Sandra Manning-Jones

As the project officer for the Sussex Flow Initiative (SFI) I was very pleased to meet with members of TTL recently to discuss ways that Natural Flood Management (NFM) can help reduce flooding across the Ouse catchment and help with flood resilience for Lewes town. SFI is hosted by the Sussex Wildlife Trust in partnership with the Woodland Trust and the Environment Agency – a partnership that started in 2012 with the Trees on the River Uck (TrUck) pilot project.

NFM is an approach to flood mitigation that’s been researched, developed and implemented relatively recently and which works alongside man made flood defences (rather than replacing them), harnessing natural processes to slow and store more water upstream to reduce the peak of flooding downstream. Natural additions such as floodplain woodland, across slope hedgerows and shelter belts or leaky dams have all been shown to help to slow the flow of water, making the landscape better able to cope with heavy rainfall and reducing the power and magnitude of flooding. With over 50 different NFM techniques to choose from, these natural enhancements can be tailored to fit a given site, providing a range of further benefits for wildlife, water quality and river health.

Over the last 10 years or so there has been a change in approaches to river management, with greater recognition that the whole area that a river drains (or ‘catchment’) is a whole unit, and that work in one place may influence conditions in other places. This means that when using a Natural Flood Management approach we must look at the whole of the river catchment, and gain a detailed understanding of its unique characteristics.

The River Ouse catchment drains an area of over 600km2 of Sussex, emerging in headwater streams which span from Haywards Heath in the west to Crowborough in the east, and flowing through the High Weald and the South Downs National Park to its river mouth at Newhaven. Much of the upper Ouse is dominated by impermeable clay soils, which means that during extreme rainfall much of the water rapidly runs off the land, creating large and devastating flooding. Water is further sped up by an extensive network of ditches and sub surface drainage. The Ouse catchment has high woodland cover (at approx. 17% of the total area) relative to the national average (suggested to be approx. 13%). However when you look more closely at these woodland patches, there is much less cover on the floodplain. This is in common with the national picture where floodplains have been cleared and drained for in order to utilise their rich soils for farming. This highlights the potential for us to plant more woodland on the floodplain to help slow the flow of flood water.

NFM is not a panacea for flooding in Lewes, but is a significant component of flood resilience. It works better for smaller scale flooding, and requires a large number of interventions to make an impact. It does need good understanding and careful planning – in the wrong places some measures could increase flood risk, so careful placement of natural flood management actions is essential. Also some of the measures will take time to become effective, for example new woodland will take around 20 years before it will bring full benefits for flood mitigation. This means we need to start this work now!

Through Sussex Flow Initiative, and its predecessor project Trees on the River Uck, we have been working with landowners and locals across the Ouse catchment to add measures where possible to slow the flow, and promote NFM. We have also conducted a great deal of analysis, modelling and surveying to ensure that we can target measures to the best places and reduce risk. We are a voluntary scheme, which means that we need to ensure that our additions have benefits for landowners, and needs to fit within the current framework of rural payments, not all of which are well suited to our aims.

We are pleased to say that we have also just received funding from Lewes District Council to forward our work and influence across the Ouse, and continue our innovative work. Over the next few months, along with the Ouse and Adur Rivers Trust and others, we will be developing a range of projects that have benefits for flooding, river health and wildlife, providing invaluable trial sites for new measures and engaging with landowners and stakeholders across the Ouse. We will also be working with TTL and other local groups to help promote and adopt an NFM approach, providing guidance and support where possible. We know that it will take time for the benefits of our work to be fully felt, but by starting now we can guarantee a legacy for future generations.

Take a look at a map of the River Ouse catchment…
And a map of the River Ouse detailed river network…