The site was situated in a remote river location with narrow roads that limited access - this restricted on-site plant use and required logistical planning prior to the scheme start date. In addition, a detailed Construction Phase Health & Safety Plan and a series of concept designs were developed to minimise in-river working.
The preferred solution of an up-and-over (pumped) eel pass was chosen with consideration to the proximity of working in and alongside the river - more specifically, to avoid disruption to the water supply and any impact on the quality of water entering the WPS.
CAT scans were completed to confirm the location of existing underground services, after which the eel passage route was marked out and excavated. The site team then installed 35 metres of by-pass channel, flow splitter chambers, flushing inlet and return outfall pipework, and power cable ducting.
Stonbury elected to use precast concrete modular trough panel sections, prefabricated channel lids and HDPE dual-sized eel tiles. This method reduced plant and equipment requirements on-site, eliminated risks of delay associated with casting concrete in adverse weather conditions and reduced on-site activities’ duration by maximising off-site fabrication.
Once the complete passage system was installed, final electrical works were subcontracted to supply power from the existing WPS building to a submersible pump and sensor. This was used to pump flows along the eel pass during the hours of dusk when the eels would be moving.
The ground was graded to the top of the trough, and the existing boulders were replaced along the sides of the pipe. The pipe was then backfilled to the original levels as per the client’s instruction. All works were completed in three weeks, without disruption to normal operating procedures of the WPS.
Although the fish pass design minimised work within and close to the watercourse’s edge, the site team – made up of competent personnel trained in Swiftwater rescue – deployed small cofferdams around the inlet and outlet pipes to ensure safe working conditions. For some of the works, they were also able to use battery-operated tools designed for water use - this kept heavy plant to a minimum within a working area restricted for space and reduced overall carbon emissions.