Location: Somerset. 0.5km of the River Avill near Dunster.
Issue: A straight, trapezoidal flood relief channel not functioning to prevent flooding, suffering from siltation and therefore dredged to improve conveyance. Devoid of habitat and trout.
Objective: Create habitat for sea trout. Increase channel sinuosity, reduce siltation, use a two-stage channel to improve flood conveyance and increase resilience to low flows.
Method: flow deflectors and brushwood berms.
WTT role: Lead partner — project design and delivery.
Partners: Crown Estates, Environment Agency. 40+ volunteers from Dunster village, Bridgewater and Petroc Colleges, Exmoor National Park and Somerset Wildlife Trust.
Project date: November 2014, 7 days.
The village of Dunster, near Minehead in Somerset, is protected from flooding by a hard-engineered flood relief channel which diverts excess flow from above the A39 near Dunster Castle. A gravel trap at the head of the flood relief channel had been poorly maintained, causing the majority of flow to be diverted even during relatively dry periods and depleting the natural river which still runs through the village. At the very bottom of the main river, the once meandering channel has been straightened and a tidal flap installed to reduce flooding around farmland, a golf course and holiday chalets. The depletion of flow contributed to excessive sedimentation in this lower section of the River Avill, resulting in vegetation encroachment and reduced channel capacity and conveyance; once winter flows returned, this subsequently contributed to flooding of the adjacent farm. In order to restore the capacity and conveyance of the channel, the river was dredged, resulting in the total removal of all in-stream habitat.
In light of this series of unfortunate events, the Wild Trout Trust was approached to see if a functional balance between flood conveyance and habitat for sea trout could be found. In the meantime, the Environment Agency would undertake a review of the flow split and maintenance régime at the head of the flood relief channel in order to maintain a better flow through the main River Avill.
There was neither the budget nor stakeholder support for a full restoration of the channel back to its original meandering planform. Habitat would have to be improved within the confines of the straightened and uniform trapezoidal channel, a task compounded by a paucity of bankside trees.
The task was to design, consent and deliver a project to restore approximately half a kilometre of river with a budget of only £3500 and no material to win on-site. However, support from the Crown Estates in the form of as much timber as we could cut and transport and over 40 volunteers made this an achievable challenge. Unfortunately, the majority of the timber was straight sections of larch, not necessarily ideal for curvy river habitat features. However, as much as could be harvested in one day from a nearby woodland was transported to site by tractor. The remaining woody material was cut from nearby hedgerows.
With volunteers from Dunster village, Bridgwater and Petroc Colleges, Exmoor National Park, Somerset Wildlife Trust and the Environment Agency, a series of flow deflectors and brushwood berms were installed to kick the flow laterally across the channel and down into the riverbed. Over time, a sinuous ‘thalweg’ (low-flow trench) began to form and the riverbed was on the move.
The end result was a two-stage channel incorporating a significantly narrower, deeper and more sinuous low-flow channel within the constraints of the wider, straightened channel. This would maintain good quality habitat and connectivity for running sea trout as well as improved biodiversity. In addition, the faster flowing thalweg would help to limit marginal encroachment, maintaining long-term flood flow conveyance and channel capacity.
One year later, a comparison of EA cursory fish surveys suggests an improvement for some species and sea trout smolts were captured where there had previously been doubts as to their continued presence in the catchment.
A comparison of electrofishing surveys before and after restoration:
|No. caught 2015
|No. caught 2016
Three years later and the channel is flourishing with improved diversity of marginal flora and swathes of water crowfoot established on what was an impoverished river-bed.