Resetting the River Aller on Exmoor
A three-year floodplain reconnection project on the National Trust’s Holnicote Estate in the west of Somerset was successfully completed in July 2023.
Using the innovative ‘Stage 0’ river restoration technique, first pioneered in the US state of Oregon, the Trust created a new waterscape.
Main moves included:
- filling in a 1.2km section of the River Aller (that was managed, straightened, deepened) with over 4,000 tonnes of earth
- pinning or part-burying 700 tonnes of fallen Holnicote timber within the floodplain, to significantly slow flows of water downstream and help develop a variegated wetland (with more channels, pools and marsh) that is good for wildlife
- sowing 250kg of floodplain wildflower seeds
- planting 25,000 native trees such as willow, bird cherry and black poplar
The project was the first in the UK to attempt to reset a main river on such an ambitious scale. Part of the National Trust’s multi-million pound Riverlands initiative, funding also came from the EU’s Interreg 2Seas Co-Adapt programme, the Environment Agency, Somerset Rivers Authority (SRA), the Green Recovery Challenge Fund and Frugi. Text continues below images.
It followed on from a series of smaller natural flood management schemes locally, all part-funded by the SRA, and it has been further complemented by a recent 125-hectare extension of the River Aller project upstream at Tivington Farm, again part-funded by the SRA. The combined impact of all these works has been monitored since 2019 by researchers from Exeter University. At sites across the River Aller catchment, they used level sensors and flow meters to capture data every 15 minutes, so by the end of March 2024 they had 52,099 datapoints from a period which endured 87 rainstorms. They also gathered rainfall data from gauges and radar, and conducted regular drone surveys.
Preliminary analysis carried out in April 2024, using a variety of techniques, showed a 38% reduction in peak flood flows after the completion of the Stage 0 works. That benefits nearly 100 properties downstream in places such as Allerford and Bossington, which have historically been vulnerable to flooding, and roads such as the A39 between Porlock and Minehead. Results also show that the increased floodplain connectivity and water storage offered by the Stage 0 works are slowing the flow of water downstream. There is more of a lag between rain and surge. The researchers note that as the Stage 0 works were completed quite recently, more changes can be expected as the site develops, for example as many thousands of trees grow.