Water cannon are undoubtedly old tech. As a crowd control method they evolved from fire hoses, with the first truck-mounted versions appearing in Germany in the 1930s. At heart, that's what they still are.
So the question is, can they still cut it in an era of social media, when flash mobs can form anywhere in an instant?
Although modern water cannon have some enhancements, the core is still a high-pressure pump jetting out thousands of litres of water a minute. They need a large reservoir – typically 9000 litres – so it has to be housed in a large vehicle. The risk of attack with bricks, petrol bombs and other projectiles means they have to be well armoured. They are even designed to resist concrete slabs dropped from above – although one German water cannon was reportedly disabled by a washing machine filled with concrete tipped off a building.
Good for dominating the immediate area around them, the effective range is supposed to be between 50 and 90 metres, but they are used closer. They can have multiple nozzles for full front coverage; some have a rear nozzle too. The stream of water can sweep backwards and forwards in an arc to repeatedly knock rioters off their feet, while the operators sit inside an armoured cab.
The cannons can cause bruising, and even broken bones if a person is thrown against kerbs or bollards and benches, but are rated among the less hazardous non-lethal weapons. A British government sub-committee examining medical aspects of water cannon concluded in 2002 that they were safer than baton rounds (rubber bullets) and found no evidence of fatalities. Perhaps the biggest risk is of being run over by one in an unpredictable crowd situation. In protests in Turkey last year, participants claim at least one person was hurt this way.
Fresh unrest
Ultimately, given the need for a heavy vehicle to cope with the armour, water, pumps and nozzles, some have questioned whether they are agile enough to deal with the type of widespread disturbances seen in London and other major cities in 2011, which first sparked calls for their use in the UK. Now this week police are calling for them to be more widely available fearing fresh unrest as the government continues to cut public spending.
The 2011 riots in England were notable for the use of smartphone messaging for rallying and co-ordinating participants. And smartphone apps are already a key tool for anti-austerity activists and protesters. They allow protesters to communicate, to exchange images of what is happening, issue warnings and coordinate actions.
"Water cannons, as a defensive repellent against mass groupings of people, will still be an effective counter rioting strategy if the rioters plan on waging their assault in the traditional manner," says Avraham Cohn, a New York-based consultant on digital security at Digital Development Consulting. But he agrees that social media change the situation. "The ability to send out messages in real time and in accordance with how they perceive the efficacy of their activities on the ground, may very well contribute to lateral, clustered attacks in an effort to outmanoeuvre law enforcement personnel."
Before social media, the only way for rioters to join in was to follow the noise. Now they can create their own flash mobs wherever the police are weakest.
"Instead of hordes of people charging and rioting in one place, we may see small groups evade law enforcement by decentralising their activities and thus nullifying the water cannon's ability to rebuff them," says Cohn.
Active denial
Would more modern techniques work better? Perhaps the most advanced non-lethal one is the Active Denial system or "pain beam", which heats the victim's skin, painfully but apparently harmlessly, with a beam of microwaves. Developed by the Pentagon, Active Denial is a powerful repellent – if hit you instinctively flee its 2-metre-wide beam. It has a much greater range than a water cannon, so requires less armour, and the ammunition supply is unlimited, but the high cost and concerns about the public reaction mean that it remains unused.
High-powered acoustic blasters such as the Long Range Acoustic Device have been used for crowd control in the US. These can project a narrow beam of painfully loud noise out to at least 100 metres. They are smaller and lighter than water cannon and have been effective at dispersing crowds. However, they can't knock people over like a water jet, and determined opponents may use earplugs or other sound-deadening countermeasures.
The US military's latest development is the Mission Payload Module – Non-lethal System (MPM-NLS). This is a vehicle-mounted grenade launcher effective out to 150 metres, much further than any water cannon. The grenades are a new development of the stun grenade, which produces no shrapnel and should not cause serious injury. The MPM-NLS has different force settings, so the operator can use it simply as a warning or can dial up a higher power to leave the target stunned and dazzled.
But all are unlikely to find favour in the UK. Water cannon, despite their drawbacks in dealing with fast moving, widely dispersed unrest, have a reassuring solidity for politicians, and their visible presence may prevent a riot. Such factors are likely to trump any high-tech alternatives.
The danger though, is that the slow-moving strong points of water cannon might simply prove impregnable but useless - like a modern Maginot Line - in the smartphone age.
Profile: David Hambling is a freelance science and technology writer based in London
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
