It was caused by fluid escaping under pressure from a vent following a failure in the sewer network.
Problems arose after heavy rain this week caused the 25 million-litre flood-prevention Cavern, several metres under Snow Hill car park, to fill to capacity.
Duncan Berry, head of liquid waste management for the Growth, Housing and Environment Department, said: ‘We have experienced a deluge of rain after a prolonged dry period – almost 65mm – which is a significant amount for any system.
‘Due to this recent heavy rainfall, the cavern has fully filled with very diluted sewage.’
Mr Berry added that when the cavern reaches capacity, a penstock – a sluice gate that controls water flow – opens and allows excess water to flow out to sea.
‘However, this morning, the penstock tried to open once the cavern was full but failed, causing the foul sewer network to back up and pressurise.
‘The air vent above the penstock was able to relieve the built-up pressure and was necessary in order to prevent the manholes in the roads and pathways popping up,’ Mr Berry said.
‘When the penstock failed to open we received an emergency alarm from our telemetry system.
‘Engineers went to the scene to manually open the system up.’
