Background Information
Groundwork South, Affinity Water and private landowners worked in partnership to resolve barriers to the migration of European Eels in the Colne River Catchment in Denham. This was an exciting project to improve eel and fish passage in Colne Valley Regional Park and deliver environmental benefits for an endangered species.
Scope of the Project
The weir structure at Denham Country Park is an impressive structure controlling the flow between the Colne and Frays rivers as well as hosting the Environment Agency’s telemetry stations. These factors mean the weir cannot be altered in any way that may impact its primary purpose and changes to the water level and split between the two weirs must be minimal. Aquamaintain Ltd were awarded the design and build contract for fish and eel passes on Colne and Frays Weirs. The team had to come up with a solution that enabled, passage of both eels and a variety of fish species, did not impact the function of the gauging weirs or the split of water flow, and did not endanger the safety of the public using the park. Due to the technical nature of the passes the designs had to be approved by the Environment Agency (EA) National Fish Pass Panel.
Our Solution
To begin a full topographic survey of both weirs was completed with the use of a Leica GS18 GNSS. Aquamaintain Ltd consultants and inhouse engineer then worked with an external hydrologist to develop a suitable pass design for each of the two weirs. Initial designs were then passed to EA fishery technical, and hydrometry and telemetry specialist for informal feedback. The Aquamaintain Ltd design team and hydrologist worked closely with stakeholders from the EA and Denham Country Park to create designs that met all the key crite ria. A Larinier fish pass with additional eel pass channel was developed for the Colne weir. This required covers and an inlet screen to prevent access of the public who used the area for swimming. A notch was cut into the Frays weir with lift out eel ti les fitted along the sheet piling to enable passage of fish and eels. Hydraulic calculations were completed to ensure no change in flow split between the Frays and Colne Weirs. The change in water levels was negligible and required some minor recalibration of EA telemetry and gauging systems.
