The natural dam at Nyos : a new potential hazard ?
Lake 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.
A tentative plan for a re-enforcement of the dam
by Hydraulique Sans Frontiere ( HSF ) - http://www.hydrauliquesansfrontieres.org/
Hazards mitigation plan
Different solutions to this hazard have been investigated by the Jack Lockwood American team :
- Move the population living up to 200 km downstream to a safer place,
- Progressively remove the first forty meters of water in the lake and break the dam,
- Consolidate the upper unit of the natural dam,
- Dig a tunnel 65 m under the present level of the lake and equip it with valves to regulate the level of the lake.
The HSF proposal
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 (Figure 1)
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 (Figure 2)
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.