Our project to make it possible for eels to swim upstream past man made barriers in Denham is complete. We have successfully installed Eel Passes, to allow eels to negotiate weirs in the River Colne, at New Denham and in Denham Country Park where the Rivers Colne and Frays meet on the border of Buckinghamshire and Hillingdon.
It’s a shocking fact that eels manage to swim all the way from South America, yet they are dying out because manmade structures like weirs stop them going up our English rivers, which is why an eel pass is necessary to let the eels swim upstream past these weirs.
European eels (Anguilla anguilla) once thrived in London’s rivers but the number of young joining the adult populations has dropped so dramatically since the 1980s that they are now classified as ‘Critically Endangered’. They migrate over 3000 miles all the way from the Sargasso Sea to European freshwaters, where they live most of their adult life. However, their migration is often prohibited due to physical barriers in the river channel, such as weirs, preventing access to their living habitats which affects the resilience of eel stocks within the river catchment.
We worked in partnership with Affinity Water, Thames Water and The Environment Agency (with additional funding from Grundon) to resolve barriers to the migration of European Eels in a number of locations in the Colne River Catchment. One of the barriers was identified at Denham Lodge (also referenced as Kings Island) in New Denham where the River Colne is culverted and flows under Willow Ave, which is where the weir is located.
The other site in Denham Country Park, on the border of Buckinghamshire and Hillingdon, hosts two weirs that are located in two separate channels known as the Colne & Frays Offtake Weirs. Both of the weirs are currently used as Environment Agency telemetry stations as well as controlling the flow between the Colne and Frays rivers. For this reason, they cannot be altered in any way that may impact their primary purpose. The design and installation selected were: A) modular fed eel pass on the River Frays; B) Larinier fish pass with a separate channel for the modular fed eel pass.
Hydraulic calculations were performed for both sites to ensure suitable functionality of the proposed passes and to ensure there was no increase of flood risk. These are low impact solutions that do not significantly alter the flow of water over the weirs whilst still bringing a great win for the local fish populations.
Installation at Kings Island, New Denham
Delivery of the eel pass frame.
Installation of the eel pass frame.
Frame with eel tiles attached.
What it Takes to Install an Eel Pass
For Kings Island we designed and installed a modular eel pass, mounted onto the edge of the weir. An eel pass is a shoot with water running down with special climbing substrate. This could be made of eel tiles which have a surface like the bottom of football boots or an eel brush. This is a simple but effective solution to eel passage with minimal impact and disturbance.
To start off with, sandbags were put in upstream of the weir to enable the contractors Aquamaintain to work in safe conditions for the installation, by drying out the channel. The eel pass was then attached to the wall, and the special tiles attached to the eel passage frame.
We would like to thank all the local residents for their patience during the installation of the eel passes. This meant the contractors could install a crucial eel pass on the barrier to help support healthy eels here in the Colne Catchment now and into the future.
The bankside path at Denham Country Park was due for some restoration and this is scheduled for later this Spring.
These structures will be continually monitored to ensure they are still functioning.
This project was managed by Chloe Crompton from Groundwork South.
You can find out more here:
Installation in Denham Country Park
Pontoon installed in the river.
sandbags to dry out the river to make installation possible.
Installation complete.
