London sewer: Milwaukee residents warn of flooding
Residents of a US city with a similar "super-sewer" to the one planned for London have said several basements were flooded during downpours in 2010, despite the presence of the tunnel.
Several basements in homes in Milwaukee were damaged by sewage, they said, but officials stressed it was impossible for the sewer to back up into homes.
Thames Water said the US tunnel had achieved everything it set out to do.
The firm added that it was not designed to deal with basement flooding.
It also said the proposed Thames Tunnel would not be designed to deal with basement flooding and that it had many other schemes to address this.
BBC London went to Milwaukee, in the eastern state of Wisconsin, because Thames Water project manager John Ramage said its sewer was the example to follow.
The area has seen at least two severe storms in 17 years and the sewer "performed exceptionally" in that time, he added.
But Emmitt Ramsey, from North 19th Place, said his basement was one of several flooded with sewage in June 2010.
He believed the tunnel "had overcapacitated itself with the amount of rainfall, so instead of letting the water go out into the lakes, or whatever, a lot of this backed up in the residences".
Milwaukee Metropolitan Sewage District (MMSD) said it was impossible for the tunnel to flood basements because it was 300ft (90m) underground.
It said the flood was caused by huge amounts of rain overwhelming local sewers.
But Mr Ramsey disagreed.
"They were telling their citizens that they got 1.5in (4cm) of rainfall within an hour and the tunnel couldn't handle it," he said.
"Either they didn't have it deep enough, or it's not equipped for the amount of rainfall.
"The deep tunnel is not doing what it's supposed to do - that's my opinion," Mr Ramsey added.
Steven Cameron lives nearby and said his basement was also flooded.
The storm was probably the worst he had ever seen, with about 7in (18cm) of rain falling in about 90 minutes.
In a lot of homes water "came up through their drainage system", Mr Cameron added.
Tunnel 'was oversold'
Milwaukee radio talk show host Mark Belling, from 1130 WISN, has regularly spoken out against the tunnel.
He said it was the biggest waste of money "probably in the history of the city of Milwaukee".
"They now acknowledge you could never build a tunnel big enough to prevent all sewage dumping.
"Well, if you can't build a tunnel big enough why are you building a tunnel? It just doesn't work."
Bill Graffin from the MMSD, said the tunnel cost $1.2bn (£738m) and had cut the amount of sewage dumped by 85%, although he admitted the tunnel "was oversold".
"What it was designed to do, it is actually doing very well.
"It has improved the water quality of this region, especially in the combined sewer area, and done a fantastic job at reducing the number of sewer overflows that we have that end up going to Lake Michigan."
But Alderman Willie Hines, the president of Milwaukee Common Council, said if he had billions of dollars to spend now, a "super-sewer" would not be top of his shopping list.
"I would look at it as part of the comprehensive strategy but it would be the third leg."
The first priority would be prevention and improvement work to stop basement flooding, he said.
His second option would be stopping rainwater from entering sewers by disconnecting pipes, known as downspouts, and setting up rain gardens and green roof initiatives.
The deep tunnel would be his third option, once he knew the scale of the remaining problem with sewage overflows.
"If I were in London's situation, I would learn from Milwaukee," he added.
MMSC said it had spent more than $20m (£12.6m) on projects designed to reduce the amounts of stormwater entering sewers and had sold 16,000 rain barrels since 2004.
Thames Water said the design and construction of the Milwaukee tunnel was being used in the development of the proposed Thames Tunnel.