Working with the Environment Agency in Polperro, Cornwall

Polperro is protected by a relief tunnel, into which the river is progressively diverted as flows increase.  This is controlled by a stilling basin and a cleverly profiled weir, all buried below ground.  The stilling basin, however, accumulates sediment and vegetation, which eventually impairs the performance of the system and has to be cleared.  Because the river constantly flows through the system, previous operations have used a dive team to pump or suck the sediment into the tunnel, from where it continues its natural passage to the sea.

With the increasing cost of the diving operation, the Environment Agency's Operations Delivery Technical Support section devised a scheme to clear the material using a siphon arrangement.  By reducing the water level in the basin, Glanvilles were able to use a trained Confined Space team to enter the space and effect the clearance.  The tunnel invert is some 12 metres below the floor of the stilling basin, giving a practical head difference of around 8 metres to drive the siphon.  This was built up of 300mm pipework, primed by a pair of 150mm diesel pumps.

With the river running constantly, it was expected that the team would have to deal with more flow than the siphon itself would take, but in the event, flows were less than the system capacity and it had to be used in bursts.  Around 300 tonnes of material has been moved and the flood Alleviation Scheme is now back to its designated state.

The obvious power of the siphon itself ensured that the team made certain they kept well away from it while working!  The other risks associated with the operation were covered by a thorough assessment, communication within the team and a carefully defined system of work.  A few detail improvements are planned to the set-up so that repeat operations can be quicker and more frequent.