A river diverted to make way for a new multi-million pound underpass on a main Belfast thoroughfare was today believed to have been responsible for the flooding of the new roads scheme.
It was thought to have overwhelmed the culvert it and been channelled through when Northern Ireland was hit by torrential rain on Saturday afternoon.
Motorists had to be rescued from their cars as vehicles disappeared under 20 feet of water the first time the roads scheme had to cope with a serious downpour.
The Broadway underpass on the Westlink - which links motorways leading to the north and west of Northern Ireland - filled with more than 100 million litres of water in a couple of hours.
Carrying some 70,000 vehicles a day the underpass is a central part of a £115 million upgrade of the Westlink route through Belfast.
Just last month it was opened with much fanfare a year ahead of schedule with Leo Martin, project manager for construction firm HMC, saying the realignment of the Clowney River to the south of the underpass had allowed them to complete the work early.
It now looks as if that river overflowing was the cause of the flooding.
As huge pumps were still being used to empty out the water - enough to fill 40 Olympic-sized swimming pools - HMC were looking at what went wrong.
Mr Martin said it seems clear that the cause of it was the adjacent river that overflowed. He said 57mm of rain fell in a few hours - 70 per cent of the total expected for the month - and the river diversion had been unable to cope.
Mr Martin said culverts had been created to withstand once in 100 years flooding, with additional capacity to cope with global warning. "Such was the deluge that it was greater than anyone expected."
The contractors - who are responsible for maintaining the new road system for the next 30 years - are preparing a report for the Department of Regional Development's Road Service setting out what happened - and they will have to say what measures they plan to ensure it doesn't flood again.
As the water level in the underpass dropped during pumping operations, it exposed tonnes of mud up to two feet deep.
Diggers were used to remove the mud and workmen hosed down walls and road surface of the underpass to clean things up for an expected re-opening tomorrow.
Clearing up operations continued across Northern Ireland with householders whose homes were flooded told they could apply for emergency Government aid of £1,000 to help them through the early days of their personal crisis.
Ministers don't yet know how many were affected, certainly hundreds, quite possibly thousands.
Meanwhile more rain fall and weathermen warned of yet more to come right across Northern Ireland.
As they did so the Belfast - Dublin Enterprise express train service was disrupted by flooding near Portadown, Co Armagh - where the local Craigavon Council said at least 60 homes had been flooded and there were concerns over the River Bann bursting its banks.