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Edited on Thu Jul-29-10 02:00 PM by NoNothing
The problem is not really the deep tunnel, the problem is combined sewers. This is a failure of local politics. Let me explain.
In most cities, there is a sanitary sewer, which goes from sinks and toilets and drains to a treatment plan. Separate from that and completely disconnected is a storm sewer, which is usually much larger, and collects water from streets and parking lots and dumps it, untreated, in a river, pond, lake, or ocean.
In downtown Milwaukee and Shorewood, municipal sewers were installed at a relatively early time, due ironically to the forward-looking progressive German socialists in power at the time. However, these early systems had a significant design flaw: raw sewage and rainwater runoff use the *same* sewer lines.
Fast-forward to recent times. The proliferation of streets and parking lots has led to common problems during heavy rainfall: all the rain, flowing into the sanitary sewer, overwhelms the treatment facility and has to be dumped without treatment. There are two ideas for dealing with this problem: the first is to separate the sewer lines like most cities do, and the second is to build a huge holding facility to collect the rainwater and sewage during rainfall so that the treatment plant can get to it when it's not raining. They chose to build the deep tunnels.
Unfortunately, this was not the best solution because while it did decrease the frequency of dumping, it didn't help as much as anticipated because it did not increase the rate at which the treatment facility could process the sewage. Furthermore, it did not solve another serious problem: during *very* heavy rainfall, the combined sewers, being smaller than most storm drains, simply cannot drain the water fast enough, and back up through the sanitary lines into peoples' basements, sinks, and toilets. The deep tunnel doesn't help this problem *at all* - the water never has a chance to get to the tunnel because it can't even fit through the intake pipes. *That's* what caused most of the recent damage and suffering. Storm lines, even if the overflowed, would at least just flood the streets with rainwater, not back up into peoples' homes.
EDIT: Contra to the OP claims, though, what backed up into basements and what was dumped into the lake is not really "raw sewage" because it was almost entirely rainwater, with a little bit of sanitary sewage mixed in. It's not like when your drains are plugged and everything that backs up is what originally went down the drain.
So why didn't the city just separate the lines in the first place? Politics, and rather ugly ones. See, Milwaukee pushed to create a sewage district called MMSD that would operate the sewers for the entire metropolitan areas, including the relatively affluent suburbs, and the Milwaukee mayor appoints a majority of the board. The problem is, separating the sewer lines specifically in Milwaukee and Shorewood would not be *system* upgrades, but rather local ones, which means that those cities would have to pay for it themselves. On the other hand, the Deep Tunnel, being a *system* improvement, could spread the financial burden across the entire district. As a result, they chose to do the Deep Tunnel, hoping that Milwaukee and Shorewood could avoid having to pay to separate their own lines.
Unfortunately, it didn't work as well they'd hoped it would, and now we're having to pay the price for their lack of vision.
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