Work to improve the upper Avon in 2018 included green bank protection, tree planting, fencing and drainage works. Find details of each of these works below.
Brash bank protection
This is the process of pinning brash (bits of tree) to an eroded bank. This brash acts like a cushion – slowing flow and encouraging the deposition of sediment which builds the eroded bank back up. the use of willow in this structure can result in rooting and help protect the bank for the future.
Two volunteers from Slamannan Angling and Protective Association and four from SNH planted 150 trees. Unpalatable and grazing resilient species (alder, downy birch, hawthorn and blackthorn) were planted without protection in an effort to reduce materials. The new stock fence here will prevent grazing from sheep but there are a few deer in the area. This is a bit of an experiment that will be monitored over the next few years to see which species have thrived/survived without protection. Biodegradable mulch mats were used to reduce competition from the grasses, docks and nettles present on the site. The trees were planted around the brash bank protection and on two other sites to provide riparian habitat and, longer term, bank stability.
Dry-stane headwall workshop
Drainage and rivers are intrinsically linked. This is particularly prevalent on farmland where fields have been created from floodplains through historic dredging and straightening of river channels and the creation of field drains. These field drains take many shapes and forms – including open ditches, stone filled ditches, buried clay pipes, and the more modern perforated plastic pipes. The brash drain. an interesting old technique I learned about on our recent workshop, involves digging a ditch, filling it with brash (butt end uphill so that water flows easily through it) and covering over the top with earth. The brash structure creates enough space for water to run through it. Field drains are just one of many examples of the innate relationship between land and rivers.
The RiverLife team are working with farmers and local volunteers on the Upper Avon to deliver habitat and management improvements. The river at Dyke farm has been dredged and partially straightened in the past and some of the drainage outflows are now blocked behind vegetated bars. To avoid the regular clearing of vegetation and bank material from these outflows, which damages riparian habitat and releases excess silt into the river, we decided to extend the outflow into the river channels and build a dry-stone construction to secure it in place. Dry-stone walling or pitching is a traditional erosion control technique used to protect land from meandering rivers. While in many places this has been superseded by green bank protection methods and we now understand the importance of erosion as a natural river process, it seemed an apt technique in this occasion and one that could add to the habitat diversity on the river bank.
So, on a rainy weekend in August – after a sustained dry spell – 6 volunteers (including 5 Slamannan Angling and Protective Association members), myself and Peter Holmes (our workshop tutor and rural skills expert) gathered on the banks of the Avon – armed with shovels, pinch bars, lump hammers and a van full of sandwiches – ready to build.
The drain itself was extended by six meters, through the bank and into the channel. While the extension pipe and the four posts and wire pinning it down were brought in for the job, all the stone used for the dry-stone structure was gathered from the corners of nearby fields using a telehandler from the farm.
The principals of dry-stone walling that we learned that weekend – use larger stones at the bottom, maintain an even batter (slope), avoid running joins, use stones that reach into the bank/wall and remember that friction holds it all together – were used to create a structure that will protect and mark the new drainage outflow. Furthermore, dry-stone walls are home to many plants and animals, so this structure could provide a habitat niche that was not previously present on this stretch of riverbank, which is mainly inhabited by tall herbaceous vegetation. The top of the dry-stone structure was then covered with earth containing roots from these tall herbs that will consolidate the structure and ensure that it lasts for years to come.
It is around the edges of this structure where erosion is most likely to occur as water is deflected from the hard stone onto the soft bank. Therefore, it is important to have a strong and resilient root structure at these edges. The rank vegetation present on the banks will soon colonise these disturbed edges but, after monitoring through the winter, we may return in the spring to deploy a few willow cuttings.
The five local river stewards and our volunteer visiting from the Almond catchment who attended the workshop now have another string to their bow when it comes to rural skills and riparian management.
2018 also saw the construction of c.450m of stock fence along the banks to protect the planted trees from grazing encourage regeneration of shrubs and trees in the future.