Case Study 4: Volcanic Hazard Advice for Mount Cameroon
Mount Cameroon has had a seismic swarm of 40 small events per day at 5–15 km depth for six days; SO₂ flux is 1,200 t/day against a baseline of 200 t/day. Prevailing winds are from the southwest. Three settlements are at risk: A, 4 km southwest of the summit, 1,200 people, one road out; B, 11 km southwest, 3,000 people, two roads and a 20-bed hospital; C, 6 km south, on a ridge 200 m above the valley floor, 800 people, one footpath. A fissure eruption on the southwest flank is considered possible. Recent lava flows followed valleys, and rainfall is about 3,000 mm/year. The regional disaster committee has fuel for only 30 bus trips and asks you to rank the hazards and advise on evacuation priority and timing.
1. Rank lava flow, tephra fall, volcanic gas and lahar for settlements A, B and C. Explain how topography and wind control your ranking.
Read the worked answer
Ranking by settlement:
- A: lava high - 4 km southwest and downslope, and recent flows followed valleys; lahar high in heavy rain; gas moderate in valley bottoms; tephra low–moderate because the SW wind carries ash northeast.
- C: lava moderate - the 200 m ridge is above the valley, but a fissure upslope could feed a different valley; gas low because CO₂ is dense and drains off the ridge; lahar low; tephra low.
- B: lava low; tephra low–moderate; gas low; lahar moderate along river valleys. The hospital makes B strategically important. Topography controls lava and gas; wind controls tephra. A is most exposed to fast-moving hazards; C is relatively protected but has only one footpath.
2. Recommend evacuation priority and timing given 30 bus trips. Estimate whether 30 trips can move A and C, stating assumptions.
Read the worked answer
Priority: evacuate A first, then prepare C; use B as reception.
- 30 bus trips at 50 people = 1,500 people. A needs 1,200 (24 trips), leaving 6 trips (300 people). C has 800, so buses cannot move everyone.
- A has one road and is closest to a possible fissure; if lava cuts the road, evacuation becomes impossible. Move A now, as a precaution, while the swarm is active.
- C can use its footpath and is on a ridge; issue an alert and evacuate vulnerable people by foot, keeping buses for A.
- Do not wait for an eruption: lava is slow but the road can be lost early; false-alarm cost is lower than a trapped population. Assumption: 50 people per bus and drivers available; if capacity is 30, only 900 people can move, so A still comes first.
3. What monitoring changes would make you (a) cancel evacuation or (b) expand it to B? Explain.
Read the worked answer
(a) Cancel or downgrade evacuation if: seismicity falls to baseline, SO₂ returns to about 200 t/day, tilt shows no inflation, and no new fractures open. Keep a watching brief; do not cancel on one quiet day. (b) Expand to B if: tremor becomes continuous, tilt shows accelerated inflation, SO₂ rises sharply, or a fissure opens upslope of A. B is 11 km away and mostly safe from lava, but ash could affect the hospital and roads; if wind shifts to the northeast, tephra could reach B. Expand monitoring to include real-time GPS/tilt, SO₂, seismic amplitude and rainfall for lahars.