Large Woody Debris (LWD), or leaky dams is one of the most utilised Natural Flood Management technique. In the right locations they can provide a cheap and easy way to reducing flood risk, particularly when applied at a catchment scale. The range of benefits these structures provide, is only matched by the diversity in their style and construction method. Ranging from naturalistic dams utilising materials from the site in which they sit, to heavily engineered structures made from fabricated materials. They all aim to reduce flood risk by intercepting the flow of water in watercourse and helping to restore river-floodplain connectivity, thereby reducing flood peaks, slowing water velocities and attenuating flow by storing water on the floodplain.
At the Sussex Flow Initiative (SFI), we are interested in all of the benefits that leaky dams provide, but we are particularly excited about their potential for making our landscapes more resilient to the extreme and unpredictable events of climate change. The benefits wheels below (taken from the Environment Agency’s Working with Natural Processes evidence base) helps to illustrate just how many benefits leaky dams can have across a whole range of ecosystem services.
SFI aims to create naturalistic structures that blend into the landscape, utilising local material where possible. There are a variety of types of the LWD, the three main types used by the project are outline below. Further information on these can be found in 'SFI's Leaky Dam Guidance Document'.
Types of LWD
Large woody debris is positioned and fixed across the banktop in streams and ditches, engaging high flows, holding back water and encourage it out onto the floodplain.
In addition to a banktop diverter, woody material is added into and secured within the channel. This therefore means the structures are active earlier, at lower flow rates. The leaky construction maintains the watercourses base flow and ensure there is still fish passage.
Woody materials is positioned and secured longitudinally within the channel to slow the flow of water and trap sediment. This type of LWD is typically used within relic drainage ditches within heathlands and woodlands.
These naturalistic structures emulate the natural process of wind blown trees or the dams that would have been created by beavers before they were lost from our landscape 400 years ago. The images below show how these structures can could just be a collection of sticks to fall length trees secured in place, but they are all:
SFI is delighted to have been awarded a Silver award at the Countryside Protection for Rural England (CPRE) Sussex Countryside Awards 2020. The award recognises the partnership projects work with landowners and local people to reduce flood risk within the River Ouse catchment through working with and resolving natural processes.
Congratulations to all the amazing projects and individuals who won awards - more details can be found by click here. Thank you to our project partners, landowners, local communities and volunteers for their support, passion and hard work in protecting and enhancing Sussex.
The Sussex Flow Initiative has been running since 2012, working with and restoring natural processes to reduce flood risk. Each year we produce an annual report on what the project has delivered. These reports focus primarily on the amount of habitat created and restored, as well as the amount of water stored through the variety of natural flood management measures.
Natural flood management measures undertaken by SFI not only help to reduce flood risk and increase drought resilience of our landscape, but they also provide a whole range of other ecosystem services. An example of the ecosystem services delivered by SFI is the regulating service of pollination, through the planting of our native flowering trees/shrubs to slow the flow and connect habitats. Ecosystem services are flows of benefits, which could be compared to the money payed into your bank account, creating a stock 'your bank balance'. Natural capital is the stocks generated by through the flows of these ecosystem services.
Storage ponds created to intercept land drains within the upper catchment of the Ouse.
What is social and natural capital?
Social capital is the benefits delivered to human health and wellbeing, and natural capital is the additional benefits delivered to nature and wider society, such as carbon storage for climate regulation, pollination for food production, and water purification.
Working with New Economics Foundation, we attempted to document the true societal value of the partnership project work delivered by the project.
Social Capital Benefits of Sussex Flow Initiative
Using bespoke surveys and spreadsheets, we found some really interesting insights into the true societal value of this project, here are some of the findings:
Volunteers creating leaky dams and hedgerow planting to slow the flow. © SFI
Natural Capital Benefits of Sussex Flow Initiative
The UK government’s Ecosystem Services Databook was used to help calculate the wider natural capital benefits that the project has brought to society. In the eight years of the project, it has generated an incredible £1 million of public and natural capital benefits. This equates to a whooping £125,000 of benefits for every year of the project since 2012.
These figures are based on seven key ecosystem services; timber provision, air pollution removal, carbon sequestration, flood regulation, water quality improvements, biodiversity and volunteering. Due to insufficient information/data it was not possible to calculate all the multiple societal benefits of all the work carried out by the project. Therefore, the true value will be greater than the £1 million. A significant portion of the work undertaken by the project delivers benefits that are incremental and will accrue greater value each year, for example the carbon storage will accrue over the lifetime of trees planted.
Having this evidence is an incredibly powerful tool, and a fantastic step to demonstrate real and wider value of the nature-based solution work of SFI.
What a year it has been!! The 5th wettest winter, the wettest February of records, and five named storms. As a result, the impact of our work was seen almost instantly.
A few highlights from 2019/20:
The above is just a selection of the work undertaken in 2019/20, the full annual report and the summary report go into great detail on what has been achieved and its impact.
Thank you to all our landowners, volunteers and partner organisation, without them we wouldn't be able to have delivered what we have.
Hedgerows do much more than just mark field boundaries. Some of the benefits they provide are easy to see – they provide shelter for livestock, food for wildlife, carbon storage, corridors for species to move between habitats, and are an important habitat in their own right. What is less obvious is the important role they play in the wider landscape, and in particular how they help to improve the quality and quantity of water in our rivers and streams.
Image: Mature hedgerow © SFI
By acting as a physical barrier to overland water flows during heavy rainstorms, and with their roots helping to increase water infiltration rates in the soil adjacent and under them, hedgerows help to slow and reduce the amount of water that reaches our rivers, contributing to flood risk reduction downstream. In the process of slowing overland flow, hedges also help to reduce the erosion of soils, and the delivery of sediment and contaminants to our rivers and streams.
During World War II, food shortages lead the government to incentivise hedgerow removal to increase food production. This practice continued for several decades due to agricultural intensification; with the size of farm machinery increasing, hedgerows were grubbed out to create larger fields to accommodate these machines and maximise productivity. This intensification has also lead to more soil compaction from large machinery and high densities of livestock. The result is that hundreds of km of hedgerow, in Sussex alone, are now missing from the landscape and our fields are more susceptible to surface run-off and erosion. Thankfully, existing hedgerows now have more legal protection and many farmers see the benefits of planting new ones to help protect valuable soil and water resources, and to provide forage and shelter for livestock in an unpredictable climate with more extreme drought and storm events. In the Ouse catchment, with the help of funders and volunteers we’re supporting landowners to plant hedges in the right places.
Image: Hedge planting © SFI
In the short term, we can’t replicate the structural and biological diversity of the missing ancient hedgerows, but by utilising historic maps, where possible we can plant new hedgerows in the same locations, and perhaps an ancient seed bank will germinate when conditions become favourable. We can also identify fragmented habitat that would benefit from being connected by hedgerows, encouraging wildlife to move throughout the landscape. For the Sussex Flow Initiative, we support the planting of hedges, and we can provide maximum funding when they are located in key areas for Natural Flood Management – specifically when the hedgerow will be positioned across a hill slope or on a floodplain.
Although we know that hedgerows interact with water and can help to slow overland flow, exactly how much, which tree/shrub species are most effective, and at what scale hedgerows need to be restored to begin to reduce flood risk downstream, is still uncertain. What research has shown, is that infiltration rates can be up to 60 times greater in fenced woodlands/shelterbelts1, compared to adjacent pasture, but we don’t know how transferable this is to hedgerows.
Research is now underway at Bangor University, where PhD student Bid Webb is investigating how trees and hedgerows influence infiltration rates compared to adjacent pasture. They are also investigating whether tree age and species play a role. So far, the group’s results suggest that Fraxinus excelsior (Common Ash) has the greatest potential (of the seven species being studies; Alder, Ash, Beech, Birch, Chestnut, Oak and Sycamore) to increase soil infiltration, due to it having the highest fine root biomass, with over 50% of this biomass in the top 10 cm of soil, and the greatest proportion of large pore sizes in the soil. The large pores enable water to quickly infiltrate into the soil, reducing overland flows. The findings suggest that loss of Common Ash from the landscape due to the spread of Ash dieback (a fungus called Hymenoscyphus fraxineus), may have implications for our landscapes’ flood resilience.
Image: Ash tree © Tim Haynes
We’re eagerly awaiting the results from the group at Bangor University, and will use the findings to inform our NFM delivery. We will also be encouraging and supporting universities in the South East to carry out similar research in the lowlands, on the local soils, and focusing on the root morphology of species commonly found in our local hedgerows.
1. Carroll, Z.L., Bird, S.B., Emmett, B.A., Reynolds, B. and Sinclair, F.L. (2004). Can tree shelterbelts on agricultural land reduce flood risk? Soil Use and Management, 20, 357-359.
On Thursday 6th December 18 enthusiastic volunteers gathered in Fore Wood to start work on Natural Flood Management (NFM) as part of the Sussex Flow Initiative (SFI) project in the Powdermill Catchment. This semi‐natural ancient, 52-acre woodland has a SSSI designation and falls within the Area of Outstanding Natural Beauty of the High Weald. The site is situated on the edge of Crowhurst village, 1.5 miles south of Battle and is owned and managed by RSPB.
The Powdermill catchment is naturally steep and fast flowing. Localized flooding to properties in Crowhurst has occurred regularly in recent years. During times of high rainfall, too much water flows down the river too quickly, and creates flood surges which cause risks to people and property. SFI is working in this catchment to slow down this flow of water.
Fore Wood’s steep ghyll woodland streams and sandstone ravines, where rare ferns and bryophytes grow, offer a great opportunity to create natural leaky dams that slow the flow of water to hold it back in times of high flood. This also helps improve the local hydrology to create a wetter woodland, and mimics the natural process of wood falling into the ghyll. By creating these woody features, we aim to slow the flow of water through the ghylls, diverting flood flow paths into minor washland flood plains. This increases the humidity and enhances the unique ghyll flora of the streams and biodiversity of the woodland.
We are very grateful to SFI and RSPB staff, interns and volunteers who worked throughout the day to construct over 20 woody debris features in the steep ghyll streams, and we will visit again on the 7th February 2019 to install another 25 natural leaky dam features.
The dams are made to look as natural as possible and include using techniques such as whole tree head felling, gulley stuffing and ditch top diverters. These impermanent woody features are designed to be active only in times of heavy rain or peak flow, but we hope they will significantly help to reducing flood peaks in Crowhurst village.
If you would like to get involved as a volunteer for the SFI or have any questions about our work in this catchment please email Rina Quinlan, Project Officer, Powdermill Catchment firstname.lastname@example.org.
Natural Flood Management (NFM) works with nature to help hold back water and slow the flow of river catchments throughout the UK. This can help to reduce downstream flooding, increase biodiversity and prevent drought at a relatively low cost. This approach is rising in popularity among NGO’s, governmental organisations and landowners throughout the UK as a method in dealing with localised flooding, whilst improving wildlife habitats and providing multiple other benefits to society.
The majority of NFM work is focused on building partnerships where rural landowners help to mitigate against flood run off from rural and urban areas, including their own land. There are many different land uses that can contribute to localised flooding, but some, such as horse grazing are rarely considered in terms of what they can contribute to reducing flooding.
With an estimated 944 000 horses kept in the UK (BETA, 2018), the British countryside is dotted with equestrian facilities both large and small. Horses are continuous grazers and need a constant supply of forage throughout the year. This can lead to horse pasture often being overgrazed and threadbare and can ultimately lead to muddy gates, soil erosion and nutrient run off via drains and surface run off channelling towards the nearest river tributary.
Good pasture management can help horses stay fit and healthy and can reduce their parasite burdens, and usually means rotating paddocks to limit the use of flood prone fields during times of heavy rain. Systems such as the Equicentral System encourage horse owners to become “grass farmers” promoting a more biodiverse grass and herb mix which has multiple benefits for horses and wildlife. Horses benefit from a more nutritious diet whilst higher level vegetation and wildflowers will encourage a greater number of invertebrates, which in turn attracts birds and bats that also help keep down numbers of problem horse flies. In addition, deeper plant roots absorb more water and nutrients into the soil which helps to prevent run off and erosion.
Planting trees and hedgerows on equestrian land can also create multiple benefits for the horse by helping to absorb water to prevent muddy paddocks, creating shade and supplying natural forage and nutrients missing from their regular diet. When available horses will actively nibble on bark, berries and leaves from hedgerows, reflecting their role as both browsers and grazers in a natural ecosystem.
Our Natural Flood Management project, Sussex Flow Initiative (SFI), carries out a huge range of Natural Flood Management work across Sussex each year. Recently, we facilitated the planting of hedgerows, shaws and woodland comprising over 12,050 trees across a 39 hectare site at the Sussex Horse Rescue Trust (SHRT) near Uckfield with a further 8,000 additional trees planned. This project was designed to create shade and shelter for the rescued horses, as well providing extra forage and enhancing local pollinator and ecological networks. SFI have also worked with other landowners to create woody debris features in water channels which help slow the flow upstream to reduce areas prone to flooding.
Managing your land to help prevent downstream flooding, can ultimately also help enhance your paddocks for equestrian use. For more information and free advice on how to make your horse pasture more water friendly please do not hesitate to contact us.
British Equestrian Trade Association (BETA) (2018) Market information, National Equestrian Survey 2015 [Online]. Available at www.beta-uk.org/pages/industry-information/market-information.php (Accessed 13 December 2018).
Since 2012, the Sussex Flow Initiative (SFI) project has been using Natural Flood Management (NFM) to help local communities to reduce flooding. What most people don’t realise is that our work to store and slow down flood water in the landscape also helps to improve water quality.
Did you know that one of the most widespread forms of freshwater pollution is particles of sand, silt and clay, known as ‘fine sediment’? Although it’s a natural and important component of freshwater ecosystems, human activities (e.g. agriculture, forestry, urban areas and wastewater treatment plants) often lead to elevated levels of fine sediment in surface waters, impacting the physical, chemical and biological characteristics of these ecosystems.
Elevated fine sediment levels can have direct detrimental effects on aquatic organisms, by burying or dislodging them; by clogging gills or feeding appendages; and indirectly by introducing contaminants attached to the fine sediment particles (either adsorbed to the surface of particles or absorbed in them). It can also limit light penetration, reduce oxygen concentrations in water and substrate, and more. The type of contaminants attached to fine sediment varies depending on the activities upstream, but in catchments which are primarily agricultural, the most abundant contaminants are likely to be nutrients (i.e. nitrogen and phosphorous compounds). In water, these nutrients can lead to eutrophication and oxygen depletion, and can even be directly toxic to aquatic organisms. However, on land and in wetlands these nutrients can be locked up in soils, or more rapidly transformed and removed/utilised by plants and microbes.
These are all good reasons for reducing soil erosion and fine sediment delivery by embracing more sustainable land management practices. Soils are some of our most precious natural resources, sometimes taking hundreds of years to form, and yet we have lost unprecedented levels of them in recent decades. In addition to tackling the sources of the problem, we also need to look at restoring the natural processes that would normally be helping to regulate and remove sediment once it’s in our watercourses.
This is where Natural Flood Management can help, and why it’s considered a more holistic approach compared to more engineered flood risk management. NFM includes a range of techniques that work with natural processes to slow and temporarily store water in the landscape. This is good news for water quality, because by slowing water velocity, the ability of sediment to remain suspended in water is reduced, leading to it becoming deposited along with any contaminants that are attached to the sediment.
For centuries human civilisations have exploited river floodplains and wetlands for agricultural production due to the naturally nutrient rich soils, which are the result of sediment and nutrient deposition from flood waters. These floodplains and wetlands are vital for healthy rivers and streams, as they can act as vast stores for large proportions of a watercourses sediment and nutrient load, as well as carbon sinks. However, many wetlands have been drained, flood embankments built to reduce localised flooding, or levees accumulated over time from dredging spoil, resulting in many rivers being cut off from their floodplains, and the movement of water through the catchment being sped up.
By carrying out NFM we can lower embankments and levees and partially block drainage ditches, with the aim of reconnecting rivers to their floodplains and restoring wetlands. In doing so, not only do we open up large areas for flood water storage, we allow the important water purification processes to occur.
Woodlands can also be important areas for water purification, but these too have often been ditched to rapidly transport water from the land, reduce the height of the water table and maximise accessibility. This combined with historic watercourse management practices which included clearing woody material from streams and ditches, reduces the ability of woodlands and its spongy, organic and microbe rich soils to store and breakdown pollutants in water. The good news is that NFM can again help to restore these processes, by using woody debris dams that slow the flow, and encourage water out of stream/ditch channels during flood events.
Many watercourses have been significantly modified (e.g. channelised) to the point where instream habitat and sediment deposition are relatively uniform. More natural streams have a sequence of erosional and depositional areas, providing greater habitat diversity. By reintroducing woody material to these streams we can encourage greater hydraulic diversity and the return of these natural processes, allowing fine sediment to be deposited and stored in some areas (rather than along the entire length), but also creating areas which are scoured of sediment and can support organisms which are less tolerant of this type of substrate.
All of this means that by using NFM to reduce flooding in towns and villages, we can also be reducing the impacts of water pollution on aquatic organisms and the effort needed to clean our drinking water (saving money and energy!). Nevertheless, despite the amazing ability of the natural world to purify water, there is an upper limit to its capacity to do so, and whilst we should be working with, and attempting to restore the natural processes that regulate water purification, this doesn’t reduce societies responsibility for minimising the inputs to the environment. NFM is just one of the useful tools that can help us begin to reduce water pollution, but to protect this important resource we need to do much more!
Droughts are natural events, and there are lots of streams or sections of stream that we would expect to naturally flow intermittently due to climate, weather patterns and fluctuations in underground stores of water. Such streams are inhabited by organisms with adaptations that allow them to subsist in these habitats. For example, many aquatic invertebrates lay eggs that are covered in a gelatinous membrane, preventing the embryos from drying out, ensuring that they can survive without water during dry spells. Other aquatic animals actively seek refuge during droughts, burrowing in wet muds or sheltering in damp areas.
However, not all aquatic organisms are well-adapted to cope without water, which is one reason why droughts, particularly those brought on by, or exacerbated by human activities, and in streams for which drying up is uncharacteristic, can be problematic for wildlife (not to mention us). In recent weeks you may have seen images of the Environment Agency working hard to rescue fish left without enough water to survive. In the UK, our native fish have few adaptations to these conditions, other than the ability to move and seek pools for refuge. In other more arid countries, some fish have evolved adaptations that allow them to travel over land to find suitable habitat, or burrow into mud and produce a mucus cocoon.
There are lots of human activities that increase the likelihood of rivers and streams drying up or experiencing low flows. One of the most significant is our conventional approaches to land and water management. Historically, land management has considered water somewhat of a nuisance, and every effort was made to encourage water off of the land and out to sea as quickly as possible. After removing trees and hedgerows, digging ditches, installing land drains under fields, constructing river embankments, and dredging, channelising and clearing debris from our rivers, water now travels quickly downstream.
In most cases, Natural Flood Management is focused on reversing these activities and restoring the ability of the land to slow and store water. In doing so, water is once again allowed to infiltrate into soils and slowly drain into surface waters, or percolate deeper into soils and replenish groundwater stores, resulting in a more steady supply of water to rivers and streams. By reintroducing woody debris to streams, not only does it help to ‘slow the flow’ but it also encourages a more diverse flow pattern (which is often lacking due to channelisation and lack of debris) and influences geomorphological processes, including the formation of scour pools. These pools can be an important refuge for aquatic organisms during droughts, and if the stream does completely dry up, woody debris is likely to benefit burrowing invertebrates by providing heavily shaded areas where sediment remains moist for a longer period of time.
These benefits for wildlife were visible when we visited some of our woody debris dams earlier this week. It was easy to see the changes in streambed level, and the scour pools that have been formed since we installed the features. The stream was not flowing and had largely dried up, but there were numerous scour pools downstream of woody debris that were visibly sheltering a (crowded) community of aquatic invertebrates. It was great to see that our NFM work is bringing benefits year-round. The multiple benefits provided by NFM, are why we at the Sussex Flow Initiative are so passionate about this approach to reducing flood risk, by restoring and working with natural processes.
In April 2018 I started my role as a Natural Flood Management project officer for the Sussex Flow Initiative (SFI) in the Powdermill Stream Catchment in East Sussex. SFI has had some fantastic results with ‘slowing the flow’ of the River Ouse catchment since 2014 and as a result, the partnership has been extended to this new river catchment.
My job is to help alleviate the rising amount of localised flooding, by using natural measures such as hedgerow planting. In this role I advise on natural flood management (NFM) and work with natural processes (WWNP) to help landowners and communities make their land and homes more water resilient in times of heavy rainfall. More importantly at the moment, it also helps us to be more resilient to drought.
The Powdermill stream runs between historic Battle to the north and Crowhurst village to the South. It sits within the High Weald Area of Outstanding Natural Beauty (HW AONB) and its undulating hills, wild flower meadows, ancient and ghyll woodlands are familiar characteristics of this landscape. Water, however, flows rapidly down these steep valleys, causing streams to overflow and high surface water run-off from hard urban surfaces such as roads and houses.
Engineering solutions have tried and failed to improve the flooding situation for this catchment and historic drainage practices and canalising of rivers, as well as increasing intensity of land use for urban development and agriculture have exacerbated the scale and severity of flooding. However, recognition for the multiple benefits NFM and WWNP can provide to local communities and wildlife is gaining more and more traction throughout the UK amongst many stakeholders.
My role is relatively simple. By creating woody debris dams, tree and hedgerow planting, pond and washland creation, this helps reduce flood peaks and slow the flow of water before it reaches homes at risk. Crucially WWNP also makes our landscapes more resilient to drought by storing more water for longer on the land.
If that wasn’t enough, NFM also greatly improves wildlife habitat for a multitude of species, provides shade and water for grazing animals, mimics natural processes, stores and cleans water, and much more. As a passionate conservationist and advocate for ecosystem restoration, I take great delight in explaining to others that much of my role entails mimicking the things that a beaver - one of our natural ecosystem engineers – would do!! It’s a great job, and I’m looking forward to seeing what we can achieve.
Rina Quinlan can be contacted at email@example.com or 07595 452038