Gas Outburst Proccss of Lake Nyos, Cameroon

Abstract

A large volume of lethal gas was released from lake Nyos in the North-West provine of Cemeroon on 21 August 1986. Some 50 days after the event, a hydro-physical and -chemical survey of Lake Nyos was carried out by four japanese scientists including the author. Vertical density distribution of lake water showed remarkably stable stratification through whole lake water column. The result of chemical analysis showed high concentration of dissolved CO₂ and Fe which contribute stable stratifications. Assuming initial saturation of dissolved CO₂ in lake water, released gas volume was estimated to be 0.86(km³) as volume difference between the saturated gas content and the residuals in the present lake. Quasi-adiabatic coolng due to expansion of CO₂ gas during ascent through the water column, led to temperature drop of water, which was estimated for every 1Om leyer thickness assuming acomplete thermal exchange between expanding gas and lake water. The calculated temperature profile assuming initially uniform distribution of temperature at 23.38℃ agreed well with the measured profile by CTD. The height of gas cloud over the lake and flow velocity of draining gas from the rim of the lake were evaluated using a simple geometrical flux model. In the case of sinusoidal change of ejection with equal volume mixing and head loss coefficient of C_ｄ=0.3, the maximum gas height reached to more than 100 m and the corresponding drain velocity was 19 m/sec.