Unlocking the Severn – Holt


Unlocking the Severn was a huge-scale river project, restoring connectivity for migratory fish on the UK’s longest river and engaging millions of people with the fascinating natural, cultural, and industrial heritage of the Severn.

Located next to a very steep-sided river cutting, the fish pass at Holt was the most challenging to construct during the Unlocking the Severn project. It took two years in total to complete (including a break during winter) and as a result was the final fish pass to be opened to the river. It is a deep vertical slot fish pass, the same as at Diglis, albeit with interlocking concrete blocks (weighing 2 tonnes each!) forming the outer walls rather than steel sheet piles.

The biggest challenge at Holt was complete lack of road access, meaning that all the materials, equipment, and people had to be transported to site
by boat! This included all the diggers and a huge 100 tonne crane which was carried down the river on a massive floating pontoon. Being only accessible by
water, this site was much more sensitive to weather and flow conditions compared to the other fish passes with land-based access. If river flows were too
strong or water levels too high the team could not safely access the site or transport equipment.

Works commenced in May 2020, initially in setting up the compound 1.2km upstream of the weir before then starting to float the plant and equipment
downstream. The first task was to construct a temporary dam which consisted of over 3,500 bags of stone. Excavation of the fish pass channel then began
along with the installation of 10m long soil nails to retain the landward side embankment.

Unfortunately, works were heavily affected by summer storms and particularly the 3rd October 2020, the wettest day in the history of the UK. This
flooded the site completely and was followed by further rainfall events resulting in the 5th wettest October in over 150 years. As a result, works ceased
over winter and resumed in Spring 2021.

Upon return, there was some significant winter damage from the wettest February on record but thereafter, work progressed well during 2021 including completion of the excavation, placing of the interlocking Lego-style block walls and the fish pass channel works. The channel works in particular took
significant time, with all of the concrete required to be transported by barge in small mini skips.

The fish pass was opened to the river in January 2022, in plenty of time for the spring migration.

This project took place in various locations, please read Unlocking the Severn (Diglis) for the detailed project breakdown.

Start date: August 2016
Completion date: August 2016
Type: Children & Young People, Community Outreach, Fish Passage, Skills Development, Volunteers

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