Lake Nyos, a Multirisk and Vulnerability Appraisal

Abstract

Situated at the northern flank of the Oku Massif, Lake Nyos crater epitomizes landscape features originating from volcanic explosions during the Quaternary. The Cameroon Volcanic Line (CVL), to which it belongs, constitutes the most active volcanic region in Cameroon. In 1986, an outgas explosion occurred from beneath the lake and killed 1746 people in several neighbouring villages. The event influenced a radial area of 25 to 40 km wide, particularly in eastern and western direction. This was mainly due to: (1) the rugged nature of the landscape (fault fields), which enabled the heavier gas to follow valleys framed by faults corridors without affecting elevated areas; and (2) the seasonal dominating western wind direction, which channeled the gas along tectonic corridors and valleys. This paper assesses the geological risk and vulnerability in the Lake Nyos before and after several proposal to mitigate future outgas events. Remotely sensed data, together with GIS tools (topographic maps, aerial photographs), helped to determine and assess lineaments and associated risks. A critical grid combining severity and frequency analysis was used to assess the vulnerability of the local population. There is evidence that along the main fault directions (SW-NE), anthropogenic activities are most intensive and they may play an aggravating role for disasters. This requires the local population's consciousness-raising. The results also show that population around Lake Nyos still remains vulnerable to volcanic hazards and floods. However, the area has been safe since the last degassing and jet grouting through multiple procedures and actions proposed in the National Contingency Plan, and equally by the relief organization plan (DROP or ORSEC plan) for the Menchum Division. Another issue is that the local population is concerned with the idea of returning to the affected areas in order to stay close to their ancestors or the deceased. Therefore, even after jet grouting and degassing, the problem of risk minimization for local residents remains.

A Concise Appraisal of Cameroon’s Hazard Risk Profile: Multi-Hazard Inventories, Causes, Consequences and Implications for Disaster Management

“…To mitigate the risk of later gaseous eruptions, the Nyos and Monoum Degassing Project was instituted to degas (Tubes were installed to siphon water from the bottom of the lake to the surface, allowing carbon dioxide to escape in safe amounts.) both lakes, and it is believed the risk of later gaseous eruptions has been eliminated [63]. A carbon-dioxide warning system was installed at both Lakes Nyos and Monoum to alert the residents living within the vicinity in case of another gaseous release.…”

Post-eruption evolution of maar lakes and potential instability: The Lake Pavin case study, French Massif Central

“…Dam failure at maar rims can be triggered by five mechanisms (Jouhanneau et al, 2001;Bonelli et al, 2007;Manville, 2015;Tchindjang, 2018): (1) Mass movements induced by seismic ground acceleration and liquefaction; (2) Phreato-magmatic eruption near the dam; (3) Regressive mechanical or linear backward erosion following a flood affecting the downstream portion of the dam; (4) Cavity collapse at the dam base or top due to chemical erosion, subsidence, and seepage; (5) Failure due to seepage (piping flow) initiated by headward erosion of a pipe, often accompanied by collapse of the pipe roof to form a retreating amphitheater-headed gully, until only a thin septum of dam material remains that is pushed over by the weight of the lake water, followed by potentially runaway breach erosion.…”