On the first Wednesday of July we ran the first of four RRC Members Site Visits for 2016, to explore the Defra Multi-Objective Flood Management Demonstration Project: "From Source to Sea: the Holnicote Experience". This project was announced in April as one of four 2016 UK River Prize finalists.
The National Trust’s Holnicote Estate in West Somerset had 33 visitors for the day, including members from several wildlife and rivers trusts, consultancies, Natural England, Natural Resources Wales and others. Unusually, almost all RRC Staff and Board members also attended, as the day coincided with the RRC AGM.
The project involved over 3000(!!) small natural flood management (NFM) interventions across two river catchments, and we were lucky to have a tour of a few of these in both uplands and floodplain areas, by the National Trust Project Manager, a tenant farmer managing the land and several consultants and contractors involved in the work. This was a great opportunity to learn about all elements of the project and ongoing management and monitoring of the site. It was the first catchment-scale NFM project I’d seen close up, and there were a few things that I found particularly interesting, which I thought I’d share here.
Much of the upper Horner Water catchment is peatland punctured by steep gullies, many of which were actively eroding as rainwater was channelled down grips very quickly. We viewed several small peat dams and blocks constructed through the project which aimed to slow the flow near the top of these gullies. A secondary objective of these structures was to provide habitat to better support the characteristic peatland flora and fauna. Since the project, many of the nicks that were eroding have revegetated and settled.
Aside from the Horner Water and its short tributaries, the fastest route for water to flow through this catchment is actually the roads and tracks. Being smooth and mostly straight, they offer low resistance compared with the vegetated ground surface, and allow substantial downhill flows to accumulate. Water then spills over at bends or bottlenecks, causing severe erosion in these spots. Recognising this, the project representatives designed lots of small gutter-style interventions along the tracks to channel smaller volumes of water off the side of tracks back into the scrub, where it will move more slowly down the catchment. I hadn’t thought of this type of work as an option for natural flood management before, but it’s easy to see how they would work when you’re there.
In the floodplain, the project representatives worked with tenant farmers to design a series of low bunds to keep floodwater on fields rather than letting it flow overland to flood the villages near the mouth of the river. The farmer explained to us how he lets the water sit for a few days before slowly opening drains to let the water seep out of the fields.
It is clear when walking across this site that a catchment-scale programme of targeted interventions to slow water is an excellent way to reduce the downstream flood peak. However, one challenge with implementing such a project on a catchment scale is that it’s very difficult to demonstrate the effect of your work – at monitoring sites downstream, any improvement you have made is dampened by all the other areas in the catchment that you haven’t yet improved. However, despite this there has still been evidence that the scheme prevented downstream inundation after a 1-in-50 year storm in 2013.
Big thanks go to all those who stepped up to help out on the day. You have good reason to be proud to show off the project.
Being still fairly new to the UK river management community, I really enjoy opportunities like these to meet new people, learn about the different organisations and continue to build relationships. Also, I think the Somerset accent might be my new favourite.
Slides from the presentation by National Trust's Project Manager, Nigel Hester, are available here.
Registrations are open for our site visits to the other three UK River Prize finalists around the country (including the River Prize winner), in August, September and November. Click here for more information and to register.