Someone's lunch got water-cannon'd (and what does that have to do with Niagara Falls?)

Credit: Shutterstock

Our local radio station was "on the scene" for a couple of hours this morning with the announcement that a suspicious package had been spotted near a door of the Champaign City building, that traffic had been closed on one of our major streets for a couple of hours, and that the police bomb squad from the University of Illinois were on the scene. Employees were prohibited from entering the building until 9:00 when the situation was resolved.  The police can't take chances because the package, a brown bag, could have been lethal, but the Deputy Police Chief seemed to have a sense of humor when he announced that the scene had been "rendered safe" by turning a water cannon on to the brown bag: "The food [someone's lunch in the brown bag] is now in several pieces all over the sidewalk...I have no idea whose lunch it was."
     So, since this is a fluid dynamics blog (though I may have a bit of trouble relating this to geological fluid dynamics), what is a water cannon? What are they used for?
     They are used for delivering water at fire scenes, for riot control, and in some commercial sectors, e.g., pressure washing of houses and decks. They were first used on retrofitted tug boats in the mid-19th century, and were subsequently brought on shore for scenes where water is not available, or the fires are unaccessible. Fireboats can pump tens of thousands of gallons of water per minute over distances up to ~100 feet, and up to 400 feet into the air.  Fire Boat #2 of the Los Angeles Fire Department can pump up to 38,000 gallons per minute (2.4 cubic meters per second).
     Water cannons are also used in riot control, as has been reported from Cairo. This practice has been reduced in countries with free reporting media at the scene of riots because the scenes are bad publicity for the police departments.  Their use in America was sharply reduced after their use in the mid-20th century to disperse crowds protesting in the Civil Rights Movement.  There have been experiments including both tear gas and dyes into the water stream, so sometimes you'll see pink, purple, or green jets. These are meant to make rioters more identifiable to the police on the scene.
      It was difficult to find any specific numbers for the pressure in the jets, or the force that they exert on people in crowds.  Industrial use machines advertise pressures >3000 psi, 200 bars (200 times atmospheric).  One reference I saw for riot control water cannons mentioned 30 bars.  These pressures can do severe damage to humans. It only takes about 1 bar of overpressure to rupture an eardrum.

Recession of NIagara Falls between 1678 and 2005
From NiagaraParks.com (Canadian)

Relevance to geology? High speed water can exert incredible forces on rock. Niagara Falls, which lies between Lake Ontario and Lake Erie, is 180 m high..  Water in free fall over Niagara Falls could theoretically reach 73 mph; velocities up to 68 mph have been measured. The stagnation pressure of water at these velocities is about 5 bars.  Water landing on rocks with this much pressure--especially if it carries abrasive sediment--exerts a strong force on the rocks, causing them to break up and then transporting them away.  There is a scour hole >100 feet deep under the Falls! This scouring + transport process has caused Niagara Falls to retreat towards Lake Erie about 11 km in the past 12,000 years, or a rate of about 0.1 mm/hour! A very nice pdf of scouring at bridges and erosion by water can be found here.