Burn Weir removal opened 22 km of suitable habitat for fish migration and spawning

 

The Bronie Burn
Bronie Burn* runs close to Ellon, through Aberdeenshire in north-east Scotland and its overall length along with its tributaries is around 22 km (Figure 1). It is a tributary of River Ythan and is inhabited by several fish species, including Atlantic salmon (Salmo salar), Brown trout (Salmo trutta), European eel (Anguilla anguilla), Sea lamprey (Petromyzon marinus), River lamprey (Lampetra fluviatilis), Brook lamprey (Lampetra planeri) and Eurasian minnow (Phoxinus phoxinus). Otter (Lutra lutra) and badger (Meles meles) are also present on site with several mature broad-leaved trees with bat roost potential. Himalayan balsam (Impatiens glandulifera) and Monkey flower (Mimulus ringens), invasive non-native species, grow along the riverbanks of Bronie Burn.

* A large stream or a small river. The term is used in Scotland and England (especially Northeast England) and in parts of Ulster, Australia and New Zealand

Figure 1. Bronie Burn (indicated by the red rectangle) at Ellon, Aberdeenshire, Scotland

Barrier removal
Scottish Environmental Protection Agency (SEPA) administers the Water Environment Fund on behalf of the Scottish Government and works in partnership with local authorities, land and structure owners, fishery trusts and conservation bodies to deliver an annual programme of river restoration projects. Owners of dams and weirs have a duty to allow fish migration under The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (CAR). However, the Water Environment Fund may be available also to help owners remove an obstacle to fish passage that is no longer in active use or a commercial asset, including dormant or ‘mothballed’ assets. Barrier removal to fish migration, such as weirs, dams, and historic engineering structures (e.g., bridge aprons, culverts, etc.) is also included in the objectives of the River Basin Management Plans (RBMPs).

Bronie Burn Weir was located ~0.6 km upstream of the confluence of Bronie Burn with River Ythan (latitude: 57.359945; longitude: -2.129584) (Figure 2) and was more than 130 years old. It was a 1.5 m high and 5 m wide weir (Figure 3) with extent of apron of ~15 m (Figure 4). The purpose of its construction is unknown, but it is suspected to be associated to a mill which is no longer present. The weir was owned by two private landowners.

Figure 2. Location of Bronie Burn Weir (indicated by the red triangle) near the confluence of Bronie Burn with River Ythan
Figure 3. Bronie Burn Weir at Ellon, Aberdeenshire, Scotland (photo by SEPA)
Figure 4. Downstream view from the Bronie Burn Weir at Ellon, Aberdeenshire, Scotland (photo by SEPA)

Bronie Burn Weir fish passability was assessed through a survey commissioned by River Ythan District Salmon Fishery Board and conducted by River Don Trust and volunteers of River Ythan Trust in late August 2013 (Figure 5). Under low flow conditions the weir was classified as impassable for adult salmon and trout. Under high flow conditions it was considered partially passable but highly impactful for adult and juvenile lamprey and juvenile eel. Thus, the weir presented a considerable obstacle to upstream fish migration.

Figure 5. Fish passability survey at Bronie Burn Weir in August 2013 (photo by River Ythan Trust)

Later in 2013, the River Ythan Trust approached SEPA to consider the removal of the Bronie Burn Weir or its modification to improve upstream fish migration for up to 22 km of suitable habitat for all stages of Atlantic salmon, Brown trout, lamprey (sea, river and brook) and European eel. Excellent instream pearl mussel habitat is also present in the burn even though there is no record of the species. In May 2018 a site visit and a meeting were held with SEPA’s restoration specialists. A few months later, in October 2018, SEPA contracted cbec eco-engineering Ltd, a UK based environmental consultancy specializing in process-based river restoration, to conduct a feasibility study (desk-based study was followed by field-based fluvial audit, ecological and topographic surveys) to provide viable options for improvement of the weir’s fish passability. The report was submitted in early May 2019, and the full removal of the weir was decided as the preferred management option. Three years later, due to difficulties in establishing the legal owners of the weir, the COVID-19 pandemic, and a cyber-attack to SEPA, McGowan Ltd was contracted to remove the barrier.

“Implementing nature-based solutions places project teams face to face with the natural world, none more so than working in and around watercourses. The collaborative approach shown by all parties drove the build phase of the Bronie Burn project to a timely conclusion and successful outcome”

Sam Hesling, Contractor at McGowans Ltd.
Figure 6. Works to dewater the Bronie Burn riverbed before the weir removal in August 2022 (photos by McGowan Ltd)
Figure 7. Temporary diversion of Bronie Burn around the construction area before the weir removal in August 2022 (photo McGowan Ltd)

In early August 2022 and just before the weir removal, the River Dee Trust was contracted to conduct a fish rescue. All fish captured (440 trout parr and fry, 450 salmon parr and fry, 50 eels and several lampreys and minnows) were reintroduced at another section of the burn. The removal works initiated soon after. Approximately 440 m of riverbed were dewatered (Figure 6; Video 1) and the Bronie Burn was temporarily diverted around the construction area (Figure 7; Video 1). Works of weir demolition and watercourse realignment were completed in September 2022 (Figures 8-10; Videos 1-2). The project was fully funded by Water Environment Fund and costed £301892.91.

Video 1. Bronie Burn Weir removal at Ellon, Aberdeenshire, Scotland (video by cbec eco-engineering Ltd, SEPA, Water Environment Fund)
Figure 8. Site after the removal of Bronie Burn Weir in August 2022 (photo by SEPA)
Figure 9. Site after the removal of Bronie Burn Weir in August 2022 (photos by McGowan Ltd)
Video 2. Site after the removal of Bronie Burn Weir in December 2022 (video by SEPA)

“The Bronie Burn project was a fantastic opportunity to demonstrate how, with a 'nature-based' design approach, works to improve fish passage at redundant weir sites can also result in significant improvements to natural river processes through the reinstatement of sediment transport continuity, with consequent benefits for habitat diversity and ecological condition. It proved to be a seamless process of integrating the design and build phases of the work, working closely with SEPA, the groundworks contractor, other stakeholders and the landowners”

Dr. Hamish Moir, Principal Designer at cbec eco-engineering Ltd.
Figure 10. Aerial view of the site after the removal of Bronie Burn Weir in August 2022 (photos by McGowan Ltd)

Since the weir removal, 500 native trees have been planted in the riparian zone, funded by McGowans Ltd and Riverwoods. This is in complement to a further 4000 native trees planted by Scottish Woodland Ltd in the commercial woodland area adjacent to the Bronie Burn. Significant improvements to the channel and the habitat diversity were also evident at the site weeks after the weir removal. Sea trout redds were seen, both within the specific works site and further upstream. Monitoring surveys will be conducted following the weir removal by SEPA (morphological), River Don Trust (fish population) and the contractor and designer for 12 months to check for any defects.

“From a NEC3 contract administration perspective, the Bronie Burn should be held as an exemplar for how all the parties acted in the spirit of mutual trust and cooperation to deliver a successful project producing real benefits to the local environment and communities. Contracts relating to in-river working can carry significant risks. With the parties together taking a proactive approach to identifying and resolving risks the project operated smoothly and efficiently”

Sameer Shinh, Project Manager, Advantage RSK acting on behalf of cbec eco-engineering Ltd.

Before & After photo pair

The removal site at Bronie Burn: (left) before and (right) after the removal operations of the weir (photos by SEPA)

Written by Foivos A. Mouchlianitis

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  • Name: Bronie Burn Weir
  • Location: Bronie Burn, tributary of River Ythan, Ellon, Aberdeenshire, Scotland, UK
  • Type: weir
  • Dimensions: Height 1.5 m; Length 8.5 m
  • Aims of removal: improvement of fish passage and upstream migration
  • Year of removal: 2022