This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Sunday, January 30, 2011

Fight for Cairo on Bridge: A dangerous place--from a fluid mechanics view

The crowds on the Kasr al-Nil Bridge in Cairo on Jan. 28, 2011
Photo by Peter Macdiamid/Getty Images
Published in the New York Times on-line on the date above
A crucial moment in the unfolding events in Egypt occurred when the crowds of protestors tried to cross the Kasr al-Nil Bridge in Cairo tried to reach Liberation Square by crossing this bridge.  Riot police kept them at bay by using gas, water canons, and truncheons. The situation is described in this NYTimes article. The battle between the protesters and the police ebbed and flowed for more than 6 hours.

What does this have to do with fluid mechanics? A lot--as you can see in the picture, a large number of people are trying to squeeze from the streets into the confined space on the bridge.  There is a law in fluid mechanics called "conservation of mass."  In the broad street or park approaching the bridge, the number of people moving toward the bridge is given by their density times their forward velocity. You can think of it as the number of "lanes" of people moving forward times the velocity that they are moving in the lanes. On the bridge, the same law applies, but the number of lanes is greatly reduced.  For a steady state, the flux of people has to be the same, which means that the people on the bridge should be moving much faster than the ones approaching the bridge.

Of course, this can't happen in a crowd, and so the bridge gets jammed--even without riot police at the far end.  This situation occurs all of the time in traffic jams when the road narrows.  The problem is that any slight event can cause the jam to turn into a stampede, with injuries and deaths.  I was amazed, and relieved, that this seemed not to happen in Cairo. Just a few weeks ago at least 104 pilgrims were killed and another 50 injured in a stampede of pilgrims in India.  Stampedes have killed people in a wide variety of venues--synagogues, theaters, meeting halls, train stations, air raid shelters, stadiums and, yes, Walmart (November 28, 2008).

And yes, this does relate to fluid mechanics--one model for traffic flow is based in river hydraulics!

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