Weirs (or low-head dams) are described as being some of the “most abundant and impactful structures to freshwater ecosystems”, and over the past few decades there has been an increased effort to remove them, especially across Europe and North America.
Existing weirs that exceed their lifespan have been targeted for removal due to concerns over public safety and liability, as well as restoring water flows and the movement of sediments, materials, and species that have otherwise been impeded for up to 100 years or more.
Although many weirs have been completely removed, there is a lack of understanding about the influential factors which determine whether planned removals go ahead or not. This has prompted a research team from the UK to investigate cases where planned removals have taken alternative trajectories, such as when a removal was intended but the weir remained, and a fish pass was constructed instead.
As Januchowski-Hartley et al explain in their research published in Conservation Science and Practice, they looked at how projects are carried out, any potential areas for improvement, and the sharing of lessons to facilitate the reconnection of more rivers.
The study, described as being first of its kind in the UK, utilised mental models research which focuses on developing methods “to elicit and share people’s complex knowledge structures” and “reveal perceptions and assumptions that influence support for when, why, and how actions, such as weir removal, are done”.
To do this the authors developed a group model that could be used to support learning and communication about weir remediation projects between individuals and groups. They added that the intention of the study was to explore, learn, and communicate, and not to quantify or predict different people’s perceptions of weir remediation.
Severn River weir remediation
The Severn River is the longest river in the UK, with the Severn Estuary being designated a Special Area of Conservation. It was weir remediation in the Severn River Catchment (SRC) that was used as a case study for this work. The study focused on a six-year initiative called Unlocking the Severn under which six weir removals were proposed.
Those involved in the SRC project worked with the intention of completely removing weirs that were obstructing migratory fish species movement to historic spawning areas. However, complete removal was not achieved at any of the targeted weirs, and Januchowski-Hartley et al saw this an opportunity to work with people engaged in the remediation projects to determine how processes could be improved in the future.
Semi-structured interviews were carried out with people who had been actively involved in the weir remediation processes across the Severn River Catchment. The results from the group mental model showed that weir remediation processes are influenced by a wider social–environmental context, such as drivers of fish decline, culture and heritage and governance, and by characteristics of the structure site (such as the weir’s form and size, and river hydrology) and of interested parties (such as weir ownership, and perceived benefits and costs).
After analysing their results, the authors found that implementation of catchment-scale planning would help to improve current approaches to weir remediation, while it was also argued that a coherent national strategy should shape where weirs are removed and where fish passes are built.
Funding was perceived as a major point of failure in the projects. Some respondents said that management of the environment in the UK “moves with what are the highest priorities at the time”, while “it is easier to raise money if you’ve got something tangible at the end”. The authors also suggested that there was a need for increased and directly allocated funding from government to support public engagement throughout project processes.
Although current legislation in the UK dictates that fish passes must be built at any newly constructed weirs (including pre-existing structures with more than half the length destroyed or modified), this does not apply to existing weirs that are not otherwise partially (>50%) destroyed or modified. It was agreed that remediation projects would be more effective if existing legislation extended to weirs that are already in place and enforced the maintenance of the structure by owners in ways that promote river health and fish passage.
A major point of failure in the SRC projects was locating and gaining permissions from land and weir owners, while improved engagement was also recommended. Improvements could focus on how people are consulted, such as hosting earlier consultations to better understand and consider people’s different perceptions and preferences in removal and/or fish pass scenarios.
It was also suggested that improved partnerships between those working across the sectors involved in caring for rivers would benefit weir remediation project processes. This could include working directly between government agencies and river-charities, as well as with landowners and angling groups, to improve fish habitat. It was also suggested that contractors who design and build the weirs are involved when exploring different remediation options, working closely with landowners to anticipate problems associated with project implementation and to develop more acceptable outcomes.
As full weir removal had been limited in the SRC, concern was expressed by several respondents about weir retention and building more infrastructure alongside them. The authors state that agreement was not reached in the group as to whether full weir removal is an improvement over other remediation methods that tended to be used in the SRC, such as retain weirs and construct fish passes built alongside them. A nod of acknowledgement was also given to heritage values that some individuals hold for weirs, as a lot of weirs across the catchment have links to an industrial heritage.
In addition the research pointed towards the need for greater collaboration, and more learning, sharing, and building on the successes in the SRC, and in other catchments and countries. As one respondent remarked: “it always feels with fish pass work that every country is kind of siloed and doing its own thing, like the French do their thing their way, the Germans do their thing their way, we do our thing our way, the Americans do their thing their way, and […] there isn’t a great deal of learning from one another”.
Challenges and opportunities in renovation
There are many challenges but also opportunities when considering renovating existing micro-hydropower sites around the world, as demonstrated by recent work in Nepal.
The 2015 earthquake that wreaked havoc across the country damaged an estimated 262 micro hydropower plants in the districts surrounding the earthquake’s epicentre, impacting over 37,000 households. Small-scale natural disasters are also prevalent in Nepal, such as landslides, flooding and avalanches, and these can often affect plants due to their location in the base of valleys near water courses. Even without extreme events, commissioned sites often do not last as long as their expected 50-year lifespan, and can fail for many reasons but generally due to poor maintenance.
As Joe Butchers et al explain in research published in the Journal of Sustainable Research, such failures provide an opportunity to renovate, refurbish and upgrade hydropower sites, using updated designs and technology to provide an improved service to the community.
However, the remote location of sites across Nepal can have an impact, such as reducing the availability of trained operators, affecting the provision of spare parts, and reducing the opportunity for income generation. In plant renovation, the authors caution, these pre-existing challenges should not be overlooked.
The option of renovation and repowering a site offers the opportunity to install new turbine designs to improve the performance. As the authors explain, historically across Nepal there have been only two micro-hydropower turbine designs that are locally manufactured – the Crossflow and the Pelton which were both introduced by international development agencies. Over the past ten years though, three additional designs have been developed that can also be manufactured locally – a propeller turbine, Francis turbine and Turgo turbine.
“The Turgo turbine is especially interesting, as it can easily be retrofitted into existing infrastructure with minimal adaptation for both Pelton and Crossflow sites. Furthermore,” Butchers et al say, “it can bridge between the typical operational ranges of the Pelton and Crossflow.”
In their research, the authors investigate the use of a Turgo turbine for the renovation an existing micro hydropower installation site in Nepal. They focused on the 27kW Mangmaya Khola plant in Sangu, Taplejung district which was built in 2005. Over time, the power output of the plant had reduced to about 18kW due to leaking of the existing turbine and possible erosion of the existing Crossflow turbine blades.
After the installation and commissioning of the Turgo turbine at the plant, power output was increased to 32kW due to increased efficiency of the Turgo system over the Crossflow at the rated conditions.
As the authors reported: “The potential benefits of renovating existing sites are many, but in terms of increased power outputs, higher efficiencies and plant reliability, the options around utilising modern turbine designs in particular are essential in order to achieve higher performance at investment and operating costs which are economically viable over the plant’s lifetime.”
However, after operating for only 18 months, the refurbished Mangmaya Khola plant was badly affected by a landslide in the local area in July 2023. The penstock and powerhouse were severely damaged and Nepali project partners are exploring how it can be restored to operation.
As mentioned in their paper, Butchers et al say the incident further demonstrates the risk that landslides pose to micro hydropower plants in Nepal.
