When the Holme Sluices were built in the 1950’s, spanning the full width of the River Trent to the south of Nottingham in Colwick, they were a key part of a large-scale flood defence scheme. Sluices and weirs are the largest barrier to fish migration on the Trent, with the Holme Sluices being the largest and the only one owned by the Environment Agency.

Now Colwick is home to another impressive environmental civil engineering project, with the recently opened Colwick (Holme Sluices) Fish Pass. After two years of building work, at a cost of around twelve million pounds, this huge structure – 200 meters long and 6.5 metres wide – will allow coarse and migratory fish, such as salmon, trout, and the critically endangered European eel, to reach spawning and feeding grounds that had been largely blocked by the sluice gates.

With the Fisheries Department regularly monitoring fish stock levels it was clear that upstream of the sluice gates that there was a lack of certain species that would normally migrate. When the waters are reasonably low, you can see salmon around the front of the sluice gates getting confused because they can feel the velocity of the water, but they can't leap it. Whilst some salmon do manage to head upstream through the canoe slalom at Holme Pierrepoint, with its engineered high-octane white water, very few other fish can manage it.

Original design; Environment Agency/Royal Haskoning.

As Simon Ward, the EA’s Fisheries Technical Specialist, explains: ‘the decline of migratory fish in the Trent catchment dates back to the Industrial Revolution, when large weirs were first built to open up the river for trade. Some fish were able to overcome these barriers – whether by leaping over or wriggling through what were often imperfect constructions of stone or wood, but not all of them. We were surprised to see salmon returning to the River Trent in the early 1980s as they hadn’t been seen in the river for many decades. We were even more surprised by the amount of salmon and other fish that were reaching the EA’s Holme Sluices - the largest barrier to the natural migration of fish in the Midlands - and getting stuck there. We figured that something needed to be done, but at the time we didn’t have the resources or funding’.

Whilst a directive has since been required that any barrier to fish migration requires a pass to be built around it, this legislation did not apply retrospectively to any barriers that already existed. However, the directive also states that if you carry out major maintenance work to an already existing structure, (it’s calculated on the cost of the work), the law is triggered, and a pass must be created. Therefore, when these sluice gates had to go through a retrofit phase, which is a huge job given the importance of the structure to the river and the city, the legislation was triggered and work on the pass could begin.

If this has piqued your interest and you want to know all about the legislation around the construction of fish passes in the U.K., then here is a link to the Institute of Fisheries Management Fish Pass Manual: Guidance Notes On The Legislation, Selection and Approval Of Fish Passes In England And Wales.

This is a major environmental intervention, being the largest fish pass in the U.K. and the second largest in Europe. There are eight large weirs along the lower River Trent from Cromwell near Newark to Sawley near Long Eaton and this is the first scheme of the Trent Gateway Partnership which aims to remove all barriers to fish migration along the River Trent, provide better wildlife habitats, improve the biodiversity of the area, and to engage local communities in discovering more about the natural and social history of the river.

Given the proximity of the Meadows to the Trent, and the ecological recovery that is anticipated, I spoke to Kathryn Battelle, the Public Liaison Officer at Jackson Civil Engineering, who have been building the pass, as well as on the ongoing renovation of the sluice gates. Whilst Kathryn cannot speak on behalf of the EA, she was clearly passionate about her work, and of the potential for civil engineering projects to find ways to confront environmental and climate related challenges.

Before reading this interview, you may want to watch this two minute film, made by the Environment Agency, which shows you what the Fish Pass looks like, and how it works.

On the 28 January, a ground-breaking event was held to celebrate the start of construction of the fish pass. The Environment Agency Area Director, Louise Cresswell was joined by Tom Randall MP along with the Lord Mayor of Nottingham, Cllr David Trimble, David Kent (Trent Joint Anglers Council) and representatives from Nottingham City Council, Environment Agency.

A.B: So, how has the construction gone?

K.B: Brexit and COVID delayed the start of the work, but we finally broke ground on the 4th of January 2022. Given that we were just entering the post-pandemic world there was a lot of instability in material pricing, but once we’d started there was no going back. The original expected completion was August 2023, but with the difficulty in material sourcing we had a real battle to get the amount of concrete required as the HS2 project was hoovering up everything on the market. Steel was also difficult to source, partly because of the war in Ukraine. But most of our delays have been weather related. During its construction the pass was flooded between fifteen and twenty times, and that stops everything.

This section of the river is not a natural part of the Trent. This was dug in the fifties to divert the river, as part of the huge flood prevention scheme; the sluice gates were opened by Prince Philip in 1955. The original stretch of the Trent runs behind us and is known as the Colwick Loop. People think it's a brook but it's the real River Trent, and that old loop floods very easily, which means the access to the park also floods over.

There’s a Pathe film of the opening, with Prince Philip being welcomed by crowds on the embankment, which you can watch here.

There was flooding in February 2022 which delayed things, but the beginning of 2024 was extraordinarily unsettled. During construction the pass had stop logs on it at either end to keep the water out so that our operatives could work inside; but when the levels of the Trent came up as high as they did, the water was able to come in over the top and the fish that came in with it were stuck inside. We had to carry out a fish rescue with nets and buckets and took more than a thousand fish out of the pass. So, we know the fish are definitely in there; there is definitely plenty of aquatic life in this stretch of the Trent.

A.B: How does the fish pass work?

K.B: The final design solution was to construct a two hundred metre long, six metre deep, and six and a half metre wide pass, with twenty-one ascending chambers through which water flows through narrow slots. These have been designed to allow fish of all species to swim upstream and provide spaces where, once they have been drawn into each chamber, they can rest before continuing their journey. It’s quite a remarkable thing. The design also had to create the water turbulence needed to draw the fish into and through the pass. It has been nicknamed the Whale Pass and it is a really sizeable construction; but this was the required solution to let the fish know that the current is better this way, so they can start to move freely upstream after all these years.

Inlet into the fish pass.

A.B: Why would water levels mean you will need to close the pass?

K.B: There are regulations on how much water can be taken out of a river and this pass is third in line for water extraction.

Holme Pierrepoint gets first dibs but if river levels drop to full drought levels they have to close. The second extraction rights go to the hydroelectric plant, which was built after Holme Pierrepont, and is just downstream from the sluices. Again, if the water level drops too far, they have to turn off. If there's only enough water in the river to fuel those two the pass must close, otherwise you'd be draining the river to keep the fish pass open, which wouldn't work.

The velocity is key; if there's only a trickle coming through the fish won't like it anyway, so there's just no point. The radial gate is automated; it will close whenever water levels go below acceptable levels, which is expected to happen every summer, which is not the fish migration season anyway. However, when the radial gate is opened, and the water starts to flow into the pass, it only takes a couple of minutes to get to the point where it starts working again.