A new sustainable approach to flood risk management which utilises land management is set for much wider use as a natural flood management solution, as outlined in Defra’s recently published 25 Year Environment Plan. This catchment-wide approach aims to tackle the causes of flooding, by working with land management and use of natural features to store and slow down water that can lead to flooding downstream. Natural Flood Management (NFM) covers a spectrum of techniques from full-scale restoration of the course of a river to smaller scale measures that make use of natural barriers and storage ponds. In addition to flood risk reduction, these techniques also often contribute to improvements in biodiversity, water quality, and carbon storage. Implementing NFM in a farming landscape is a challenge for both policy makers and farm businesses, who need to consider business risk, capital costs of installation and maintenance costs. There is clearly a challenge to engage farmers who will naturally question what impacts and benefits NFM will arise for their business. Recent EA funded projects in Cheshire have allowed us to examine the costs, implementation challenges and management of small-scale NFM measures, and to evaluate how they work on commercial farms.
Case Study: Field Wetland on Rookery Brook
Field wetlands are small constructed ponds usually located adjacent to the main watercourse in unproductive areas that are typically difficult to farm, usually being waterlogged. They have multiple benefits of not only improving water quality by trapping sediments but also providing additional riparian habitat and helping to reduce downstream flood risk. Recent research on farms in Leicestershire and Cumbria has shown that field wetlands effectively trap up to 26 tonnes of sediment annually. The nutrients transported with the sediment were retained in the wetlands; water travelling through field wetlands typically has a 60% reduction in phosphorus and 35% reduction in nitrogen, which is deposited in the silts, thus filtering out nutrients that were lost from the field.
An area of land that regularly floods and is difficult to crop was identified adjacent to the Rookery Brook. The area lies at the base of a steep slope so has the additional benefit of collecting any surface runoff from the slope as well as flood water after heavy rainfall. Consent for the pond was required from the Environment Agency as the works were adjacent to the main river, but after a check for any resident water voles, and a plan for monitoring fish in the pond, consent was granted and a contractor began the works.
The wetland was designed with a new in-flow channel coming off the main brook to divert water at times of high flow following heavy rainfall. During these ‘flashy’ rainfall episodes excess water in the brook flows into the wetland, allowing sediment and attached nutrients to settle out. An island built into the design helps to slow the flow further and provides additional habitat. An outlet channel with a leaky barrier at the other end of the pond allows water to flow back into the main river whilst retaining sediment behind it. Reeds were planted in the channel and on the pond margins to stabilise the soil and further slow flows into the pond.
The outcome of creating this type of feature can be difficult to fully predict, however 12 months on from completion there is clear evidence of large sediment deposits in the pond which would otherwise have run directly into the River Weaver. The pond will require clearing out every couple of years and the sediment will be spread back onto the farmland returning some of the lost nutrients to the soil.
Cost: approximately £7,500 in survey, design, permit, earthworks and marginal planting
Area and economic impact: 1.2ha, cropped area lost < 0.25ha although yield was typically low to zero in this field corner due to the waterlogged conditions over winter. The land was already largely non-productive area, so economic impact was zero taking account of small saving from not ploughing and establishing seed on a failing field area. What was learned? Disruption to farming activity was relatively small although access along the field margin was required in early Spring due to the site being inaccessible. Ideally this installation would occur in late Summer or Autumn to minimise impact on the adjacent crop. Communication with the contractor was key, some adjustments had to be made to ensure the channels and pond functioned effectively.
Farm opinion: the wetland is working well in trapping sediment from fields upstream, it is surprising to see how much has settled out. There will be a cost in clearing it out at some point, but we can work that in when time and ground conditions allow. Overall I’d be happy to see more of these going in around the farm, if there’s funding and support for them to go in, but it has made me think about holding on to soil in the field as well. We’ve been experimenting with some cover crops and I can see us doing more of this on the maize ground (Reaseheath College Farm Manager).