Dynamics of Lake Nyos Eruption and Degassing

Since the eruptions of Lakes Monoun and Nyos in the 1980's, progress has been made in understanding the mechanisms and dynamics of the eruptions. To prevent future eruptions, scientists and engineers have designed methods to degas the lakes slowly (Halbwachs et al., 2004). In this report, I refine an earlier model on the dynamics of natural lake eruptions, and consider the dynamics of controlled pipe degassing of lakes. An earlier semi-quantitative analytical model on the dynamics of lake eruptions (Zhang, 1996) did not consider shallow water entrainment. In a new numerical model, shallow water entrainment is considered but exsolution from the entrained water as it ascends is ignored. Hence this is still a relatively simple model. The results show that the centerline velocity depends on the diameter of the bubble plume. When the diameter is very large (more than 10 km), the centerline exit velocity approaches that of the earlier model. For a realistic plume diameter of about 100 m for Lake Nyos, the centerline exit velocity would be 45 m/s, half of the exit velocity given in Zhang (1996). The real centerline exit velocity likely lies between 45 to 90 m/s, and closer to 45 m/s. The dynamics of pipe degassing differs from natural lake eruptions because the pipe provides a rigid conduit with constant radius. There is no shallow water entrainment. The eruption velocity is limited by the sound speed of the gas-water mixture. If equilibrium between the gas and liquid phases is assumed, at the exit, the gas:liquid volume ratio would be 11:1, similar to the observed gas:liquid volume ratio of 9:1. Hence the equilibrium assumption is not far off. The sound speed for a 9:1 gas-water mixture is 31 m/s, leading to an eruption column height of 50 m, similar to the observed eruption column height. Hence exit velocity of pipe degassing is the basically the sound speed of the mixture.