Small dams at high risk?

Large numbers of people could be at risk from the failure of small dams built by local communities, safety experts told a seminar of dam and irrigation engineers in New Delhi, India. Such dams, which are often built with the help of non-governmental organisations (NGOs), are at greater risk than large dams. Swiss engineer Martin Wieland and French engineer F Lempérière explained that these small dams are usually built with insufficient engineering knowledge and are mostly earth-filled.

At a seminar at India's Central Board of Irrigation and Power in February, it was concluded that dam failure could be sudden and disastrous: where there was no warning they could drown 50% of the people hit by released water. Wieland said it needed to be realised that there are now large numbers of such small community dams around the world, not just in India.

The two specialists were reporting on studies of risks and failures associated with all types of dams, with a focus on Asia. They said monitoring and safety plans need to be put in place for every kind of dam, large and small, because many older dams' construction and design parameters for earthquakes were not only outdated but obsolete.

Lempérière added that monitoring and safety measures were neither difficult nor costly but as small dam owners have little money, local authorities may need to consider a regulatory safety and maintenance framework to make sure action is taken.

The specific risks came from liquefaction where piping can lead to break up of the earthfill or where water ingress causes the dam to start to slide. Piping is a slow process so it could be spotted early and failure prevention implemented. The most dangerous small dams are those using fine sand of less than 1mm in diameter.

'Few large dams but hundreds of small dams have been destroyed in minutes,' Lempérière said of his survey of failures. Failures of tailings dams were 'very dangerous for close populations' and in the last 30 years 'such dams caused more fatalities in industrial countries than all other dams', he added.

Wieland, chairman of icold's Committee on Seismic Aspects of Dam Design, said failure could be triggered by earthquakes; as was the case during the Gujarat earthquake of 26 January 2001 in the Bhuj area of India. Two hundred earth dams were damaged and needed repair or strengthening. Luckily, because water levels were low at the time, floods did not occur, he added.

Increased risks for small and large dams generally come during floods or seasonal rain when reservoirs refill. While large dams are usually constantly monitored by trained engineers (Asia has now built 30,000 large dams and is the main dam construction region of the world) this is not the case for small dams. Unofficial estimates suggest India alone has at least 100,000 'check' dams which capture rain and flood water which filters down to raise underground water levels. They have become very popular because local people can easily build them and agricultural payoff is very high for little cost.

Asked about the risks from 'check' dams, Lempérière said: 'Small dams may be much more dangerous because collapse can be so sudden. The failure of a 15m high dam could release water at speed at 300-5000m3/sec which is often devastating.'

Wieland said that NGOs should utilise available literature when designing small dams, and seek assistance from engineers who have worked on larger, highly-engineered structures.

Lempérière, who is part of a voluntary, worldwide network of engineers promoting co-operation on hydro and irrigation data, said that five years ago the French government instituted a low cost survey of 300, 10-60m high dams in France. It used three teams of engineers spending up to three days at each site, looking for basic leaks and problems.

A couple of dams were immediately decommissioned, 20 were given immediate expansion work on the spillways and 50 more dams were put under further risk assessment.

Questions were also raised at the meeting on the need to consider action to decommission a number of check dams and irrigation dams built by local communities because of the potential risks from poor structures. But this had considerable political implications because so many people were benefiting from them in terms of water supply.

The problem with big, non-concrete lined earth and rockfill dams, Lempérière said, was that water starting to go over the top could rapidly cause a breach. One simple and cheap solution was to put a small 1-1.5m concrete or cheap steel plate wall along the top of the dam for as little as US$50 per metre. Or increasing the capacity of the spillways could reduce accidents from failure by up to ten times.

Poor maintenance of dam gates and slipways was generally behind the cause of failure, says Lempérière. He added that electro-mechanical systems were not necessarily better than simple, common sense measures and good early warnings. Bad weather and flooding were the times when failure occurs and this often means power systems for electro-mechanical services can go down and road access can be restricted. A half an hour warning system could save hundreds of lives because people have time to evacuate. Wieland and Lempérière said they expect to see an increase in the number of dam failures in the years ahead.