THE USE OF EARLY-WARNING SYSTEMS IN DETECTION OF GAS RELEASES FROM LAKES

 Nyos-Monoun Degassing Project (NMDP) Technical Report

Submitted to the Cameroonian Inter-ministerial Committee on Degassing by W. Evans, G. Kling, and G. Tanyileke on behalf of the NMDP International Advisory Committee (A. Bernard, W. Evans, M. Halbwachs, G. Kling, M. Kusakabe, J.-C. Sabroux, G. Tanyileke, K. Tietze)

1 November 2000

Yaounde, Cameroon

EXECUTIVE SUMMARY

This report summarizes a plan for installation and use of a CO₂ monitoring system to detect hazardous gas releases from lakes and provide an early warning for affected populations. The monitoring systems will be positioned near the population centers by the lakes, and will sound an alarm siren if CO₂ gas concentrations in the air around the stations reach dangerous levels. These systems operate continuously and can reduce (but not eliminate) the risk to human life from a gas release by warning people that they should move immediately to higher ground to escape a CO₂ cloud. The success of this installation relies on people understanding the reason for the monitoring system and their preparedness to escape properly. The plan also relies on the maintenance of the system, in order to prevent both false alarms and alarm failures, and on the response of trained personnel in the investigation of a gas release and in the notification of people when the affected area is again safe.

The first station will be installed and will begin recording CO₂ concentrations at Lake Monoun, but will not become fully operational with the warning siren connected until the response plan outlined below is in place (by January 2001). At that time a second station will be installed and made operational at Lake Nyos.

 The installation of an early warning system is only one part of an overall program to mitigate these natural hazards. The overall program will further reduce the risks to human life by lowering the gas concentrations in the lakes themselves through controlled pumping.

INTRODUCTION TO THE NATURAL HAZARDS OF GASSY LAKES

In this section we present background information (previously reported) on the causes of the Lakes Nyos and Monoun gas disasters. Natural hazards created by the lethal release of large amounts of gas from lakes are very rare. Only three lakes in the world are known to contain high concentrations of dissolved gas in their bottom waters: Lakes Nyos and Monoun in Cameroon and Lake Kivu in East Africa. Only two of these lakes, Nyos and Monoun, are known to have recently released gas resulting in the loss of human life. Two conditions are necessary for CO₂ accumulation in lakes to reach dangerous levels. First there must be an abundant source of CO₂, and second a lake must be strongly stratified, a situation where the bottom and surface waters do not mix, in order to trap the CO₂ in bottom waters. Geological conditions in much of Cameroon are favorable for the generation of CO₂ gas. This gas is discharged harmlessly to the atmosphere in many soda springs found throughout the country. In most of Cameroon's crater lakes, sheltering from the wind and great depth help to create stable stratification. The combination of gas-rich springs feeding into stratified lakes led to the buildup and eventual violent release of gas from the bottom waters of Lake Nyos in 1986 and Lake Monoun in 1984. Previous investigations have shown that these conditions do not occur in any other Cameroonian lakes, and no other lakes contain dangerous amounts of CO₂.

The gas contents of these two lakes continue to build, supplied by the input of CO₂ from underground springs that discharge into the bottom of the lakes. The trigger for a gas burst is unknown although any disturbance that moves deep, gas-rich water closer to the surface could result in a gas release. As that deep water rises, the weight of water above it (the hydrostatic pressure) decreases, and at some point the dissolved gas pressure will become equal to the hydrostatic pressure. At this point there is nothing to force the gas to remain in solution, and gas bubbles begin to form. This process is identical to the removal of the cap from a bottle of soda -- when the cap is removed there is no more pressure to keep the gas dissolved in the soda, and bubbles are formed. Once bubbles are formed in the lake they rise rapidly and drag the deep water toward the surface, at which point additional deep water is drawn upward and depressurized. This leads to a chain reaction that eventually results in the violent release of enormous amounts of lethal CO₂ gas. Because the gas content of these lakes is currently very high, catastrophes similar to those in 1984 and 1986 could occur at any time.

The natural hazards presented by these lakes are unique in that remediation is possible before a disaster occurs. The plan involves pumping of gas-rich bottom waters to the surface of the lakes through pipes. As the bottom water moves toward the surface the dissolved gas comes out of solution and forms bubbles. These bubbles lower the density of the gas-water mixture and the fluid begins to rise rapidly through the pipe. The energy released during degassing is sufficient to drive the pumping operation without any external power source. In addition to removing the gas currently in the lakes, it is possible to prevent future gas buildups using a pipe that continuously flushes bottom water out of the lake.

Until the lakes are degassed using the method described above, the people living around the lakes are at risk. Relocation of survivors from the Lake Nyos event has already occurred. However, people are moving back into the evacuated region, and the number seems to increase each year. There was no relocation of survivors around Lake Monoun; however, the population around the lake has increased in recent years, and new farmlands have been occupied very close to the lake shore.

There are several elements to an overall plan for the reduction of hazards from gas releases. The first element is to increase the awareness of people living near the lakes through education about what causes the hazards and how to recognize and avoid hazardous situations. A second element is to install a CO₂ monitoring system near each lake. Because CO₂ gas is heavier than air, it collects in low-lying areas around the lake or flows away from the lake in river valleys. A continuous CO₂ monitoring system can serve to warn people when a gas release has occurred so that they can move away from low-lying areas and uphill to safety. This report describes a gas monitoring system and associated safety procedures that will be used at Lakes Nyos and Monoun. A final element is to remove the gas from the lakes by controlled pumping, which is part of the overall program for degassing and hazard mitigation coordinated by a Cameroonian Inter-Ministerial Committee, Chaired by the Prime Minister, head of government.

DESCRIPTION OF THE MONITORING STATIONS

The gas monitoring stations will be identical at both lakes Nyos and Monoun. The station consists of a CO₂ detector and an alarm system. It is powered by batteries that are recharged by a solar panel. The detector will operate continuously (24 hours a day), sensing the concentration of CO₂ in the air at the station.

The stations will be permanently placed in a low-lying area between the lake and the nearby population. If a dangerous cloud of gas is released from a lake and drifts toward inhabited areas, the gas will be sensed by the detector and the alarm siren will sound. The siren will cycle on and off at intervals of a few minutes instead of sounding continuously. Cycling of the alarm will continue until the battery can no longer provide power or until the concentration of gas drops below a hazardous level. Because local inhabitants will not know why an alarm has stopped sounding, they should assume that the area is dangerous from the time that they hear the first alarm until the time that the area and the gas monitoring station have been checked by personnel trained in these procedures.

A CO₂ monitoring station alone, no matter how sophisticated, cannot ensure the protection of everyone living or working around the lake. For example, if the wind at the lake surface is blowing strongly in a direction away from the station, gas levels at the station may remain low and fail to trigger an alarm. Also, nearby lightning strikes may affect the station electronics and trigger a false alarm, or burning vegetation near the station could produce enough CO₂ gas to cause the detector to sound a false alarm. There is a trade-off between station sensitivity and the need to limit false alarms; this balance will be found during the early stages of station operation. However, because all detection systems are imperfect, it is important that local inhabitants do not rely completely on the monitoring stations for protection and that they are aware of other warning signs that could indicate a gas release.

WARNING SIGNS OF GAS HAZARDS

A large release of gas from a lake will cause several things to happen. If local inhabitants know what to look for, they could possibly recognize a dangerous condition whether or not the station alarm sounds. For example: (1) The venting gas will probably cause larger than normal waves on the lake surface. The sighting of such waves or the noise of large waves suddenly striking the shore should be a warning to people in the area. (2) The gas release at Lake Nyos was accompanied by loud rumbling noises; although such noises may signal a hazardous situation, a gas release could occur quietly as well so it is necessary to look for other signs. (3) A gas release will also stir up the lake, making the surface water reddish and cloudy. (4) Finally, the released gas will be toxic to birds, cattle, and other animals near the lake. Therefore, if people on high ground above the lake can see that the lake surface is reddish or cloudy, or that animals are dead or dying beside the shore, they should not approach the lake until trained personnel have investigated.

The gas cloud will probably be invisible, and it may be odorless, so moving downhill to help others that have fallen is very dangerous. If someone sees another person or animal lying on the ground downhill from where they are standing, and that person or animal appears to be unconscious, they should not try to help them. People should always continue to move uphill, away from the gas cloud.

RESPONSE PLAN

Below we outline a basic plan for an escape and response at Lake Monoun when the warning siren is sounded or when other warning signs of a gas release are detected. The precise details for a plan of escape and response must be determined by the appropriate Cameroonian agencies in cooperation with local authorities, and will naturally be somewhat different for Lake Monoun and Lake Nyos.

At Lake Monoun, the gas monitoring station will be located near the inlet stream, between the lake shore and the town of Njindoun. The nearest houses are about 500 meters away. If an alarm sounds, the people in these houses will hear it easily. They should immediately leave their houses and notify the chief or his representatives. The chief has a loud horn which is used to signal all the townspeople. A special signal could indicate that the gas monitor alarm had sounded. When the townspeople hear the siren or the horn they should stop what they are doing immediately, gather small children, and move up the side of the large hill (Makwet) just west of town. People should leave all their belongings behind (the belongings will not be damaged by the gas) and move as quickly as possible. People who are not in Njindoun but might be working on farms around the lake should go uphill to the highest point that they can reach directly. When they get to high ground, they should remain there, and in no case should they go down and cross a low area in order to reach the large hill west of Njindoun. Everyone should remain on high ground until designated personnel can verify that it is safe to return to low-lying areas.

A chosen messenger should go at once to report the incident to the Administration in Mbamkouop, north of Njindoun. The road to Njindoun should be closed with a barricade and a sign that warns of a toxic gas release. From the farming area on the south side of the lake, a messenger could take the news to the area of houses near the old headquarters of the C.O.C. coffee plantation, and from there a message can be taken to Foumbot. Also at the C.O.C. location there should be someone responsible for closing the road to Njindoun from this direction. This will prevent people from traveling into the area and being harmed.

It is best to have trained personnel located at Mbamkouop or nearby to be dispatched to investigate the gas release. In addition, when the news of an alarm has reached the Administration a message should be sent immediately by radio to appropriate response groups in Yaounde, such as the Fire Brigade, and to IRGM. The personnel from Yaounde are required to analyze and reset the CO₂ detection station itself, and they can provide additional assistance and backup if necessary.

Personnel who are dispatched from the Administration to investigate the alarm should be trained to work in areas where breathing the air is unsafe, and they should have access to Self-Contained Breathing Apparatus (SCBA) that would allow them to work safely in a gas-filled area. An ordinary gas mask offers no protection against CO₂ gas. As part of the CO₂ monitoring installation, we will provide a portable CO₂ sensor that can be used by the trained personnel walking into low-lying areas. These small sensors will sound an alarm when elevated concentrations of gas are detected. Investigators should use both SCBA gear and this portable CO₂ sensor to assess the situation after a release. They must check all the low-lying areas around the Panke River and the road through Njindoun for high CO₂ concentrations. If high gas concentrations are detected, the investigators should leave the area and move to higher ground. Investigators should return periodically and continue their checks until there has been enough dilution of the CO₂ with the atmosphere to lower the gas concentrations to safe levels. When the investigators find that the CO₂ concentrations have dropped to safe levels, they will report to the local authorities and the townspeople can return to the area and assist anyone who has been injured.

Part of any good response plan for natural hazards is to allow for the unexpected. For example, in the unlikely event that the personnel trained with SCBA cannot respond immediately, select people from Njindoun or from the Administration can be designated and trained to investigate whether a gas release has actually occurred. The greatest danger from these events occurs right after the gas has been released and the gas is consolidated into a large cloud. As hours pass after the release, the CO₂ gas is diluted as the wind mixes the gas into the atmosphere. Trained townspeople can investigate the event safely if they first wait for the most dangerous time to pass, and second if they follow the proper procedures.

The proper procedure for investigating a gas release without the benefit of SCBA is as follows. From the hill above the town, they should walk slowly down to the high point on the northern shore of the lake, where they can look down and observe the condition of the lake surface, the shore areas, and the animals and birds. If they see that the lake surface is cloudy and reddish, that vegetation on the shore has been pushed over by large waves, or that birds, fish, or other animals are dead in or around the lake, they should assume that a gas release has occurred and report back to the townspeople to remain high up the hill and wait for investigators with SCBA. Dangerous concentrations of CO₂ may persist in certain areas for 1-2 days after a gas release.

If the designated people see no signs that any gas release has occurred, they should report back to the townspeople and then begin to walk slowly down into the town area while the townspeople remain on the hill. Here, a lantern can provide an indication of the safety of the air; if the flame suddenly dies out, the CO₂ content in the air is dangerously high. Wild or domestic animals can give a good indication and can be observed from afar. If dogs, goats, chickens, and other animals are all fine, the air will be safe for people as well. If there are no signs of danger, the designated people should make a thorough check of the entire town and then return to announce their findings to the rest of the townspeople on the hill. If flames are extinguished or dead animals are seen, the people should not go further, but should return at once to the hillside and wait for investigators with SCBA.

PREPAREDNESS AND TRAINING

In order for CO₂ monitoring to be effective at reducing risk, all the people involved must understand exactly what they need to do and be prepared to act at once. For example, the responsibilities of communication about the gas release should be clear and well understood. Those contacted must have appropriate plans for how to proceed in case an alarm occurs at night or during heavy rains, or when some of their people are not available to respond.

Local inhabitants must also be prepared for temporary escape from a gas release. They should recognize the sound of the warning siren and be prepared to act immediately. For example, people could construct shelters on high ground that they could use in case an alarm sounds during a heavy rain or at night.

Signs need to be posted in the towns and surrounding areas that give brief information and instructions in different languages. The signs should include the following information: (1) the dangers of the gas in the lake, (2) the warning signs of a gas release, (3) the purpose of the CO₂ monitoring station, (4) the need to escape to high ground if the siren sounds or a gas release is detected, and (5) the need to allow trained people check the area before returning. Because one of the lowest and most dangerous areas is the road bridge over the Panke River to the east of the lake, signs should be posted in both directions warning people not to walk or drive down to that bridge if the CO₂ warning siren is sounding. These should be large, permanent signs that would serve to warn visitors to the area who would have no prior knowledge of the hazards.

Equipment must be kept in good working order. SCBA tanks must be kept full of air and batteries in the portable CO₂ sensors must be kept charged. A maintenance program for the CO₂ monitoring station must be followed. Specially trained people, initially from IRGM, will need to check the calibration of the detector every few months. Local townspeople will need to keep the area around the station free from brush and grass that could burn in the dry season and either harm the station or trigger a false alarm. Finally, people will be responsible to keep the station from being damaged by cattle or by vandals. Any damage to the station should be reported to IRGM at once.

CONCLUSION

The proper installation and use of CO₂ monitoring and warning systems can reduce the risk to people from hazardous gas releases. However, they can only be effective when integrated into a pre-determined plan that specifies the escape routes, the network of communication from the lake all the way back to Yaounde, and the persons responsible for deciding when the affected areas are safe. These monitoring systems are only one part of the overall program to reduce the natural hazards associated with these lakes. The systems do not replace any of the security and safety measures that have been advised and discussed in other reports. Such measures include the need to establish a security perimeter during the initiation of pipe degassing, the need to continuously monitor flow in the pipe and conditions in the lake, and the need to educate people about the risks posed by these natural hazards.

Last Updated:  November 2000