Background
Arundel Mill Pond, situated on the River Frome and adjacent to the Thames and Severn Canal, is a stunning location where the river widens and wildlife blossoms. However, in recent years the pond has undergone significant siltation and prior to works was dominated by substantial areas of mature reedbeds, Himalayan balsam and other overgrown vegetation.
Sanctus, appointed by the Wildfowl and Wetlands Trust and by working closely with the Stroud Valleys Project, have created open areas of water upstream of Arundel Mill Weir to recreate the former mill pond, whilst also making improvements in the weir area. The project forms part of a larger initiative to improve the local environment in the Stroud Valley and enhance the ease of fish movement.
The area is used regularly by dog walkers, educational groups, volunteers and the general public, whilst also being home to many residents who have access to the weir and banks of the pond. Therefore, enhancing natural beauty, biodiversity and safety of the area were paramount in this project. With the site contaminated with asbestos from the former factories upriver, the method was chosen in order to restrict public access to contaminated materials by ensuring the islands created were inaccessible by the public.
Objectives
The four-week contracting programme had two key phases, the first centred around the weir to prevent further leaking at the weir wall and repair the large scour hole to the back of the spillway which was growing with each flood event. The second phase was to undergo extensive de-siltation of the mill pond area to create silt laden islands, in turn creating open water areas, one adjacent to the garden of Arundel Mill Cottage and another at the Stroud Canal Towpath. As the country’s leading environmental contracting specialists, Sanctus were delighted to be asked to make this ambition a reality.
The remediation design highlighted the following summary of key objectives.
- The site is to be environmentally improved creating additional areas of open water and installation of a dipping pond for public usage;
- Management of the health and safety of construction workers when handling site soils;
- Management of the health and safety of maintenance workers post development; and
- Mitigation of pollution risk to the local environment.
The Programme
Phase 1: Scour Hole and Weir Wall Improvements
The prior condition of the sluice and spillway structure required repairs to prevent further erosion to the large scour hole to the rear of the weir. Originally this area was filled with silt, however, with flood events this was mobilised and taken downstream, leaving a large unfilled void. In order to fill this, Sanctus placed approximately 50 tonnes of boulders to form a rock roll mattress and rip rap which also added to the structural integrity of the weir.
The leak in the weir wall was repaired by erecting a line of Nicospan back filled to the wall with expanding bentonite pellets which seal the wall from further leaks and allow for the pond to be placed at this location without further damage.
Later, this will also include an eel pass as part of the wider project to bring fish and eels further up the Stroud Valley.
Following these works there were several areas of mildly damaged footpath along the canal by the dumpers used to transport boulders. These were repaired or replaced in order for Sanctus to leave the site better than it was found, something that the company believes is extremely important.
Phase 2: De-siltation of the Arundel Mill Pond
In the second phase of the project, Arundel Mill Pond was desilted using an amphibious excavator to create open pond areas and approximately 1m raised islands with the excavated silt. The local biodiversity will be enhanced due to the extra aquatic habitat created, along with a huge improvement in the aesthetic of the area for the residents and future site users and the improved drainage of the River Frome through the sluice and spillway structure as the river will act in a less flashy manner due to the extra water storage.
The islands were supported by a ‘wall’ of wooden stakes, Nicospan and silt tight geotextile backing which will allow the drainage of water whilst retaining all excavated silt in place. This method provides a sustainable alternative to a hard engineered wall structure. Only silt sediments which have infilled the pond over time were excavated and redistributed, which was around 1m deep throughout the site. The pre-existing riverbed sediments remained in place and untouched. After the project, the islands were revegetated with wild meadow flowers, improving the aesthetic and binding the silt together to prevent erosion by the weather and river and, therefore, the release of contaminated material. A site won log from a fallen tree was used at the entrance to the canal side pond to ensure that even in low flow situations, the pond will retain some water, ensuring that the lamprey and other small fish such as sticklebacks and bullheads will not be dried out.
The Challenges
Operating plant machinery within a wetland area
Sanctus are committed to working sensitively, safely and sustainably which formed our approach to working in close proximity to the River Frome. All earthworks that took place were designed to reduce material handling and machine movements in order to keep disruption to wildlife and water to a minimum. Biodegradable fuel was used for the plant in order to reduce the risk of pollution to the river in the event of a spill.
Prior to the works electro fishing was carried out to remove all aquatic wildlife from the working area which included brown trout, lamprey eel, stickleback and bullhead. A watching brief was then in place during excavation to protect any remaining wildlife, with a small number of frogs and toads also translocated. Stop nets were then in place for the duration of the works to prevent the return of aquatic life.
Silt curtains and sedimats were implemented to catch any sediments that would add to the natural sediment concentration in the river. This method worked alongside daily water quality monitoring (of turbidity, RDO concentration, conductivity, resistivity, dissolved solids, pH, ORP and temperature) upstream and downstream of the works to ensure that Sanctus were not polluting or disrupting river quality during the works outside of acceptable thresholds advised by our in-house Ecologist.
Working with contaminated land
Through site investigations prior to the works at Arundel Mill, Sanctus discovered that the silt across the whole site was contaminated with free fibres of Asbestos. In order to work as safely as possible all site personnel were face fitted with respirators in the chance of any fibres becoming airborne. Air quality monitoring was also in place to protect works personnel, residents and site users and found no risk of airborne asbestos fibres at any point.
Biosecurity
Sanctus operate strict biosecurity controls on all sites and appreciate the ecological harm that the introduction and / or spread of Invasive Non-Native Species namely Japanese Knotweed and Himalayan Balsam (INNS) can cause. In line with our enhanced biosecurity protocols, all equipment was disinfected with an appropriate treatment (in this case, shoe picks, hard brushes and Virkon S Aquatic). Sanctus personnel followed best practices with the Check, Clean, Dry prevention method throughout works in line with our in-house biosecurity training. All plant and machinery delivered and used on site was inspected for foreign objects by staff trained in the identification of INNS and disinfected again before leaving site.
Managing Public and Contractor Interaction
By using expert machine operators, Sanctus was able to move material along the narrow canal towpath and around the millpond safely and complete the works at minimal public disturbance with no main footpath closures. Machinery using the towpath was always guided by a banksperson in order to steward the public. Furthermore, signage and fencing were used to inform the public of the works and keep them at a safe distance from machinery.
Conclusions
Delivery of the project at Arundel Mill realises a long-awaited ambition to reopen the mill pond. The two ponds created have achieved in significantly increasing the storage capacity of the mill pond. The combination of aquatic and on land habitat provides a naturalised area for biodiversity to flourish and allow the natural beauty of the area to be appreciated by residents and future site users, namely local educational groups who use the site regularly. The rip rap installed at the weir not only protects from future erosion but is also in keeping with the site and improves its aesthetics. Sanctus are confident that wildlife and visitors alike will benefit from the betterment of the site.
