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Drainer pumps help give fish a free passage

Monitoring and maintaining water quality is not the only challenge facing the Environment Agency (EA) in its task of managing the UK's rivers. The EU Water Framework Directive (WFD) also requires the EA to ensure that migratory fish have free passage in UK rivers, to help maintain population numbers by enabling the fish to travel to their spawning sites.

A section of the river had to be drained to give access to the river bedA section of the river had to be drained to give access to the river bed

The legislation has led to a programme of fish pass schemes, with modifications to rivers to enable the fish to travel upstream. A new fish pass built in the River Bollin at Wilmslow in Cheshire provides a typical example. Working with EA framework contractor Balvac, specialist contractor CT Construction was tasked with building the fish pass, which involves installation of a pre-cast concrete weir structure below the water level.

Before construction could begin the contractor first had to drain the section of the river where the fish pass was to be sited to give the team access to the river bed and ensure health and safety prevailed on site during the programme.

Oliver Tolputt, at CT Construction, explains: “Site preparation began with the construction of a temporary dam, created by placing bags of dry stones across the river to isolate the site. Once this was ready, we could begin the over-pumping operation to drain the site by pumping the water over the temporary dam while construction took place. However, the dam could only slow the flow of water back to site during the programme rather than prevent it completely.”

CT Construction turned to pump hire specialist Sykes Pumps to help specify the correct pumping equipment and configuration and set up the equipment on site. Sykes provided five PX 30 submersible drainer pumps for the site, which were supplied and installed with 40m of hose and 12” pipework for each pump to enable CT Construction to pump the water 40m downstream of the dam.

Chris Graham, from Sykes Pumps, comments: “Our new range of drainer submersible pumps offer easy handling and excellent capacity, making the pumps ideal for dewatering at all kinds of construction sites from the smallest building project right up to the biggest civil engineering contracts.  For a project like this, where the team is going to be working on the riverbed, the pumps ensure that the project experiences no delays due to water ingress during construction and that all operatives are safe throughout the programme.”

Once the site had been drained, the pumps remained in-situ and operational on a 24/7 basis to ensure the site was sufficiently dry during the four-week construction project. Sykes also provided regular maintenance visits as an integral part of the hire contract to ensure the pumps were operating at optimum efficiency throughout the programme.

CT Construction began by excavating the river bed, creating a trench in preparation for the pre-cast concrete fish pass structure to be installed. A blinding concrete base was then laid to create a flat, even surface for the pre-cast structure.

Tolputt continues: “The blinding concrete base was essential for creating a flat surface for the fish pass. Blinding concrete can withstand much lower pressure than structural concrete but it is ideal for a lighter duty construction project like this.”

Once the blinding concrete base was laid, CT construction could lower the pre-cast concrete fish pass into place using a crane on the river bank. The man-made weir was then permanently fixed to the concrete base with concrete tie-ins to complete the salmon pass structure.

In addition to the main concrete weir structure that makes up the salmon pass, the new pre-barrage fish pass installed in the River Bollin at Wilmslow also includes an eel pass, which comprises bristles that are designed to aid the eels’ transit over the concrete and were installed on the concrete tie-ins.

Once the fish pass construction was completed, the material that had been excavated from the river bed to enable the concrete base for the fish pass to be laid was used to back-fill the site before the temporary dam was removed and the river was allowed to flow back over the site. While the scope of the original project only involved construction of the fish pass, the act of draining the water from the site revealed some erosion of the river banks. As a result, underpinning of the banks with sheet piling and concrete was also added to the scheme.

Tolputt  says: “The project has significantly enhanced this section of the river, both by protecting fish populations and by restoring an area of the bank which could have affected the water flow. An efficient pumping operation was critical throughout the scheme, and Sykes Pumps ensured that we had the efficient equipment needed to maintain a dry site until all works were finished.”

Topic: Pumps & Pumping Systems
Tags: Rivers , fish , environment agency , concrete

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