Risk Returns

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] 79

A sudden out-gassing of CO

occurred from Lake

Nyos on 21 August 1986, when a limnic eruption trig-

gered the sudden release of about 1.6 million tonnes

of CO

; this cloud rose at nearly 100 kilometres per

hour. The gas spilled over the northern lip of the lake

into a valley running roughly east-west from Cha to

Subum, and then rushed down two valleys branching

off it to the north, displacing all the air and suffocating

some 1,700 people – mostly rural villagers – within

25 kilometres of the lake, as well as 3,500 livestock.

As at May 1987, there was about 26 per cent build-up

of CO

and scientific findings in 1991 show that Lake

Nyos now poses an even greater threat to thousands

more people than before, because it is being recharged

with deadly CO

The dam at the northern end of the lake has obviously

receded since its formation some 400 years ago. The

area surrounding the lake shows traces of nearly vertical

landslides and rock falls. In addition, in the downstream

of the dam, old and recent landslides can be identified.

There are even indications that landslides will occur at

the section of the dam just east of the spillway.

The collapse of the dam would result into the release

of water at an estimated peak discharge of 17,000

cubic metres per second into the Kumbi River, and

discharge into the Katsina-Ala River in Benue State,

Nigeria. About 50 million cubic metres of water would

flow downhill, flooding the Northwest Province of

Cameroon and seven frontline States in Nigeria –

Taraba, Benue, Kogi, Adamawa, Akwa-Ibom, Cross

River and Anambra.

These massive floods would, in a matter of minutes,

submerge and swep through Katsina-Ala in Benue

State, as it hurried into the Benue River and Makurdi,

100 kilometres away. Within a few hours it would be

felt in Lokoja, Asaba and Onitsha. The destructive

impact of the flood would put thousands of lives at

risk, and over 20,000 herds of cattle and other live-

stock and 15,000 hectares of arable land would perish.

The estimated financial loss would be in billions,

comprising crops, residential and commercial struc-

tures, utilities and infrastructures including roads,

bridges and other services.

A lot of degassing activities have been ongoing to

prevent the occurrence of toxic gas release from the

lake. However, it is believed that CO

is building up

in the lake so quickly that it raises the risk of a new

catastrophic release.

If the dam breaks, the gas would spill from the

upper section of about 40 metres of the lake down-

ward to the region inhabited by hundreds of people.

In effect, Lake Nyos and its neighbourhood is still a

disaster zone, given the evidence of human activities

and the presence of farms, houses and cattle-rearing

around the lake.

NEMA has acquired and installed early warning equip-

ment that will give an alarm to warn the communities

around the lake to move before the release of poisonous

flood water into River Katsina-Ala and othe tributaries.

Early warning systems for Lake Nyos

Lake Nyos is a crater lake in the Northwest region of Cameroon,

located about 322 kilometres north-west of Yaounde and about

436 kilometres south-east of Abuja in Nigeria. It lies on latitude

6°26’17’’N and longitude 10°17’56’’E. It is a deep lake, high on

the flank of an inactive volcano in the Oku volcanic plain; a zone

of crustal weakness and volcanism that extends to the south west

through the Mount Cameroon stratovolcano. The lake covers an

area of about 1.5 square kilometres, with a surface elevation of

1,091 metres and a depth of 208-210 metres. Its volume of water

is 132 million cubic metres, with a dam length of 50 metres. The

lake is compositionally stratified, with a 200-metre deep fresh

water area in the upper 50 metres and heavier sodium and carbon

dioxide (CO

) rich water below that. The water below 180 metres

is particularly rich in sodium and CO

. Most of the sodium and

come from numerous sodium-bicarbonate bearing springs –

derived from an underlying magma chamber – feeding into the

bottom of the lake.

The dam wall of Lake Nyos consists of very poorly consolidated

pyroclastic layers of gravel, rock and mountainous soils and is

subject to erosion from rain, wind and the lake waters. This region

has an average rainfall of 2.5 metres each year. A natural dam of

volcanic rock hems the lake waters.

Flood early warning equipment

Image: NEMA 2010