Monday, January 28, 2013

Brazil Nightclub Stampede: Trampling Physics

Disaster struck this weekend in Brazil as a fire broke out in an extremely packed, overcapacity nightclub in Brazil, immediately sparking panic among the partygoers. Thousands of patrons rushed to the club's exits, and one person fell. Tramplings soon followed.

Over 230 people died from tramplings, smoke inhalation and falling debris within the club. Many of the victims were students celebrating summer break.

A number of factors seemed to contribute to the tragedy: confused nightclub staff, double capacity and sheer panic. Physicists have been studying the dynamics of such crowd stampedes for awhile now, drawing from the field of fluid dynamics. Scientists have offered a number of preventive measures in light of this research, and perhaps these studies can prevent future tragedies.

The Griffin nightclub in New York. Image Credit: VancityAllie via flickr.


"Lifted Out of Their Shoes"


Earlier research (~2007) in crowd dynamics has been inspired by similarly tragic crowd disasters. Two authors published one such paper after nearly 350 people were killed in a stampede during the Islamic pilgrimage (hajj) to the holy city of Mecca.

Physicist Dirk Helbing and his engineering colleague Anders Johansson from the Dresden University of Technology in Germany teamed up with Saudi researcher Habib Zein Al-Abideen for the study. They found interesting parallels between physics and crowds.

Looking at video recordings of the stampede, the researchers observed three types of crowd flow emerging: laminar flow, stop-and-go flow, and "turbulent" flow. These types of flow set in with increasing crowd densities.

Laminar flow occurs when a fluid travels smoothly with many of its physical properties remaining constant throughout. You can see a great laminar flow experiment in the video below.



Once the crowd became denser, the mass of people mimicked a stop-and-go fluid. This happens when density reaches a critical value and the crow flow drops to a critical value, causing waves to propagate in the crowd.

Finally, the most dangerous type of flow happens when crowd "pressure" is at its highest, leading to turbulent flow. Turbulent flow in a crowd leads to all sorts of trouble.

In their work, Helbing and his colleagues defined pressure as the crowd density (people per square meter) multiplied by how much the nearby crowd velocity varied from the average velocity for the whole crowd. High pressure spots sparked the initial tramplings during the hajj.

According to the researchers, observers of a previous hajj saw shock waves in the crowd propel people up to 3 meters forward, sometimes squeezing them out of their shoes and ripping their clothing. This extreme crowd density can cause asphyxiation even without tramplings.

To help alleviate pressure during subsequent pilgrimages, event organizers better tracked the density at all times and widened bottleneck areas. These solutions appeared to help, but can these solutions carry over to smaller settings such as the Brazilian nightclub?

Columns and Crowd Rhythm


Research more focused on indoor settings has revealed a number of other potential measures to take. Research in 2009 by a Japanese group found that adding columns along the exits of a building can help prevent stampedes.

Albeit a bit counterintuitive, this blocking strategy helps maintain an orderly flow out of a building. As Scientific American reports, these obstacles may reduce evacuation times, but the researchers cautioned that human psychology complicates the matter. You have to give the researchers credit for treating humans as more than mere particles in a simulation.

More recent work from some of the same researchers revealed another curious finding. Apparently, slowing down every individual's pace in a crowd can increase a high-density crowd's flow velocity. Coined "slow rhythm," the technique proved itself when the researchers tested it with people walking in a circle at various speeds and densities.

Participants in the experiment walked in a circle at various speeds and densities (the number of people walking in a fixed circle).
Image Credit: APS/Yanagisawa et al.


Upon reaching a critical density, the slower-walking crowd actually completed their laps faster as a group, possibly avoiding the stop-and-go and turbulent flows seen during the hajj disaster of 2006. If a DJ could somehow keep a crowd calm and moving to the beat, maybe more people could escape swiftly and safely. You'll probably never find that amount of coordination during a panic such as the recent Brazil disaster, however.

Nonetheless, this field of research has introduced some promising solutions that have worked in some applications in the past. Hopefully, tragedies like the Brazilian nightclub fire and stampede can be avoided in the future with a little help from physics.

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