Degassing Nyos home page
Disasters at Nyos: overview
In 1986, a tremendous explosion of CO2 from the lake Nyos, West of Cameroon, killed more than 1700 people and livestock up to 25 km away. The dissolved CO2 is seeping from springs beneath the lake and is trapped in deep water by the high hydrostatic pressure. If the CO2 saturation level is reached, bubbles appear and draw a rich mixture of gas and water up. An avalanche process is triggered which results in an explosive over-turn of the whole lake. Since 1990 a French team has carried out a series of tests in an attempt to release the gas slowly through vertical pipes …
The 1984 and 1986 disasters
Lake Nyos disaster, which claimed 1800 victims in August 1986, was not unprecedented, but never before one had heard of Mother Nature asphyxiating human beings and all terrestrial animals on such a scale in a single and brief event.
Two years previously however, a lethal gas burst originated from the neighbouring lake Monoun, in the same remote area of Cameroon, and killed 37 people, an odd and tragic episode that went almost unnoticed.
What happened at Lake Nyos: the limnic eruption phenomenon
Volcanoes are well known as producers of poisonous or asphyxiating gases and, in some instances, these gases kill people caught in the volcanic plumes : such was the case on Dieng Plateau in Central Java, Indonesia, where 149 people died in 1979, in the wake of a comparatively minor phreatic eruption - an eruption driven by the vaporisation of groundwater, without any ejection of magmatic material. The possibility that a phreatic eruption occurred through the Cameroonian lakes cannot be completely ruled out. However, in these cases, the culprit could well have been the lake itself : indeed, the possibility that the Monoun gas bursts originated from huge amounts of carbon dioxide dissolved in the deep layers of the lake was first investigated by H. Sigurdsson and his team, who concluded that a disturbance of unknown origin had upset the density stratification of the water column, triggering an overturn of the lake and the subsequent release of carbon dioxide. Being denser than air, CO2 flows over the ground surface, asphyxiating people unfortunately present in the gas cloud.
Most probably, the same explanation holds for lake Nyos but, owing to the much larger size of the latter lake and the topography of the surroundings (the perched at lake, c. 1,000 m , is drained by deep and long valleys, sheltering several villages), the death toll was much higher, including the last victim stricken some 27 km downstream from the lake.
The carbon dioxide which erupted from the lake water is undoubtedly of volcanic origin, continuously seeping through the sediments of the lake bottom, most probably as a cluster of warm springs of CO2-bearing water. Due to the high solubility of carbon dioxide in water, a lake can dissolve a volume of CO2 more than five times its water volume. The stability of such a "time bomb" stems from the fact that CO2-rich water (e.g., soda water) is denser than pure water, as long as gas bubbles do not nucleate. The horizontal layering of the water column is due to the differential diffusion of CO2 and heat but, contrary to salt (which stabilises the thermohaline stratification of the oceans), carbon dioxide has a solubility that is limited by temperature, making the stratification intrinsically unstable. Thus, there is even no need of an external trigger (landslide, earthquake or heavy rain) to upset the stratification of the lake. Once CO2 bubbles nucleate within a saturated layer of the lake water, they rise and grow, attracting in their wake deeper water available for ex-solution, feeding the chain reaction process : the entire lake overturns through an ascending column of rising and expanding bubbles.
The term "limnic eruption" was first coined by J.C. Sabroux at the UNESCO Conference on the lake Nyos disaster, in March 1987 at Yaounde, in order to take into account the analogy with volcanic eruption, also powered by gas bubbles ascending and expanding in a liquid (the magma). The limnic eruption, the mechanism of which was further refined by K. Tietze, explains satisfactorily the sequence of events and consequences of the Monoun and Nyos disasters, and suggests a possible mitigation of this new geological hazard.
Degassing Nyos home page
Degassing Nyos and Monoun "Killer Lakes"
The degassing procedure principle
Thorough investigations of the physics and chemistry of lakes Monoun and Nyos quickly revealed that both lakes still contain huge amounts of carbon dioxide (10 millions m3 and 300 millions m3 in Monoun and Nyos, respectively) and that this gas is being added at such a rate that saturation could be reached within years in the deep layers of the lakes. Since it is impossible to guarantee the perennial stability of the lakes, it has been proposed to make these lakes safer by extracting, in a controlled way, the carbon dioxide they contain. The process is no more than a limnic eruption brought under control ; it is inspired by the industrial process known as "gas lift" and, more precisely, by the methane (and CO2) extracting unit which had been operating at Gisenyi, Ruanda, on the shore of the African lake Kivu (another gas-bearing lake, but far from saturation).
The method consists of a pipe set up vertically between the lake bottom and the surface. A small pump raises the water in the pipe up to a level where it becomes saturated with gas, thus lightening the water column; consequently, the diphasic fluid rises to the surface. Therefore, once it has primed the gas lift, the pump is not needed, since the process is self-powered : above the saturation level, isothermal expansion of gas bubbles drives the flow of the gas-liquid mixture as long as dissolved gas is available for ex-solution and expansion.
In 1992 at Monoun, and in 1995 at Nyos, M. Halbwachs and J. Grangeon demonstrated the feasibility of such a process. In both cases, the eruption of a gas-water mixture was primed through a 140 mm diameter pipe (made of high-density polyethylene). The measured flow-rate matched the results of a numerical modelling of the diphasic flow, as carried out by G. Kayser, and the reliability of the remotely operated control valve for stopping the flow on request was also demonstrated.
These successful experiments pave the way for an operational scheme of degassing lake Nyos and lake Monoun down to a level that would rule out the possibility of a limnic eruption.
Historic of the Nyos degassing project
modest self-siphon experiment was carried out at lake Nyos and Monoun in March
1990 by means of a 9 mm inner diameter PVC pipe.
A little soda geyser (1 m high) was primed for the first time on both lakes indicating the feasibility of the controlled degassing plan.
The experiment was also designed to provide Cameroonian scientists with a simple means for determining, in the field, the CO2 concentration profiles of the lakes.
A large-scale self-siphon experiment was conducted in March April 1992 at lake Monoun, after determining an up-dated profile (fig. 1 b). Two polyethylene pipes (50 mm and 140 mm inner-diameter, respectively) were fixed between moorings on the lake bed and a floating raft, at a distance of 100 m or so from the meteorological monitoring platform set up by the US team (fig. 1 a). The main results of the experiments are presented in table. Two to three 140 mm diameter pipes (fig. 2) could readily remove in one year the carbon dioxide dissolved in Lake Monoun.
Jacques Grangeon is engineer at the University of Savoie (electronic, micro processing, sensors and signal conditioning). He worked out all the details of the degassing equipment tested at Monoun in 1992 and Nyos in 1995.
Michel Halbwachs is professor of physics at the University of Savoie and engineer in Sciences of materials. He has worked for 25 years on instrumentation, data collecting and remote controlled system on volcanoes. He supervised the experiment surveys on the Cameroonian lakes in 1990, 1992 and 1995.
Gaston Kayser is a retired engineer from EURATOM (working in the French Atomic Energy Commission) and specialist in the critical problems of the nuclear reactor. As such, he has at highest level the mastery of mechanical, material strength and biphasic flow hydraulics.
Jean-Christophe Sabroux was the pioneer of the lakes degassing project in 1986 while he was expert at the Major Hazards Delegation of the Ministry of Environment. He is an engineer and works at the Atomic Energy Commission. He has a wide range of competences in fundamental geochemistry, thermodynamics, mechanics and physics.
Brice Wong is a retired engineer from the French Electricity Company. He is a specialist in dam building and related expertises. He is at the head of the NGO Hydraulics without Frontier (HSF) and designed various tentative solutions to the tricky problem resulting from the unconsolidated natural dam at Nyos.
Our main scientific counterpart in Cameroon is Dr. Gregory Tanyileke, a high level geochemist and a long time specialist in Lakes Nyos and Monoun. He will be assisted by Jacob Nwalal, Engineer in Hydrology, who acquired during our past surveys a profound competence in all the parts of the degassing equipment.
by Bernard Cannet
Copyright Magma Production
Monitoring of the diphasic stream
Surveillance of the de-stratification
The first permanent degassing column will be assembled and set up in January 2001 at Nyos. The main part of the funding comes from the US Office of Foreign Disaster Assistance ( OFDA ). The French Embassy in Yaounde and, of course, the Cameroonian Government also participate in the financial and logistical support.
The OFDA contracted the designing and assembling of the column, its ancillary instrumentation and the satellite remote control system to a French Company, Data Environnement. The raft construction was contracted to a Japanese Company, Yoshida Consulting Engineer Office.
The overall system is rather similar to the one experimented with at Nyos in 1995. Special attention has been paid to the instrumentation ( see below ), both for the purpose of technical surveillance of the two-phase flow instabilities, and for the collection some more fundamental parameters .
The polyethylene bars which constitute the column will be electrically soldered on the spot. The pipe external and internal diameters are 180 mm and 145 mm, respectively. The depth of the inlet removal mouth will be as close as possible to the bottom of the lake, say, around -205 m.
It is to be noticed that the dissolved gas concentration considerably increased at 200 m depth since the last experiment in 1995 ( 9 lgas/lliquid as compared to 5.5 lgas/lliquid ): the expected self-sustained fountain will be at a much higher energetic level. The gas release rate will largely overcome the natural CO2 recharge (from 2 to 5 Mm3/year).
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.
Different solutions to this hazard have been investigated by the Jack Lockwood American team:
The following proposal is still an indicative draft as no field investigation has yet been carried out by any expert from HSF. The idea is to consolidate the lower unit and to protect the upper unit as detailed below.
1) Consolidating the lower unit and preventing the leakage
In order to prevent the leakage, one, two, or three rows of concrete columns could be put in place using the "jet-grouting" technique. The jet grouting consist in drilling vertical holes inside of the structure and injecting an extremely high pressure (2000 bars) pure cement jet. This high speed jet (500 km/h) violently mixes the surrounding granular material and naturally results in a well-proportioned concrete.
The columns would be drilled next to one another to result in a concrete seal on the upstream side of the dam, and therefore prevent leakage. Steel bars have to reinforce these concrete columns.
On the down stream side of the dam, concrete columns would be drilled several meters apart in order to allow the drainage of the residual leaking.
2) Protecting the upper unit
A concrete spillway would be laid over the upper unit. It would be anchored to the upper unit and to the bedrock through the reinforced concrete columns. This spillway would prevent the upper unit from superficial erosion when the stream floods during the rainy season.