35W storm drain geyser

On a stormy night in 1999, a MnDOT employee has pointed a traffic camera at a huge round grate in the median of Interstate 35W.  Occasional cars pass by and lightning flashes.  Then without warning, a geyser of water blasts high into the air.  The cloud of mist obscures our view of a pickup truck crashing into the heavy steel grate, relocated to the center of the northbound lanes by the force of the water. 

As round two bursts forth, a person can briefly be seen running for high ground before the mist once again obscures our view.  With the protective grating now removed, getting washed "down the drain" by the receding water would certainly be a fatal trip.




The video has made the rounds on the web, and many people have seen it.  I doubt many people know that this is actually right here in Minneapolis. The video is from MnDOT traffic camera #622 at Interstate 35W and 35th Street in south Minneapolis.  Here's the intersection as seen from above:



View Larger Map

The grating in question caps the top of a ventilation shaft for a major storm drain tunnel that runs far beneath 35W for many miles.  The tunnel serves as a main arterial route for stormwater runoff from the entire freeway to drain into the Mississippi River.  The main pipe deep underground is large enough to drive a bus through.  The outfall for this drain happens to be located directly beneath the 35W bridge over the river, about 3.5 miles from the location of the geyser in question.

So why does this happen?  There appears to have been an engineering error somewhere along the way.  As more and more feeder pipes drain into the single main pipe, it needs to get larger to accommodate the extra water.  It seems that the sizing isn't quite able to keep up with a torrential downpour, and the pipe fills to the ceiling somewhere downstream from this point.  The water continues to flood into the pipe further upstream, and the excess has to go somewhere.  In this case, it gushes up a deep shaft and blasts into the air at 35th street.  Safety concerns aside, it's quite a show. 

With the recent (and ongoing) reconstruction of 35W in this area, I am not certain if the plumbing has been changed to correct this issue, or if the grating still exists as seen here.