Gateside Mills Weir, River Eden, Scotland

Removal of an old mill weir restores fish migration to upper River Eden, Fife

The River Eden
Eden is a 48-km long river in Fife, Scotland with a catchment area of 60 km², running West to East through the peninsula of Fife before entering the North Sea (Figure 1). The river is inhabited by several fish species including the iconic Atlantic salmon (Salmo salar) which has seen drastic declines across the North Atlantic and Scotland, the European eel (Anguilla anguilla) which is classified as Critically Endangered by the IUCN Red List of Threatened Species, the sea trout and resident brown trout (Salmo trutta), and the European brook lamprey (Lampetra planeri). The River Eden has diverse fauna present including species directly connected with the river such as otters (Lutra lutra), Eurasian kingfisher (Alcedo atthis), white-throated dipper (Cinclus cinclus) and occasionally osprey (Pandion haliaetus), which is a specialist fish eating raptor.

Figure 1. (left) River Eden catchment area and (left and right) location of the Gateside Mills Weir (indicated by the red triangle)

The Gateside Mills Weir
The Gateside Mills Weir was 3-m-high and 22-m-long located on the River Eden near the village of Strathmiglo, in Northern Fife, Scotland (Figures 1-2). It was constructed between 1854 and 1896 to provide water power to Gateside Mills industrial site. The water was diverted by the weir into a channel known as a ‘lade’ where it turned an eight-foot (~2.5 m) water wheel powering machinery. The mill produced shuttles and bobbins which were used by 19th-20th century industrial textile weaving factories, most notably in India that was world leading in jute manufacturing. By 1939 the mill had been converted to electric making water power obsolete. Indian independence in 1947, led to a ban on the import of bobbins which impacted Gateside. Diversification into new markets allowed it to continue up to the 1980s, when the mill closed as a bobbin mill (Figure 3).

Figure 2. Location of the Gateside Mills Weir (indicated by the red triangle) in the River Eden catchment

Figure 3. Gateside Mills as a working industrial bobbin mill site in the 1960s. The lade outfall on the left is where there the original water wheel sat that provided water power to the mill pre 1939 (photo from A.G. Highland (1989) – Gateside Mills: The Scottish Bobbin and Shuttle Trade in its British and International setting, 1860-1960; Available at: https://research-repository.st-andrews.ac.uk/handle/10023/2689)

By 2018, when the project for the removal of the Gateside Mills Weir initiated, the weir had been redundant for decades (Figure 4) but continued to be a full barrier to fish migration resulting in the disconnection of 17.6 km of river habitat from the rest of the River Eden catchment. In addition, the barrier restricted flows and natural sediment transport affecting biodiversity and quality of habitat in the immediate area and downstream. The weir and lade were overgrown with vegetation and required maintenance (Figure 5). Additionally, the low-flow sluice in the middle of the weir attracted debris leading to blockages and flood damage to the property owner.

Figure 4. Gateside Mills Weir in January 2022 (photo by SEPA)

Figure 5. Gateside Mills Weir and lade overgrown with vegetation in August 2022 (photo by SEPA)

Dam removal
Owners of dams and weirs in Scotland have a duty to allow fish migration under the Water Environment (Controlled Activities) (Scotland) Regulations 2011. The Scottish Environment Protection Agency (SEPA) administers the Water Environment Fund (WEF) on behalf of the Scottish Government to enable fish passage improvement work on redundant fish barriers, and these barriers are objectives within Scotland’s River Basin Management Plans (RBMPs). Before progressing a WEF project, it is made clear to barrier owners that SEPA prioritises full removal wherever possible. SEPA’s WEF team works in partnership with local authorities, land and structure owners, fishery trusts and conservation bodies to deliver the annual programme of fish barrier and river restoration projects.

The Gateside Mills Fish Barrier Removal Project (Video 1) began in late 2018 when the weir owner agreed to fish passage improvement work using the Water Environment Fund. The work was directly commissioned by SEPA’s WEF team and the key project outcome was to restore natural levels of fish passage to 17.6 km of upstream river habitat. The project went through an options appraisal and design stage before the full removal of the fish barrier in 2022. The options appraisal was carried out by Royal HaskoningDHV in 2019 and having assessed several potential solutions, recommended full removal as the preferred option. Removal would secure: permanent fish passage, no future maintenance requirements and natural sediment transport and flows. The relatively small cost difference between removal and modifying the weir, the short lifespan estimates of the weir without maintenance (10-25 years) and potential risk of beavers damming the low flow channel of the weir also contributed to the decision to fully remove it.

Video 1. The Gateside Mills Fish Barrier Removal Project (video by Ebsford Environmental)

In 2020, Royal HaskoningDHV were commissioned to develop a design fit for construction. Sustainability and restoring a ‘close to natural’ channel were a core part of SEPA’s requirements. The main challenge for the design team was designing a close to natural river that would not experience significant channel instability. The site had a 2-m bed level difference between upstream and downstream of the weir, that would require 285 m of channel re-profiling to achieve a natural gradient. This was deemed excessive in-channel work and would have controversially required the clearance of a lot of mature riparian trees. Therefore, the design proposed a reasonable compromise of restoring the river over 80 m at a steeper gradient of 2.5%. The channel was designed to be two-stage with constructed/rock step-pools in the central channel to achieve a fitting river type for the steeper gradient involved, reduce velocity and the risk of instability.

The works contractor, Ebsford Environmental, was appointed in 2022 and works began in August to remove the weir. Before any work could begin at the site, the Tay District Salmon Fishery Board were sub-contracted to carry out a fish rescue. The results highlighted the range of species being impacted by the barrier: salmon (15), trout (220), freshwater eel (5) and lamprey (2) in addition to minnows and stickleback.

The first major works task was to excavate 92 m³ of sediment contaminated with invasive non-native plant Himalayan balsam (Impatiens glandulifera) upstream of the weir and bury it on site following biosecurity best practice (Figure 6). Access to the burial locations required a path closure and close communication with the local authority and its users. Once, the sediment behind the weir was removed deconstruction of the weir began (Figure 7, Video 2). This was achieved by pushing the river to the right bank and through the low flow channel in the weir. This allowed demolition of the left side of the weir in the dry. Once this was complete, the contractor pushed the water through the demolished left side of the weir allowing the demolition of the right-hand side of the weir in the dry. Suitable material for reprofiling the new channel and building step features was stockpiled. Part of the right-wing wall of the weir remains on the river bank at Gateside Mills. This now serves to retain a link to the former industrial heritage of the site for the local community (Figure 8).

Figure 6. Large colony of invasive non-native plant Himalayan balsam growing out of impounded sediment and very small channel, upstream of the Gateside Mills Weir in August 2022 (photo by SEPA)

Figure 7. Removal works of the Gateside Mills Weir in August 2022 (photo by Royal HaskoningDHV)

Video 2. Removal works of the Gateside Mills Weir in August 2022 (video by SEPA)

Figure 8. Removal site after the demolition of the Gateside Mills Weir, in September 2022. The Hazel tree on the far bank is growing out of the remaining right-wing wall of the former weir which provides a link to the industrial heritage of the site (photo by SEPA)

After the weir was removed (Figure 9), the reprofiling and construction of the close to nature step-pool channel was carried out by excavator over 80 m of river length. The groundworks took around 5 weeks to complete, and all site operations were completed by the end of September 2022.

Figure 9. (up) Removal site after the demolition of the Gateside Mills Weir, and (down) upstream view from the removal site, in September 2022 (photos by SEPA)

Two months following the completion of works, in November 2022, a large flood event passed through Gateside Mills and re-adjusted the constructed channel. The constructed floodplain berms suffered erosion, two large sediment bars appeared and a sub-channel that bypassed a section of the constructed channel had formed.  None of these changes affected fish passage, however, the changes to flow and potential for further erosion did present a risk of damaging the landowner’s building on the right bank and gabion baskets protecting a sewer pipe on the left bank (Figure 10). The site was monitored and following a similar sized flood event in December 2022, little geomorphological change occurred providing confidence that the site was stabilizing.

The project had sufficient contingency budget for some minor remedial work to remedy some of the post-construction flood damage. In 2023 the Designer, Royal HaskoningDHV and works contractor, Ebsford Environmental, worked with SEPA and the landowner to produce a solution focused remedial design in-keeping with our project outcomes including natural channel form and function. The remedial works carried out by Ebsford Environmental used spare rock from groundworks in 2022 (still on site) to protect the bank toe below the building and planted 30 willows (Salix viminalis) on the bank face to provide further stability for the future (Figure 10). The bypass channel was formalized into part of the constructed step-pool channel and excess material in the mid-channel bar (formerly the constructed bank toe) was removed to allow water to dissipate freely across the channel width. This approach helped stabilize the channel and reduced erosion risk. All material for remedial works was sourced from site except the willow saplings.

Figure 10. Following the large flood in November 2022 there was a potential risk of erosion to the right bank below the Nissen Hut and gabion baskets on the left bank that protect a sewer pipe and thus remedial works occurred to achieve a more stable channel with reduced erosion risk to property: (up) before remedial works, and (down) after remedial works (photo by (up) SEPA and (down) Royal HaskoningDHV)

Overall, the project cost £387,000 including the remedial works in 2023 and was completely funded by the Water Environment Fund. Support from our designer and works contractor allowed a carbon calculator to be used to estimate and track emissions. The groundworks and remedial works had an as-built carbon footprint of 7.13 tCO2e (tonnes of carbon dioxide equivalent), which is 59% of the mean footprint (per annum) of 1 person in Scotland (12.1 tCO2e). Sustainability focused procurement, design and contractor innovation helped to deliver a 35% reduction in the as-built emissions compared with the estimated carbon emissions at end of design (11.01 tCO2e). The project in general was low emissions, very low waste and applied circular economy principles to reuse material throughout.

There was a small local community of residents and businesses at Gateside Mills and in the local village, some of which held an aesthetic attachment to the mill. During the project the community were generally supportive of fish passage improvement work at the weir and there were no complaints during works. A former mill worker lived in the local village whom the project managers met just before groundworks began. At the time, the mill worker was sad to hear the weir would be removed but understood that it was the right thing to do and was supportive of the project. In 2023, a year after works (Figure 11), the SEPA project manager met the mill worker again near the restoration site. During their conversation, he shared fond memories of salmon leaping at the weir in years gone by. However, more powerfully, he stated that he looked forward to more salmon returning now the weir was gone. Given he was a former employee of the mill with strong nostalgic connections to the site, this was great to hear and provided re-assurance of positive local opinion.

Figure 11. (up) Removal site after the demolition of the Gateside Mills Weir, one year after the removal works, in September 2023 (photos by (up) Royal HaskoningDHV and (down) SEPA)

Challenges
At options appraisal stage of the project, the consultant highlighted the risk that the weir was likely built on a natural bedrock “step” in the river that may limit fish passage. An intrusive coring survey of the weir was carried out to understand this risk better during design. Five cores across the weir surface provided confidence that the bedrock step was unlikely to be an issue to fish passage and informed channel design and deconstruction. However, uncertainty of actual bedrock depth due to weir masonry and bedrock being of the same sandstone and the possibility of variation between coring locations meant groundworks could still be impacted by unexpected bedrock levels. To manage this risk, trial pits to locate bedrock and a 3-day re-design period were included in the groundworks programme. Unexpectedly and somewhat thankfully, no bedrock was found during groundworks.

The potential presence of water voles (Arvicola amphibius) pre-works posed a risk to groundworks being delayed by a year. Water voles are a protected species in Scotland and it is a wildlife offence to damage, destroy or obstruct access to any structure or places they use for shelter. A water vole survey was not carried out the year before works. By Winter 21-22, it became apparent that best practice timings for pre-works survey, obtaining a protected species licence and carrying out mitigations were not feasible with groundworks occurring in 2022. The primary water vole mitigation is translocation which requires a lot of preparation time that unfortunately we did not have. The issue was discussed with specialists in SEPA’s Ecology team and NatureScot who have remit for protected species. An approach was formed to survey for water voles in early spring outside of the good practice period on the basis it may still inform us of the risk and allow early decision making. The survey found signs of American mink (Neovison vison) that are an efficient predator of water voles, which meant they were highly unlikely to be present. This provided the project confidence to proceed with groundworks in 2022. To ensure compliance, a follow up protected species survey was carried out pre-works confirming no water voles were present.

River Eden had the highest level of water scarcity in Scotland in August 2022, during the first few weeks of groundworks. At this time, the river reached its second lowest water level on record and the situation was so severe that water abstractions from the river were suspended affecting farm businesses. This drastic situation meant that aquatic ecology in the river would be extra vulnerable to additional stress. For Gateside Mills project, this presented a risk that groundworks could cause harm to fish and breach the project’s (CAR) license for in-channel engineering work. As a result, precautions to protect fish and manage silt during works were given the utmost importance. The lack of river flows and the project site being in a confined valley consisting mostly of fine material made this site extra challenging (e.g., diversion channel was not an option). In order to mitigate this risk, project managers worked closely with the contractor, site supervisor and ecological clerk of works to ensure no harm came to fish and that fish rescue and relocation happened as early as possible during works. This included water scarcity being added to daily weather checks, trial pits planned for in-channel being done in the dry and silt monitoring and mitigations being in place. Installation of a timelapse camera allowed for remote viewing of higher risk activities and promoted close communication and vigilance amongst SEPA, the contractor and design site supervisor to ensure risks were being monitored and managed.

Ecological benefits
The removal of the Gateside Mills Weir had multiple outcomes (Table 1). The weir removal has restored natural levels of fish passage at Gateside Mills for migratory fish species including Atlantic salmon, sea and resident trout, eel and lamprey. As a result of this work, 17.6 km of River Eden habitat have been ecologically reconnected to the rest of the river for the first time in over 120 years. Because the weir was removed, longevity of fish passage has been secured at the site without maintenance requirements. In addition, the return of a free-flowing river has restored natural sediment transport processes that will benefit aquatic habitat and biodiversity in the River Eden.

Attempting to restore rivers as close to natural when removing fish barriers comes with the potential for some river response post-works, as evidenced in this case study. However, the benefit of this approach is that the River Eden at the Gateside Mills now benefits from a more natural form including natural banks and varied in-channel habitat and flow.

The path network near Gateside Mills is well used by the local community and local stakeholders have been enthusiastic about the improvement of biodiversity and wildlife along the River Eden.  It is expected their experience with nature will be improved by the fish barrier removal work as migratory fish reconnect with upstream habitats and begin to contribute to the local food web.

Table 1. Outcomes for Gateside Mills Weir Fish Barrier Project

In 2023 following the removal of the weir, SEPA carried out an electrofishing survey in the River Eden around Gateside Mills and recorded Atlantic salmon for the first time at survey locations upstream of the former weir (Figure 12). The furthest site was 2 km upstream indicating migratory fish utilizing the reconnected habitat within one year. When SEPA surveyed the same sites upstream of the weir in 2021 no salmon were found, which was a clear indication the weir was impassable to this species. Therefore, these salmon may be the first recorded salmon to utilize the newly reconnected river habitat in over 120 years.

Figure 12. Atlantic salmon fry (up) and parr (down) recorded upstream of Gateside Mills Weir removal site in 2023, one year after the completion of the removal works (photo by SEPA)

Before & After photo pairs

The removal site at River Eden: (left) before (January 2022) and (right) after (September 2023) the demolition of the Gateside Mills Weir (photos by SEPA)

The removal site at River Eden: (left) before (October 2019) and (right) after (October 2023) the demolition of the Gateside Mills Weir (photos by (left) Royal HaskoningDHV and (right) SEPA)

The removal site at River Eden: (left) before (August 2022) and (right) after (September 2022) the demolition of the Gateside Mills Weir (photos by SEPA)

With many thanks to Lawrence Belleni and SEPA for providing the information and the visuals presented herein.

Written by Foivos A. Mouchlianitis