As part of the Environment Agency River Avon restoration package, Cain Bio-Engineering undertook restoration of approximately 1km of the River Nadder, a tributary of the River Avon. The restoration was completed in two sections; Daslett Hatches (downstream) restoration was completed in Autumn 2014 and Sutton Mandeville (upstream) was completed in Autumn 2015. The key project driver was to improve the ecological status for this section of river in line with Water Framework Directive objectives. This case study provides details of the works completed at Sutton Mandeville.
The Sutton Mandeville river section has an operating mill wheel incorporating inline water control infrastructure (weir and hatches). Mill operation requires impounding of the river for powering the mill wheel. The river had been historically dredged and straightened to deepen and widen the channel providing increased water storage capacity. Impounding of the river had lead to a reduction in flow velocity causing siltation of the main channel and preventing fish passage (the River Nadder is an important spawning ground for the River Avon chalk stream Atlantic Salmon).
The River Nadder restoration issues
Before work began, the Environment Agency – in conjunction with the landowners – lifted the hatches to reduce upstream water levels to a more natural height. This exposed steep vertical banks, leaving the river largely disconnected from its floodplain. The following key issues were identified following removal of the hatches:
- Overwide channel with steep cliff banks leading to a lack of marginal zones;
- Sub-optimal flow velocity; and
- Heavily silted and homogenous river bed.
A range of river restoration techniques were employed to overcome these issues, including:
- Large Woody Debris (LWD) installation and tree hinging to narrow the channel (increasing flow velocity) and helping create sinuosity;
- Bank grading to narrow the channel and re-connect the floodplain;
- Riffle creation to add spawning habitat and increase bed diversity;
- Berm and backwater creation; and
- Wet woodland habitat improvement with the creation of scrapes to enhance wetland habitat and to provide increased flood water storage.
Restrictions to works
Two ecological surveys were carried out prior to the start of the project, by Environment Agency ecologists, to identify any protected species on site. The first pre-construction survey identified very little in the way of protected species, however the second one identified active water vole burrows, many within areas identified for bank grading. This restricted the extent of bank grading carried out in these areas and required the use of water vole friendly techniques instead. Bank grading was completed, along other banks (with Ecologist approval) to provide adequate re-connection of the river to the floodplain.