Flood Risk Management in North Yorkshire - The Real Story
North Yorkshire is seeing the employment of Natural Flood Management, in addition to traditional engineered defences, in order to protect the region from a range of flood risks.
Water has a crucial role in North Yorkshire’s iconic landscapes, stretching from the formation of its characteristic valleys in distant geological history to how people interact with the land today. In fact, many of the Yorkshire Dales are named for the waterbodies that intersect them. The county contains over 1700km of main rivers, the larger rivers and streams which have the greatest importance for local drainage, and over 23,000km of smaller ordinary watercourses.
North Yorkshire is home to the headwaters of the Rivers Wharfe, Aire and Nidd and contains several river catchments: the Derwent, Wharfe, Swale, Ure, Nidd and Ouse. The topography of the county is extremely varied, ranging from steep uplands to low-lying valleys. North Yorkshire is known as a rapid response catchment, meaning that there is often little warning before flooding happens because the water runs quickly down the steep valley sides. River levels increase quickly and can remain high for several days after flooding. North Yorkshire has therefore experienced many significant river (fluvial) floods, including several in recent years such as in 1999, 2000, 2004, 2007 and 2012.
Although rainwater would usually infiltrate through the soil, this may not be possible if the soil is already saturated after previous heavy rainfall events. In these instances, the water ponds on the surface. Surface water generally flows along simple routes such as roads and pools in topographic low points. This can flood roads and properties in these areas. As surface water flooding can be so localised in nature, it can be difficult to predict. North Yorkshire has experienced several surface water flooding events in decent decades, a notable example being in 2012. In some areas, such as Malton, Norton and Old Malton, the river levels in the River Derwent can interact with surface water to cause a ‘flood locking’ effect. If the water levels exceed a certain depth, this blocks the outlet for surface water into the river so it cannot be discharged.
After heavy rainfall, when large volumes of water have infiltrated into the ground, the water table rises. If river levels are high, water can soak into the banks, which also causes the water table to rise. Like water above the ground level, groundwater tends to flow from higher to lower ground. When the groundwater reaches these low-lying areas, it can cause the water table to rise above the surface, causing groundwater flooding. As water moves more slowly underground, this can happen days or even weeks after heavy rainfall. Some localised parts of North Yorkshire, such as Malton and Norton, are susceptible to groundwater flooding after long wet periods. If groundwater levels are high, this can also infiltrate into the local combined sewer network, which can reduce the capacity for wastewater and storm water during flood events.
Due to the range of flood sources affecting North Yorkshire, a variety of different approaches to flood risk management are used, from flood gates and pumping operations around Malton to natural flood management methods and tree-planting around Pickering. These methods help to reduce the risk of flooding to North Yorkshire’s residents.
It’s important to understand the flood risk to your property and the steps that you can take to prepare and adapt for flooding to minimise the risk of damage to your home or business. You can find out more about how to prepare for flooding here: https://flood-warning-information.service.gov.uk/plan-ahead-for-flooding
Malton Flood Defences
Malton Flood Defences
The historic town of Malton, North Yorkshire, grew and thrived as a market town from the 18th Century after early navigation improvements to the River Derwent enabled extensive transport of goods and produce. However, despite being an important part of the town’s picturesque charm and success, Malton’s close connection to the River Derwent means that the town faces flood risks from a variety of sources and has a long history of flooding. Records of major floods in the area stretch back as far as 1867 and many floods have occurred in and around Malton in recent decades.
The River Derwent flows through the town after being joined by the River Rye four miles upstream. When river levels are high, the local drainage system cannot discharge into the river as the outlets become blocked when the water level rises above them, causing surface water flooding as the water cannot escape and drains begin to back up. In addition, parts of Malton experience flooding from rising groundwater levels following wet winters and increasingly extreme summer storms. In nearby Norton, surface water again poses a risk, and there is also a risk of sewer flooding if the drainage system becomes overwhelmed, whilst in Old Malton, there is river flooding from Riggs Road Drain.
Defending Malton, Norton and Old Malton from this broad picture of flood risks requires interlinked approaches at various levels. Methods ranging from traditional engineered defences around the River Derwent to pumping plans are already in place and have helped to minimise the impact of flooding in Malton, Norton and Old Malton. Improvements are now being sought to enhance the efficiency of these responses and ensure their sustainability in the long term.
The Malton, Norton and Old Malton Surface Water Management Project aims to address the residual surface water flood risk and groundwater flood risk in these areas. Without measures in place, this could have a 10% chance of occurring each year. The scheme combines property flood resilience (PFR) measures and improvements to the existing pumping response works, and was funded by North Yorkshire County Council, Ryedale District Council, the Environment Agency, the York, North Yorkshire and East Riding Local Enterprise Partnership (YNYER LEP) and the Local Growth Fund, a government fund for projects that will benefit the local area and economy.
When the risk of flooding is high in Malton, Norton and Old Malton, an emergency pumping response is conducted by an experienced team involving North Yorkshire County Council, Ryedale District Council, the Environment Agency and Yorkshire Water. The team have delivered this response for 25 years and are confident in carrying it out efficiently. The plans are regularly refined and have been accredited with averting severe flooding in 2018. However, their implementation of has sometimes required closure of highways to discharge the water to a suitable location, causing transport disruption. Also, the frequency and duration of the response may make this challenging to maintain in the longer term as flood risk increases due to climate change. Improvements are therefore being delivered in five locations to create permanent infrastructure that will help to improve response times and minimise disruption when emergency pumping is required. These works are being carried out by WSP, global specialists in engineering and consulting.
Pumps are located upstream on either side of County Bridge. Permanent brackets are being installed on the bridge to hold the pipes in a fixed location to pump out water over the flood wall. The pipes are angled into the river to reduce the potential for scouring caused by pumps discharging floodwater at the base of the wall. During a flood, pumps can then be connected to pipes which are already in position rather than needing to roll out temporary piping, helping to speed up response times. A permanent pump is also positioned in the yard outside Tate Smiths brewery. Following the improvement works, the pipework will rest on a permanent steel platform and the pump will be raised on a concrete platform base, enabling pump operatives to work at a safer height when conducting maintenance.
Lascelles Lane in Old Malton often experiences surface water flooding on roads. North Yorkshire County Council has an existing pumping response system in place to minimise this, although this has sometimes involved running pipes down the road, resulting in disruption to traffic. The pump is therefore being moved to nearby allotments and will be sited on a permanent stand. As there is an existing manhole in the allotments, the pipes from the pump will run straight into the drainage system instead of down the road, reducing equipment needs and the impact on traffic.
The Cat Well in Old Malton fills up rapidly with water during a flood. Sluice gates are installed, although without pumping, the water is at risk of overflowing onto the road and into nearby properties. When the water rises, pumps are rolled across the grass nearby to access the well. Depending on the severity of the flood, between one and six pumps may be required. However, as the field is regularly used for access by pedestrians, the movement of trucks and trailers used to relocate the pumps can churn up the ground, causing a slip hazard. Hardstanding is therefore being installed so that the pumps can be rolled out safely. Marker boards and CCTV cameras are also being installed at the Cat Well to allow the water levels to be monitored remotely. Additional CCTV is being installed at two other key locations to improve flood monitoring and the provision of warnings for all types of flooding, enabling assistance to be prioritised and provided in a timely manner.
The Malton Property Flood Resilience (PFR) Scheme
Malton, Norton and Old Malton residents at the greatest risk of recurring flooding are also eligible for a government grant scheme to fund PFR measures. This can help to reinforce and enhance the benefits of the pumping improvements by providing additional reassurance at the property level. The PFR works aim to identify approaches that can be delivered at individual properties to reduce the physical impact of flooding and the emotional impact it can have on people’s lives.
North Yorkshire County Council are working with the Environment Agency to administer the PFR scheme. Firstly, JBA Consulting, expert consultants in environmental and weather-related risks, worked across Malton to survey eligible properties and determine which PFR measures would be most effective. 149 properties were eligible in total. Just as every home is different, so are its needs for PFR, so the solution for each property needed to be carefully considered. This was particularly evident in Malton, where the eligible properties included homes and businesses, clubs, listed buildings and even a brewery and a supermarket. Fortunately, property surveys continued as essential works despite the Covid-19 restrictions in place. Survey practices were adapted to ensure the safety of the occupants and surveyors. Property exteriors were able to be surveyed as usual, with images of important interior features such as changes in floor levels being shared virtually. After the consultants completed their surveys, their recommendations were delivered to the occupants and discussions with the surveyors were held via telephone or videoconference. This allowed the occupants of each property to explore the options available based on the surveys, their needs and the costs involved, to select their preferred measures. The scheme covered resistance approaches such as flood barriers and doors, non-return valves and self-closing airbricks.
You can find out more about these PFR measures here: Yorkshire Flood Resilience | Examples of measures
North Yorkshire County Council received grant funding from central government for every eligible property. Once the PFR measures had been selected and agreed, residents informed the council of the measures they wished to install and these were sourced by the council from industry-accredited suppliers. The PFR measures are installed by experienced and qualified contractors. Installation commenced in February 2021. Once installed, the contractor demonstrates the use of the measures to the occupants and the work is signed off. JBA Consulting will return to the site in early summer 2021 to carry out post-installation assessments to ensure that the measures operate effectively as designed. From this point, the occupant become responsible for their PFR measures, including maintaining them and ensuring that they are operated in the event of a flood.
By improving awareness of how and where flooding is likely to occur, the Malton, Norton and Old Malton Surface Water Management Project aims to enhance the focus on monitoring water levels where flooding is expected so that warning services can be improved for all types of flooding. This could have a crucial role in informing people when they may need to operate their PFR. One of the key goals of the Malton, Norton and Old Malton Surface Water Management Project is to involve communities more actively in managing their flood risk. Through the installation of PFR, the scheme empowers residents and businesses to reduce the risk of flood damage to their properties.
The Malton, Norton and Old Malton Surface Water Management Project provides short, medium and long-term solutions to the area’s residual surface water and groundwater flooding risk. By reinforcing a robust flood response framework and further empowering residents to take action at the property level, the scheme aims to reduce anxiety around flooding in communities. The delivery of the scheme has also helped to boost community flood awareness and preparedness by engaging locals about developing emergency flood response plans and improving awareness of what to do when a flood is likely and where to seek support.
Malton’s local businesses are at the heart of its community and its popularity with visitors, and are likely to benefit considerably from the Malton, Norton and Old Malton Surface Water Management Project. It is estimated that preserving the value of existing businesses in flood-affected areas could bring economic benefits of £589,000 to the area by reducing the risk of costly flood damage to stock and premises and the threat that this poses to jobs and income. As well as helping to protect communities and their livelihoods, the scheme also brings hope for the future. By reducing the threat of flooding to local properties, it is hoped that the scheme will unlock the area’s economic potential, creating new opportunities for investment and business growth and establishing Malton as a desirable residential and tourist destination with a thriving economy and community. The measures delivered through the scheme were carefully considered from the outset to fit sensitively with the town’s heritage and conserve and enhance its environmental characteristics. This investment in flood risk management in Malton, Norton and Old Malton could also encourage the integration of flood risk management into future developments in the area.
The range of measures being delivered through the Malton, Norton and Old Malton Surface Water Management Project highlights the importance of a varied approach to managing flood risk. Whilst the improvements to emergency pumping help to reduce the risk of flooding across the town by enabling more rapid responses, PFR measures further help to reduce the risk of flood damage to individual properties and enable individuals to take a proactive role in managing the flood risk to their homes and businesses. By developing resilient responses to flooding both within the community and from individuals, Malton’s flood risk management measures complement each other to provide protection for hundreds of homes and businesses.
Natural Flood Management in Pickering
Natural Flood Management in Pickering
Throughout Pickering’s history, the town has experienced flooding from Pickering Beck, including four incidences between 1999 and 2007, the latter of which incurred £7 million in damage. The source of the beck originates in the North Yorkshire Moors and flows down into the town.
Defending Pickering with traditional concrete walls would have been unfeasibly expensive and residents were concerned about the visual impact on the historic and picturesque town. As a result, an innovative nature-based solution was trialled, and the Pickering Slow the Flow project began.
The Pickering Slow the Flow project is based around Natural Flood Management (NFM) which works with natural processes to help reduce the risk of flooding, for example, by slowing down floodwaters or storing water safely upstream on floodplains. Following severe and widespread flooding across the country in 2007, a series of government recommendations were made to manage flood risk effectively and sustainably in the future. Following this, Pickering was chosen as one of three locations in the UK to host an exciting pilot project for NFM to investigate how land management affects flood flows, as well as the wider benefits that it can bring to local communities.
The scheme commenced in April 2009 and was funded by Defra and led by Forest Research. Other partners include Forestry England, the Environment Agency, the North York Moors National Park Authority, Durham University, Natural England and local authorities including North Yorkshire County Council and Ryedale District Council. The project was divided into two phases, the first of which ran between 2009 and 2011, and the latter between 2011 and 2015.
Opportunities for NFM around Pickering were mapped by Forest Research and Durham University carried out modelling to find places where the beck could be re-connected to its floodplain. Using a new opportunity mapping approach, the first time that this method was applied in Yorkshire, ideal locations for woodland creation to manage floodwater were identified. This scientific approach was important when planning the most effective locations for the NFM measures. The measures are mainly located in Cropton Forest in the steep upper course of Pickering Beck as installing NFM measures in the upper catchment of a watercourse can make a significant difference in reducing flood flows. The more NFM measures are installed across the catchment area, the greater effect they have on flood flows in total.
As the project developed, the plans expanded to cover Sinnington, a village in the upper River Severn catchment which has also experienced repeated flooding. As a small, remote village, engineered flood defences were not economically viable, hence the expansion of the Slow the Flow scheme provided an ideal opportunity to reduce the area’s flood risk using nature-based methods.
Community involvement was crucial in the development of the project and was a key element in its success. Community information days and events were held at local village halls and local residents had an active role in consultations. The project also led to the development of new strategies to work with landowners to encourage and support them to install NFM measures on their land.
For further information about the background of the Pickering Slow the Flow project and how it is being delivered, visit the Forest Research website: https://www.forestresearch.gov.uk/research/slowing-the-flow-at-pickering/slowing-the-flow-at-pickering-about-the-project/
The Pickering Slow the Flow measures were delivered by Forestry England, including tree-planting, in-stream structures such as woody debris dams and smaller check dams that block ditches running off the moorland into the beck. To support these measures during major floods, a large earth bund was constructed at Newbridge, upstream of Pickering, which considerably increases upstream water storage. The construction of the bund was led by The Environment Agency. The bund can hold 120,000m3 of water, with the other NFM measures complementing this to further reduce the risk.
129 woody debris dams have been constructed in the Pickering Beck catchment and 38 in the River Severn catchment. These are built from loose frameworks of logs and branches positioned across the channel and secured into place to slow the flow of water downstream by obstructing but not completely preventing it. This helps to store more water upstream, reducing the speed and volume of floodwater flowing through Pickering. Moorland grips, ditches that areas of bog or heathland, were also blocked using bales cut from local heather. The water running along the grips infiltrates into the bales, where it is temporarily stored and eventually drains out over the surrounding land, rather than flowing down the grips into the beck. 187 heather bale check dams have been installed.
Planting trees on floodplains and on the banks of watercourses, an area known as the riparian zone, can help to form a natural barrier to flood flows. Trees obstruct overland flow paths, slowing the rate that runoff water flows into the channel. Also, trees help to improve the quality of the soil around them, which can allow more water to infiltrate into the ground instead of flowing over the surface. By stabilising the soils they grow in, trees can reduce soil erosion into river channels, which may otherwise have their capacity reduced by silt. 19ha. of woodland in the Pickering Beck catchment and a further 10ha. in the River Severn catchment were planted during the project.
‘No-burn’ buffer strips were established alongside watercourses draining Levisham Moor. Heather burning can increase runoff by removing flow-resisting vegetation and altering the soil’s properties to make it repel more water. Steps were also taken on farms to manage surface water runoff, including check dams to block runoff pathways and ponds and vegetated channels (known as swales) to hold water and store sediment washed from the fields.
As a pilot project, Pickering Slow the Flow also helped to develop new approaches to natural food management and learning from the earliest measures has informed new methods of NFM delivery. In Sinnington, two large leaky woody bunds have been constructed. This applies the concept of woody debris dams on a larger scale to span the width of the entire floodplain with a wall of stacked logs, which are secured to tree stumps and posts. A more engineered approach to debris dam construction was also developed to enable the construction of larger dams on the main Pickering Beck by fixing the stacks of logs to the bankside using wiring, posts and bankside trees.
To find out more about the NFM approaches used and to see examples, visit the Forest Research website: https://www.forestresearch.gov.uk/research/slowing-the-flow-at-pickering/slowing-the-flow-at-pickering-about-the-project/slowing-the-flow-at-pickering-catchment-measures/
Like all flood risk assets, once NFM measures have been delivered, they must be maintained so that they continue to function as intended. To date, the Pickering NFM measures have been maintained by Forestry England. Debris dams are typically expected to last at least 10 years and those in Pickering are surveyed every year to check their condition. The Cropton Forest beaver trial is the next step in the maintenance of the Pickering Slow the Flow measures. Beavers naturally construct woody debris dams from logs and branches after felling trees with their large teeth. The beaver trial area is situated within the Slow the Flow project area and the beavers’ interactions with the existing debris dams are being monitored to see whether they will maintain, reinforce or add to the structures or alternatively if they will build their own structures that will slow the flow in the same way. The trial commenced in 2019 and will run for 5 years.
With the pilot project for the delivery of the NFM measures now complete, Forestry England and Forest Research continue to gather data to monitor ongoing impacts and evidence the longer-term benefits of this pioneering scheme. The learning from Pickering Slow the Flow has supported a range of scientific studies into sustainable and natural ways to manage flood risk and been used to deliver similar projects elsewhere in the UK as natural flood management becomes increasingly recognised.
Results indicate that the Pickering Slow the Flow project has reduced the probability of flooding from a 25% chance in a year to less than a 4% chance. In Sinnington, the river typically breaks its banks for 1-2 hours during peak flow, so NFM helps to control this and reduce the risk of the river overtopping into the village. In winter 2015, the Slow the Flow measures reduced the flood peak, the maximum volume of water flowing past a point in the river at one time, by 15-20%, helping to reduce the risk of water reaching nearby properties. The benefits of NFM can be reinforced by flood risk management approaches at different scales, such as property flood resilience to provide additional reassurance to individual homes and businesses. By helping to reduce flood peaks to more manageable levels, NFM can also boost the effectiveness of other flood risk management measures in the catchment, making it an important element of catchment-based approaches.
“The ‘Slowing the Flow’ natural measures play a significant role in reducing rain water runoff from the Pickering beck upper catchment. Natural measures can support traditional engineered features and play a significant role in protecting communities downstream where there are practical and financial barriers to implementing engineered solutions.” said Forestry England’s Yorkshire District Forest Management Director, Alan Eves.
In addition to flood risk management, NFM can also provide further environmental benefits. The Pickering Slow the Flow measures have created a wide range of habitats within the rivers themselves, around the banks and on the floodplains, helping to boost local biodiversity. A total of 49.9ha. of woodland habitat has been planted or improved and 3.9ha. of heathland habitat has been restored. The extensive woodland planting delivered through the scheme may also bring benefits through carbon storage; research estimates that the trees planted in the Pickering Beck catchment could take up an average of 21,000 tonnes of carbon dioxide in 100 years.
Through the Slow the Flow project’s pioneering pilot study of NFM measures, Pickering and Sinnington demonstrate the growing benefits of nature-based approaches as part of a sustainable solution to flood risk management. With research ongoing and the beaver trial in progress, the Pickering Slow the Flow project will continue to inform the latest developments in nature-based flood risk management solutions. The project has gained a strong national profile and its outputs have guided government policies on flood risk and land use management. As the delivery of natural flood management expands, learning from the Pickering Slow the Flow project goes on to inspire and inform sustainable and catchment-based approaches to managing flood risk across the country.
Misconceptions - Beavers
Misconceptions – Beavers
A pair of Eurasian Beavers from Scotland were introduced into a 10-hectare area in Cropton Forest, North Yorkshire. The 5 year trial began in April 2019 and aims to see how well they maintain debris dams installed to slow river flow. The pair now have four young, called kits. Unlike species such as grey squirrels, the Eurasian Beaver is native to Britain, although they have not been seen here for around 400 years after they were hunted to extinction. Centuries later, the first UK beaver trial started in 2009 in Knapdale, Scotland. Eurasian beavers continue to live in their natural habitats in other European countries including Germany, the Netherlands and Norway to this day.
Eurasian Beavers are herbivores and eat aquatic plants and grasses and the bark, twigs and leaves of trees. Fresh water surrounded by woodland is an ideal beaver habitat as it provides food and shelter. Beavers are crepuscular, so are most active at dawn and dusk. They are more confident moving in deep water than on land so usually build dams in small shallow streams to make the water deep enough for their movement. Using their large teeth, they fell trees and split them up into smaller branches which they drag into place to build dams. These restrict the flow of water and create ponds of still, deep water where beavers build their lodges.
Beavers are sometimes dubbed ‘ecosystem engineers’ because of their impact on river ecosystems. Beaver dams are more watertight than other debris dams found in rivers and streams as they use the sound of running water to find gaps in their dams and work throughout the night to repair them. Instead of flowing uninterrupted downstream, the water flows into the pond behind one dam, where it is stored for a while, before eventually escaping through and being stored behind the next dam downstream, and so on. Therefore, when water leaves a beaver habitat, it flows slower and the volume of water is lower than when it flowed in. This can help to reduce the risk of flooding as slower, shallower flows are less likely to overtop the riverbanks downstream. In Devon, where a beaver trial started in 2011, the beavers have now constructed 13 dams which store an extra 1 million litres of water. When the water flows out of the beaver trial area, its maximum flow rate is 30% lower than when it flows in.
By delaying the flow of water downstream and storing it in ponds, beaver dams encourage water to flow out of the river channel behind the dams. This can help to increase the river’s connectivity to its surrounding floodplains, which allows water from high river flows to be stored naturally instead of causing deep, rapid flood flows downstream. Permitting flooding on upstream floodplains also helps to store floodwater away from downstream residential areas. The storage of water on the floodplain also helps to create diverse wetlands, which have great ecological benefits as well as helping to reduce flood risk. Beavers create bank-side tunnels, canals, lodges and burrows to provide access to foraging areas, which help to create varied habitats that attract biodiversity.
By building dams, beavers can affect other species in the river. Although beavers do not eat fish, some concerns have been raised about beaver dams blocking the migration of salmon and trout and about sediment accumulation behind dams damaging spawning sites. However, in the Scottish Beaver Trial, fisheries experts agreed that beavers brought more benefits to their fish populations than disadvantages. Beaver dams and felled trees provide fish with areas of refuge from predators and when beavers fell trees into large watercourses, this can create areas of shallow flow called riffles that provide fish spawning areas. Beaver dams also create areas with different depths of water which provide a broad range of habitats for both fish and a variety of insects that the fish eat.
To find out more about the Cropton Forest beaver trial and the latest news about your local beavers, visit the Forestry England website: https://www.forestryengland.uk/beaver-trial-cropton-forest
Misconceptions - NFM
Misconceptions – NFM
Natural Flood Management (NFM) methods work with the natural features of a river catchment to slow down or store flood water. NFM aims to reduce the maximum height of the water, known as the flood peak, downstream, or delay its arrival to give you more time to prepare for floods. There are many different approaches that we can use to manage flood risk naturally.
Planting trees and creating woodlands are common approaches used in NFM. You may wonder how something as simple as trees could make much of a difference compared to the huge scale and power of flooding, although there are many ways that trees can work to reduce flood risk. When surface water is flowing over land, trees can obstruct this. This increases the roughness of the catchment’s surface, which slows the flow of water over it. This helps to slow the rate that water enters rivers or sewers, reducing the risk of flooding due to rivers overtopping their banks or sewers backing up. Fast-flowing surface water can also carry sediment into rivers and can fill their channels with silt. This leaves less space to accommodate water and therefore increases the risk of the river flooding. Planting trees to slow surface water flow can therefore help to maintain the capacity of the river channel as well as slowing the flow of water into it.
As trees grow, their roots break up the soil beneath them. This creates more gaps, or pores, in the soil, which improve its ability to store water. After heavy rain, water infiltrates through the ground and is held in the soil pores. In forests, infiltration through the soil can be as much as 67 times higher than on grazed land. When it rains, water also lands on leaves in the tree canopy, where some of it evaporates. This reduces the amount of rainfall reaching the ground and flowing over the surface and also helps to keep the soil beneath the trees drier, leaving more space for water storage in its pores. Woodland creation is a long-term approach to flood risk management. As the effects of the trees on water take-up increase as trees grow, it takes around 15-20 years for their benefits for flood risk management to become established. However, after this time, these benefits can be permanent. Research by the Environment Agency has found that planting woodland in a river catchment can reduce the peak flow of water, the highest rate at which it flows, by between 5% and 65%. Planting trees on a river’s floodplain could delay the arrival of a flood peak by as much as 2 hours, giving you extra time to move your valuable possessions to safety.
To find out more about the role of trees in managing flood risk, visit DEFRA’s website:
Leaky woody debris dams consist of loose frameworks of logs or untreated boards that are placed into small rivers or streams to reduce the rate that water flows downstream. Leaky dams work by restricting but not completely preventing the flow of water in a channel. Whilst a limited volume of water is able to pass through the small gaps between the woody debris, most backs up behind the structure, which reduces the flow speed of the river and the volume of water flowing downstream. This can help to reduce the risk of flooding in downstream communities. Leaky dams can therefore bring great benefits to downstream communities, but only if they are used well.
As simple as building a leaky dam might sound, creating a successful system of leaky dams is more complex than blocking rivers or streams wherever the opportunity arises. The location of these blockages needs to be planned very carefully. As water backs up behind the dams, reducing the speed and volume of water flowing downstream, the depth of water upstream of the dams increases. This can cause the water to spill out of the channel and onto the floodplain upstream. Reconnecting rivers to their floodplains in a strategic way brings great benefits for floodwater storage when planned carefully upstream of residential areas. However, if the locations of leaky dams are not properly planned, this can cause unintended flooding and potential damage. As leaky dams encourage rivers to flow out of their banks, they are ideal in the upper catchment to channel water into woodland, scrub or rough grassland, where it would be stored safely away from downstream settlements. Blocking streams in residential areas could instead encourage the river to flow out of its banks into surrounding homes. These areas would benefit more from leaky dams installed upstream. As well as thinking about the effect of a leaky dam on its surroundings, it’s also important to consider how it could affect other leaky dams in the area. If leaky dams are too close together, the water trapped behind one could submerge and reduce the effectiveness of the one behind it. Leaky dams can also be washed away by deep and rapid flows in larger, wider channels, which could worsen damage caused by flooding if large, heavy debris was to be swept towards buildings or block the channel further downstream. Narrower, shallower channels are therefore an ideal location for leaky dams.
Find out more about leaky dams in this information sheet from the Flood Hub: https://thefloodhub.co.uk/wp-content/uploads/2019/03/Leaky-Woody-Dams-Natural-Flood-Management.pdf
If you would like to know more about the different types of leaky woody dams, how they work and key considerations, check out this information guide from the Woodland Trust: https://www.woodlandtrust.org.uk/media/1764/natural-flood-management-guidance.pdf
or the ADEPT Assessing the Risk guidance at:
As NFM measures have been increasingly publicised in recent years, this may seem like a fairly new approach, so how do we know if it works? With flood defences most commonly being viewed as the large-scale infrastructure projects we might see driving through city centres or in the media, are these natural methods as effective?
It’s important to remember that flood risk management isn’t a case of one approach or the other. If we want to manage flood risk sustainably, we need to look at the bigger picture, involving a range of methods that work together to reduce flood risk. NFM works best within a catchment-based approach, where natural methods are used upstream to complement other defences in more populated areas. NFM is thought to help most with reducing the risk of the smaller, more frequent flood events which might happen every few years or decades. In Pickering, after the set of NFM measures were installed, a flood that had a 25% chance of happening each year is now only 4% likely to occur annually. NFM also works with the larger-scale engineered defences downstream to reduce the threat posed by the larger exceptional events.
Using NFM alongside other defences elsewhere in the catchment could help to reduce the required height of flood walls or embankments or increase the lifespan of existing defences. Using natural methods to reinforce existing flood defences can therefore help to reduce our reliance on engineered measures, making catchments more resilient and helping them to prepare for a future of increased flooding. At the Beam Washlands in Dagenham, London, ponds, reed beds and wet woodland were created to store floodwater and streams and rivers were re-aligned and re-profiled. This created 25,660m3 of floodwater storage in the area, which provided a 4% (or 1 in 25 year) standard of protection to downstream properties. This also complements a pumping station in the area. Together, the NFM and the pumping station reduce the flood risk to 570 homes and 90 businesses and provide benefits worth an average of £591,000 per year in avoided flood damage costs. Using such combinations of flood risk management methods that support and reinforce each other is likely to become increasingly important in the future as climate change and its effects on flood risk may leave our existing defences unable to provide the level of protection we would require in future decades.
If you would like to find out more about different NFM approaches and evidence for their effectiveness, see this collection of brief reviews by the Environment Agency: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/654440/Working_with_natural_processes_one_page_summaries.pdf
To learn more about NFM as part of a catchment-based approach, visit the Catchment-Based Approach website: https://catchmentbasedapproach.org/learn/what-is-natural-flood-management/