The natural dam at Nyos: a new potential hazard?

 

Lake Nyos is bounded on it northern part by a narrow natural dam consisting of poorly consolidated material. If the dam happened to collapse it could lead to devastating floods which could affect a downstream area as far as Nigeria, 100 km away.

The dam is about 40 m high, having a width of 45 m in its narrowest part. It consists of slightly consolidated pyroclastic material in it lower part (the lower unit) covered by 6 m of harder material (the upper unit). Water from the lake is continuously seeping through the lower unit digging galleries of regressive erosion up to 3 m in diameter and going up to 10 m inside the structure. This leakage and the resulting erosion greatly threaten the stability of the dam.

A 20 m wide spillway flooded during the rainy season, exposing well-developed joints and tobel-like holes (marmites).

In conclusion, the natural dam does certainly not fulfil engineering norms and it is the responsibility of the scientific community not to deny the potential hazard until clarity has been gained through careful investigation by geotechnical specialists.

 

Courtesy : Jack Lockwood, Bull. Volcanol., 50, 1988

Courtesy : Jack Lockwood, JVGR, 39, 1989