Diglis Fish Pass

Find out all about our biggest fish pass

Diglis Fish Pass – Facts & Figures

This is the first first fish pass that twaite shad and other fish migrating up from the sea meet on their way upriver. It is the biggest of all the fish passes on Unlocking the Severn, and the biggest fish pass of it’s kind in England and Wales at the time.

It contains a unique underwater viewing window that allows us to monitor the endangered fish migrating upriver to spawn and also to give members of the public their chance to have a face to face encounter with wild fish of the Severn swimming by!

This pass is 100 metres long, 8 metres wide and 5 metres deep.  It is easy to lose the sense of it’s size now that the water is flowing through it but do check-out some of the construction pictures in the photo story below where you see how small the construction workers look in the bottom of the pass!

Work began in 2019 and finished after 18 months.

The construction at this site was impacted by two major flooding events in November 2019 and February 2020 and the first COVID-19 lockdown that caused site to close for 5 weeks  in March/April 2020.

Severn Trent Water provided critical access to the construction site through their water treament plant.

The fish pass was designed by FishTek and Kier was the main contractor for the build.

The Diglis Fish Pass was commissioned in April 2021 when it was opened to the river; water flowed down the pass, and almost immediately we saw fish swimming up it!

The viewing gallery was officially opened on 8th October 2021 by HRH The Princess Royal and then public tours began.

About this Deep Vertical Slot Fish Pass

A type of pass, called a deep vertical slot fish pass was selected for this site alongside Diglis weir in Worcester.  The urban location meant there was not enough room to create a long, sweeping by-pass channel around the weir.

Deep vertical slot fish passes are also ideal for coping with the wide range of river levels that we see on the River Severn.  And they also create flow conditions that are suitable for a variety of fish.

The pass is formed by a series of 11 ascending pools.  Each pool is just 20cm higher than the previous one.  This breaks one big jump in the water level below to above the weir into a series of more manageable steps for the fish.

The design of our deep vertical slot fish pass channels on the River Severn incorporates special augmentation channels on either side of the pass.  These help us move a large amount of water down the sides of the pass, to create a strong outflow that is attractive to fish looking for a route upriver, without affecting the speed and churn of water in the pools that the fish swim through.

A layer of stones across the base of the fish pass serves to add roughness, that slows the water down right at the bottom.   Ideal for smaller fish who like to wriggle along the river bottom such as  smaller eels and lampreys.

Read more about how a fish pass works HERE.  

A lens into the River Severn

Rather excitingly, Diglis Fish Pass boasts an impressive underwater viewing gallery! This allows a view into the fish pass, where fish can be seen swimming upstream through the River Severn.

The gallery is important for scientific monitoring of the twaite shad during the spring fish migration. Between late April and early June, the monitoring team record the number of twaite shad using the pass, and also take note of the other species of fish seen during this period. By the end of 2022, an incredible 25 different species had been recorded using Diglis Fish Pass!

As well as providing a space for scientific monitoring, the viewing gallery also allows visitors and school groups to get up close to the wildlife of the River Severn. The window gives people the opportunity to view shimmering shoals or solo swimmers in their own habitat.

Unlocking the Severn run tours of Diglis Fish Pass on select days between March and October. Click on the button below to view the available dates. Please note, tours cease over winter, so between November and early March, the calendar may not show any tour dates available.

25 Fish Species have so far been recorded using the pass!

Watch how the build unfolded.


How this pass was designed

How this pass was built

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