VolkerStevin develops one of a kind gravity eel pass at the Pevensey Levels
VolkerStevin was contracted by the Environment Agency to upgrade the current water control structure at the Pevensey Levels in East Sussex.
The structure had reached the end of its working life, and a new solution was required to improve water level control and eel passage. A collaborative approach was taken to develop an innovative solution for a new eel pass, JBA Consulting led on the design, supported by VolkerStevin and internal Environment Agency specialists providing advice and technical support.
The Pevensey Levels, East Sussex, is a wetland designated as a Site of Specific Scientific Interest (SSSI), a Ramsar Site and a Special Area of Conservation (SAC). It is 3,600 hectares tract of low-lying grazing meadows, intersected by a complex system of ditches that sustain several nationally and internationally significant plant, invertebrate and bird species. Star Inn Gates is the main water level control structure on the eastern Pevensey Levels. The structure has reached the end of its working life and presents a barrier to upstream eel migration. The project consists of the design and installation of replacement gate structures, as well as the incorporation and compliance with eel passage measures and regulations.
A specialist eel pass solution was required as the upstream water levels vary significantly, around 1500mm between the summer and winter months, meaning a static gravity fed eel pass would not have been suitable. A static pipe would have spent a significant portion of time as either a downflow pipe or completely out of the water and therefore provided no access for the eels. As it is necessary to retain high water levels upstream of the gate, the flow through the eel pass must be kept to a minimum, ruling out a bypass channel.
A pumped eel pass was considered, but due to the likelihood of pump failure, power consumption, long-term maintenance of the eel pass, efficiency of operation and the desire to provide a passive solution, it was not taken further.
Working with the Environment Agency’s fisheries specialists, the team developed an innovative gravity eel pass, that required no power source and minimal maintenance in operation.
The eel pass exit comprises a floating articulating swivel pipe, which can adapt to seasonal variations in the water level. The novel adaptability is achieved by incorporating ballasted floats to the pipe, which control the degree of submergence regardless of the angle of the pass exit pipe. This floats above the water level, preventing it from becoming a downflow pipe, but still allowing sufficient water to enter the system and eels to ascend the pass. The downstream entrance of the pass is fully submerged and fixed, passage media is provided using mussel rope, commonly used in the aquaculture industry.
The design is entirely passive and will ensure a consistent flow rate, without any manual or electrical operations. The team also prepared a maintenance plan to ensure the asset is maintained appropriately over its design life.
The articulated arm is housed in a protective structure to protect it from damage during high flow periods, the location of this also minimises the amount of construction and maintenance needed in or above water. The remainder of the pass is constructed using standard drainage products, pipes and manholes, and the use of the rope allows the pass to be cleared of any debris easily and without water entry.
Once completed, the pass’s operation will be monitored, with the aim of using the data collected for similar projects.
Matthew Alcock, VolkerStevin Senior Project Manager, commented: “We're proud to be delivering a solution that both protects the environment and addresses the waterflow management needs at the Pevensey Levels. We have installed an innovative gate, which provides the flexibility needed to control water levels throughout the year.
“An important part of the project has been working collaboratively with our supply chain partner JBA, on developing an innovative gravity eel pass that requires no power source and minimal operational maintenance. The solution developed is one of a kind, adapting to the changing water levels at Pevensey Levels.
“With the first gate successfully commissioned, we are now focused on completing the second gate, ensuring long-term resilience for this vital wetland."
Daniel Sargent, Flood Risk Management Advisor at the Environment Agency, said: “The delivery of this project has enabled us to continue to manage the water levels for Pevensey Levels, a wetland Site of Specific Scientific Interest (SSSI), as well as developing an entirely innovative design for an eel and elver pass.
“We look forward to seeing the eel pass operational and seeing how this innovation can influence other eel pass designs across the country.”
