Part 2: Nature-based Solutions
The challenges outlined above are complex and inter-linked. They have been growing in complexity and impact over many years and the activities that generate them have become embedded in many people’s way of life. For almost as many years we have, as a society, been trying to tackle these impacts, often by building higher flood defences, for example, or adding air conditioning to our homes. However, these approaches can be both expensive and inflexible. Our research shows that there is a range of more natural approaches that can help us adapt to climate change in the Culm catchment at the same time as making our landscape richer in wildlife and better for people.
Working with natural processes and “working with the grain of nature” is an approach that is now accepted as an essential part of our response to the climate and ecological emergencies. Nature-based solutions cover a wide range of options and you can find out more in the Environment Agency’s Evidence Base which details 14 different measures (see Fig 11) and provides 65 case studies. You can also read a quick summary elsewhere on this website: https://connectingtheculm.com/what-are-nature-based-solutions/
The following overview summarises our findings from the last year of research. This includes the development of a hydrological model for the Culm catchment that allows us to input various scenarios for both climate change and adaptive measures that we might take. The text below describes the outputs from this model as a way of assessing the cost-benefit of various options.
Connecting the Culm is not just about working with nature to adapt to flood and drought. We are also focused on maximising the many other benefits of nature-based solutions for the area. These include improved water quality and aquatic ecology; more, better quality, connected and larger habitats for wildlife; more carbon sequestration in the soil and in trees; more accessible nature-rich areas for people to enjoy; and a more resilient food supply because our pollinators can flourish.
2.1 Soil management
As set out above, how we manage the soils of the catchment is the primary solution for restoring the natural function of the catchment and reducing the risks of flooding and drought.
Well-structured soils are a classic ‘win-win’ scenario for the environment and farmer. Landowners will see improved yields and increased grazing time at the beginning and end of the year. Local watercourses will see reduced peak flows and more consistent flows through prolonged dry spells, and there will be less risk of soil erosion and pollution, all of which will benefit river ecology. Every field, or part field, that has an improved soil structure will have a positive influence on water flow. At an individual level this may be minimal, but collectively at a catchment scale this can lead to significant reductions in volumes of rainwater reaching the watercourses shortly after rainfall, lowering the risk of flooding and making the landscape more resilient.
The priority area for intervention in the catchment is the farmland of the clay plateau where intensification has resulted in compacted soils across much of the area. Here farming needs to be extremely sensitive to soil compaction and, where possible, heathland and woodland restoration would be hugely advantageous.
In the lower catchment, on the Triassic mudstones and Permian sandstones, we need to target areas where soil loss is occurring and take opportunities to enhance soil permeability wherever possible, by reducing the risk of erosion, and capping or compaction.
Maize is generally a high-risk crop for the whole catchment and due to the economic conditions it is currently grown in many places that are relatively unsuitable and harvested late, during October days that are typically very wet in the west country – which should be avoided (see Fig 12). The likelihood of needing to harvest during wet weather, making soils liable to compaction and therefore leaving them in an unsuitable condition over the winter months is simply too great.
We have modelled the impact of improving soil health and infiltration in the catchment and this shows that this intervention is the most significant for reducing downstream flood risk. It also could provide other benefits including resilience to drought; plus new and enhanced habitat, including woodland, and on the clay plateau species-rich meadows and heathland.
2.2 Runoff attenuation and floodplain reconnection
As noted above in section 2.5, the disconnection of the river from the floodplain is one of the un-natural processes at work and a good nature-based solution is to reconnect the river with its floodplain. This can be done by creating high-flow pathways onto fields that have been selected for this purpose, using bunds or the natural topography to temporarily store water (Fig 13).
Another technique is to use tree-planting in areas that are already prone to flooding. When these areas flood, the trees help slow the flow, which then creates a backwater effect, whereby upstream flow depths increase and can be used to promote pathways to temporary storage in the landscape.
A further technique is to identify where there are well-draining soils and create pathways for floodwater onto these soils, where the water can readily drain away.
In the river valley bottoms historic networks of water channels could be restored to create a more diverse wetland landscape. Flooding here is a natural event and could be allowed to happen where it does not threaten property.
Runoff can also be held back before it reaches watercourses, helping to slow the flow of water down the catchment. This can be achieved by introducing bunds or barriers which intercept flow pathways in the landscape, causing water to infiltrate through the soil or be released more slowly.
Our modelling of the impact of these features to reconnect the river to the floodplain and attenuate runoff in the catchment shows that, if fully implemented in suitable locations across the catchment, they could help to reduce peak flows by 5% in severe flood events like the one experienced in November 2012.
2.3 Tree-planting and hedges
Trees have multiple benefits in the landscape and have a specific role to play in natural flood management. Soils within woodlands are up to 67 times more absorbent to water than a compacted field surface, because of the dense root systems, rich humus layer and the lack of overall disturbance. Woodlands act like a giant sponge, holding water back in flood conditions and helping replenish groundwaters to supply the river during droughts.
Tree-planting alongside rivers and streams is particularly beneficial because the belt of trees can help buffer any run-off from adjoining fields, filtering out soil and other pollutants. The trees can also physically slow the river down when it overtops its banks, again reducing flood risk downstream.
Tree planting away from rivers and streams is also beneficial for improving infiltration of water into the ground, as well as improving the water quality of downstream water courses, and the whole of the Culm catchment is designated as a Water Quality priority zone for tree planting.
Our modelling of riparian tree-planting over 125 ha of the catchment shows that this measure is the least effective of the three scenarios modelled at reducing flood risk (approximately 1% reduction in peak flow in a flood equivalent to November 2012). However, it does offer potential for localised flood risk reduction and multiple other benefits, including an excellent habitat corridor for wildlife adjacent to the watercourse, and reducing pollution impacts on water quality by filtering out pollutants before they reach the watercourse.
More details in this article: https://connectingtheculm.com/using-trees-to-slow-down-water/
Beavers have attracted a huge amount of attention as natural flood management engineers. They build dams to create pools that hold back water in flood events, which is then released slowly to replenish the rivers below.
The first officially permitted colony of wild beavers in England are now resident on the River Otter, our neighbouring catchment. We fully expect the beavers to spread from this river into the Culm at some point in the coming years. The River Otter Beaver Trial, led by Devon Wildlife Trust with the University of Exeter, has researched many aspects of the impact of beavers and how they can co-exist with landowners. Their conclusion is that beavers have a huge potential to provide a range of catchment-wide benefits for the environment, including to natural flood management, biodiversity, fisheries and water quality. However, there can also be negative impacts in some instances and these need managing.
Further modelling and work to build understanding of how beavers could operate in the landscape of the Culm would be needed to explore this further as it has not been undertaken to date. Any future activities will be dependent on the national situation of how beavers are managed in England, and Defra are due to consult on this in 2021.