Stop 14: Investment banks
Bank protection provides structural support to the canal, protecting it from erosion and preventing leaks.
When the canal was first built traditional materials and methods like timber piling and masonry were used. In restoring the canal back into navigational use, easily accessible materials and modern techniques such as steel sheet piling were preferred. In isolation, this hard edge provides very little value to wildlife; creating a physical barrier between aquatic and terrestrial habitats and preventing the creation of a vegetation fringe.
Sheet piling often continues to be used along the towpath to provide structural integrity and protection from erosion by towpath use and boating traffic, and leaks. In its rural setting, the offside comprises mostly soft earth embankments, encouraging a natural and diverse fringe of vegetation.
The biodiversity value is centred on the transitional zone between the aquatic and terrestrial habitat. Colonising from surrounding wetlands since the waterways were built, these narrow fringes are significantly biodiverse. Dragonflies and damselflies with aquatic and terrestrial life stages, amphibians and reptiles, water birds, as well as water voles which require soft banks for burrowing and reeds and rushes for food and shelter are all dependent on this vital habitat resource.
However, these natural banks are at risk from erosion, particularly from adjoining agricultural land use. Stock grazing can be beneficial in controlling scrub encroachment, maintaining a diverse sward and creating shallow pools. Yet, grazing directly on the banks leads to poaching, bank erosion, trampling of vegetation, and increased sedimentation and nutrient enrichment of the water. A further initiative of the Severn Uplands project was to introduce measures to help protect these banks through fencing livestock out of the riparian zone, thus encouraging vegetation to naturally stabilise the soil and buffer impacts from pollution sources.
As the biodiversity value of the canals has become more widely acknowledged, alternative ‘bioengineering’ techniques have been encouraged, favouring more sympathetic ‘green’ solutions, which balance conservation, recreation and engineering needs.
Look out for nicospan along the offside bank. This is a flexible and lightweight woven geotextile material which is usually backfilled with soil or dredgings to create the required profile and help to stabilise eroded banks. This material is permeable to water and allows vegetation to grow through it, encouraging the establishment of reed fringes behind which help to create a naturally stabilising bankside habitat.
Coir rolls are another more environmentally friendly and sustainable solution, based on the principles of the natural stabilisation properties of vegetation. The rolls are constructed from biodegradable coir fibre which provides initial structural stability for the banks and a platform to aid natural vegetation establishment. As the vegetation matures, it roots into the banks, creating a sustainable form of erosion control and bank support, whilst providing significant biodiversity interest and helping to improve water quality.
Last date edited: 17 July 2015