Early in 2016 an oak tree on the river bank was uprooted by raised water levels and high winds. The tree fell and the stem and branches were cut and winched away, leaving the stump and roots at the edge of the river. By Dartmoor standards, the summer was relatively dry but, during the autumn, periods of heavy rain raised the river level and began to undermine the river bank at this point. The stump then dropped into the water and the exposed soil along the bank, leaving the recently resurfaced riverside track at risk of erosion. There was also the potential for a large volume of material to be washed into the river. This could cause problems for the aquatic life downstream so, to protect the track and the wildlife of the Teign, urgent action was needed.
The root ball, in its position in the river, was compounding the problem by diverting flow towards the river bank and another sudden storm or high rainfall “event” could cause significant damage if repair work was not carried out. Track work along the river would usually take place between April and September but these “emergency works” were needed for the safety of vehicle movements and walkers.
Site Manager, Dave Rickwood, had to move quickly, explaining that “all work along river banks must be approved by the relevant flood management authority. Interestingly, this section of the Teign is not classified as a main river, it is an ordinary watercourse and we need to discuss our plans with the flood risk team at Devon County Council”. The Environment Agency is responsible for flood risk assessments lower downstream and they take over responsibility for the Teign from Steps Bridge to the coast. Their website states “Environment Agency powers to carry out flood defence work apply to main rivers only. Lead local flood authorities … carry out flood risk management work on ordinary watercourses”.
Having proved to be successful elsewhere along the river, rock armour was chosen to reinforce the damaged area. Once the old tree stump was lifted out of the river, large granite boulders were selected as the best option to build up the bank. A total of 50 tonnes of granite boulders were brought in, some weighing up to 1 ½ tonnes each and these will shore up the bank without affecting the river chemistry. Watching the machines at work has demonstrated the level of care and accuracy required to lift these monoliths into place without disturbing the river bed. Again, this has helped to prevent additional siltation downstream.
Careful repositioning of topsoil has meant that the riparian woodland vegetation can re-establish itself to further reinforce the bank. Plants such as the wood rush and the ferns play an important role in consolidating the soil structure.
by Matt Parkins