Fragile dam - a time bomb that could kill thousands, but when will it go off?

When the natural dam at Lake Nyos in north west Cameroon collapses, tonnes of water will course through the surrounding valleys and 10,000 villagers could be killed, geologists say. Those not swept away or drowned in the careering wall of water could risk suffocation by a poisonous gas cloud released from the bowels of the lake that sits atop a dormant volcano. “There are two outstanding dangers if the dam breaks - the release of carbon dioxide that will certainly kill the surrounding population and the water running downstream which will kill 10,000 people in Cameroon and Nigeria,” said Isaac Njilah, a geologist at the University of Yaounde. The problem is, no one knows when the dam might break. Njilah has just come back from studying the dam which lies just over 300 km from the capital Yaounde. He says that that the volcanic rocks holding back tonnes of water could give way at any time. “If forces [eroding the dam] are not checked, the dam could collapse imminently,” Njilah told IRIN this week. He says fractures along the dam are increasing in size and number and the dam is leaking water. In the rainy season, which runs from June to October, the lake is so high that water runs over the dam, washing away topsoil and loose rocks. “But worse still,” said Njilah, “any earthquake or volcanic eruption around the area could force the fragile dam to give way letting water loose.” “And if the dam gives way, accumulated carbon dioxide under the lake will escape just like gas comes out of a newly opened bottle of Coca-Cola,” he said. The dam plugs Lake Nyos, which is in the throat of a volcano and sits on a fault line that runs from the town of Tiko on the western coast, north-east across the country towards Chad. A string of active and inactive volcanoes run along this seam in the land and the area is prone to earthquakes. Though the volcano below Lake Nyos is dormant, it leaks carbon dioxide, which is absorbed into the deep waters of the lake. One disaster already Cameroon has already witnessed the deathly consequences of an eruption of these gases. Without warning, on an August night in 1986, the Lake Nyos belched a toxic cloud of carbon dioxide and water droplets that suffocated up to 1,800 people in their sleep. So much gas was ejected that the surface level of the lake - which has a surface area of more than 1,000 km sq - dropped by a full metre. Because the gas cloud was heavier than air, it hugged the ground through the valley on the north side of the lake for 23 kilometres, travelling faster than residents could run to escape it. The cloud also killed an estimated 5,000 livestock and forced between 4,000 and 5,000 people to abandon their homes. The tragedy, which scientists say is the worse gas disaster on record in the world, sparked a series of studies of Lake Nyos, one of which also identified the weakness of the natural dam. Though a project to siphon the toxic gases from the base of the lake is up and running, nothing has been done to strengthen the natural dam. “There is a need to fortify the dam with an artificial dam made of solid concrete,” said Njilah. But not everyone agrees with the predictions of imminent disaster. Geologist Gregory Tanyi-Leke, who works for the Institute of Mining and Geological Research under the government’s Ministry of Mines, agrees that the dam is fragile and that 10,000 lives would be at risk from any dam burst, but thinks that that is unlikely to happen tomorrow. “The dam is certainly fragile, but saying its collapse is imminent is very alarmist,” said Tanyi-Leke who was part of an international team of experts that studied Lake Nyos after the 1986 disaster. “Observing the dam from above the waters and saying that it will soon collapse is misleading,” said Tanyi-Leke. “It requires at least a month of careful studies of the lake to make any assertion.” Because the dam is made up of debris from a volcanic eruption, it is easy for water to erode weaknesses in the rocks, but a thorough investigation is required to assess the extent of the damage caused, he explained. Tanyi-Leke wants the Cameroonian government to construct roads to make this forested corner of Cameroon more accessible and so speed ease an evacuation of the area if it became necessary. Though the government evacuated the region following the 1986 disaster, people have begun moving back to take advantage of the fertile volcanic soil that provides lush farming and grazing land. The United Nations has been in touch with the Cameroonian government, and offered technical support for any investigations of the dam.