Sewage has spilled into the sea 40 times since the Cavern was temporarily closed

Sewage has spilled into the sea 40 times since the Cavern was temporarily closed

In May, engineers moved into the 80-metre-long, 17-metre-wide subterranean facility – which is 60 metres beneath the Fort – to complete a £533,587 project to install a new cleaning system. The work is aimed at reducing the number of trips workers have to make into the chamber, which can hold 25 million litres of water.

Normally, the water that has spilled out into the sea in recent months would have been collected by the Cavern. However, as this has not been possible, it has been diverted to an outfall near the Elizabeth Marina. The government say that the sewage that flowed into the sea was ‘extremely diluted’.

The Cavern cleaning-system project was deliberately timed to begin at the start of May to minimise the number of spills, but the Island has faced repeated heavy rain in recent weeks, with last month being the second-wettest October on record.

It follows three unconnected spillages of untreated ‘extremely diluted’ sewage in three months from a pumping station adjacent to the Dicq Slipway in St Saviour. During the first spillage, in August, used sanitary products were found on the beach nearby.

In a statement, a spokeswoman for the government said: ‘Since the Cavern has been out of action for refurbishment there have been 40 recorded spill events. This is not raw sewage, but extremely diluted sewage.

‘The Cavern is a great asset and is due back in service in December, when it will once again be collecting the majority of these flows.’

She added: ‘As part of the sewage treatment works upgrade, we are building additional storm storage tanks at Bellozanne, so will have additional capacity in the future.’

Prior to the 1950s, at times of heavy rain, the Island’s Victorian sewers, which carry both foul water and rainwater, drained directly into the sea. Because of this, the government began work to build separate foul-water and surface-water sewer systems. But due to the built-up nature of town, this was not possible in much of St Helier.

As a result, in the early 1990s, engineers – using a similar type of machine used to construct the Channel Tunnel – began building the Cavern.

Normally, water from the sewers travels from town along a network of pipes to First Tower pumping station and onwards to Bellozanne for treatment.

However, at times of heavy rain, the water reaches the Weighbridge, where excess liquid falls over an underground weir into a large chamber and then drains into the Cavern through a pipe.

When it stops raining, water from the chamber is then gradually pumped – normally over a number of days – to the sewage treatment works at Bellozanne to be dealt with.

Despite the construction of the Cavern, for years a large section of the town drainage system remained unconnected to it.

And on several occasions – including in August 2015 – Beresford Street and the surrounding area flooded, causing thousands of pounds of damage to some businesses.

Because of this, a £5.6 million project was launched in 2012 to build a 25-metre deep shaft below the former Ann Street car park and a network of underground tunnels to connect it to the Cavern.

And despite the recent extended period of persistent heavy rain, the area has not flooded.

A Growth, Housing and Environment Department spokeswoman said: ‘The shaft at Ann Street, which takes more surface water flow, has reduced the risk of flooding during the recent heavy rainfall. Without the shaft, there may have been flooding in the area.’

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