The banks of the middle reach of the Allan Water are susceptible to bank erosion due to the sandy low resistant soil present, historical practices have also lowered the watercourse level and increased bank heights. Historic human events which have exacerbated this issue include alteration of a natural rocky pinch point at Kinbuck bridge to reduce the water level for land use purposes and decades of river bed dredging. This has led to several issues:
- High input of fine sediment from bank erosion affecting the aquatic environment, fish habitat and water quality;
- Unvegetated steep/vertical bank faces that provide no resistance to flood flows and increase water conveyance speeds and downstream flood risk;
- And an unnatural looking landscape without complex bankside vegetation.
The Allan Water Improvement Project based at the Forth Rivers Trust (FRT) has been running in the Allan Water catchment for two years now, and its project officer, Lawrence Belleni has been working in the Allan Water to identify and implement Natural Flood Management Projects, water framework directive actions and projects that deliver multiple ecosystem benefits for the wider environment. Previous bank protection works on this reach included tree and willow cut planting on bank faces to using a brash bank protection technique funded outside of SRDP. Identifying suitable new sites is the project officers’ remit, the land occupier at Nether Cambushinnie Farm, was interested in bank protection work and could pay up front for the work and apply for funding retrospectively through SRDP’s Agri-Environment Climate Scheme (AECS).
Description of bank
The restoration site is typical of eroded banks on the Allan Water, it is 95m long with a bank height varying from 2m to 3m in height. The bank is being eroded at the bank toe, causing the vertical faces of the bank to slip into the Allan Water. This process is described as slab bank erosion in the SEPA Engineering in the Water Environment Good Practice Guide- Bank Protection-Rivers and Lochs, 2008. Due to the height and gradient of the vertical faces, very little to no vegetation was present to help protect the bank from this process. In addition, sheep grazing is present onto the bank edge, however it does not appear to be the main cause of the erosion at this site. The considered methods for bank protection on this project were: willow spilling, plant roll revetment, Hurdle and Coir matting, and engineered log-jams. Out of the above methods only willow spilling and engineered log-jams would protect from bank toe erosion and could be applied in a large waterbody, such as the Allan Water, and a river bank of this scale. The willow spilling option was selected for this bank out of the available Restoring River Bank methods, which will protect the toe from erosion, and discontinue this process. It has also provided the land owners and local community with an example of a natural flood management technique. Delivering a basis to teach and train any local land occupier the necessary skills, thus becoming a restoration option to attempt for themselves in the future.
To stabilise the bank toe, willow spilling has been put in place. The spilling was installed slightly below low summer water level to ensure permanent wetting of the willow, facilitating survival. Therefore, in high water level events the spilling would be submerged as it will not be higher than 60 cm.
Spilling was completed by driving in 2m long wooden posts every 70 cm along the former bank toes that have been eroded. The posts follow a line along the eroded bank toe and all sit at a similar height level. 3m willow rods have been densely woven around the posts with the basal end of the willow rod facing upstream. The rods were tamped down during the process of spilling to ensure the bank toe is protected from erosion. To increase the willows survival rate and growth, earth was placed and compacted behind the willow spilling.
The restoration site was a very steep (almost vertical) bank, therefore it required its bank to be reprofiled to a slope less than 1:1 (45 degrees), otherwise slab erosion issues may remain due to the high and steep unvegetated banks. To stabilise the bank face behind the spilling, a live willow mattress revetment was created. Coconut geotextile was used on the reprofiled slope to protect the bank from erosion. The geotextile has been anchored in a trench around 20 cm deep and attached with wooden stakes (approx. 30 cm) behind the willow spilling. The willow faggots were placed vertically up the bank face with their basal ends in the trench behind the willow spilling (approx. 20-30 willow branches per meter). The trench was filled with ground and a thin layer of ground was added above the willow (5-10 cm depth) on the bank face with grass seeds sown over that layer. Post and wire were then used to attach the geotextile to the top bank surface.