks to manage erosion, and vegetation can help to improve local habitats. *Remember that safety is key when working near water – look for organised work parties or local volunteer groups to get involved with if you are interested in riparian planting. Your local council or Lead Local Flood Authority may also be able to assist. Find out more about the benefits of riparian vegetation and some suggestions for planting some in your area from the Woodland Trust: https://assets.sussexwildlifetrust.org.uk/wt-trees-for-water-leaflet.pdf
If your community is interested in having a larger NFM project, such as leaky dams or a riparian planting scheme, contact your local authority for further details.