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Beneath a parking lot in St. Helier, a giant feat of engineering is increasingly at the forefront of Jersey’s response to climate change.
Carved out of granite in the mid-1990s, the cave stores excess sewage and rainwater during storms, stopping pollution in St Aubin’s Bay.
It is currently used an average of 50 to 80 times a year.
With more extreme weather expected in the future, the huge space could be used more often.
A smaller, second cavern is due to be built by the end of 2023, while the government is considering the need for additional rainwater storage tanks.
Dimly lit and clad in damp, dark concrete, the underground structure can hold about 25,000 cubic meters of rubbish and was largely completed in 1997.
Since then, it has helped Jersey’s Bellozanne Sewage Treatment Works deal with wet weather conditions that are adding pressure to an aging drainage network already battling the demands of a growing population.
“When we get a lot of rain, we get high levels of sewage that Bellozanne can’t handle. Before the cave was built, that would have gone into the sea,” says Duncan Berry, head of liquid waste for the Jersey government.
“Now the cave will collect and store those flows and we can empty it back into the sewer at a later date if there is capacity.”
Berry said a second cave, which can store 8,000 cubic meters of waste and will open in Bellozanne by the end of the year, as part of a new sewage treatment plant, will help the island weather future storms.
After delays and after the main contractor went bankrupt, the entire project is now expected to cost £83.3m, down from £75m.
“Obviously we want to invest constantly, we need more money to develop the island,” said Berry.
“A lot of new houses are being built, so as we build a new sewage treatment plant, we have to look at the network now – to expand the network, to increase the capacity – so there will probably be a lot more mini houses.” caves around the island, but only for dirty water.
“We are currently reviewing all of our surface water management plans to account for climate change in the future.”
As tall as Jersey’s air traffic control tower and so tall you’d have to flush your toilet almost three million times to fill it, the cave is accessed through large double doors in the center of Snow Hill’s parking lot.
It undergoes an annual clean-up, with a team of up to eight people dressed in full protective gear spending about three weeks spraying the surfaces with fire hoses.
Using automated pumps, the sludge is then collected and transported to La Collette for incineration.
Staff said the process often revealed multiple items hidden in the murky water, from food wrappers to wet wipes, while a cannonball and live toads were among the more unusual discoveries.
“Cleaning it was very difficult in medieval times,” says Rob Lightfoot, operator of the pumping station, who has worked in the cave since its construction.
“It was a lot of manual operation and a lot of staff.
“Now the team is relatively small and we have great equipment to help us get the job done.”
Construction of the cave ended up costing Jersey taxpayers £22.4 million, which was more than double the initial estimate.
And while the project initially proved to be controversial, it has more recently been praised.
In January, politicians gave credit for mitigating the effects of flooding on the island after heavy rainfall.
“It saved hundreds and hundreds of spills to the island,” said Berry.
“It was a brave decision to build it. We don’t have a lot of land in Jersey so it’s probably the only solution we had.
“This is an asset that will last for hundreds of years.”
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