Geological hazards and risk management
Domain 7 - Geological hazards and risk management. The exam tests the risk equation, the physics of each hazard, and the difference between structural and non-structural mitigation. Valencian floods are the most likely regional case study.
The framework
- Risk = hazard × exposure × vulnerability. Hazard is probability and magnitude; exposure is people and assets in the area; vulnerability is the damage susceptibility.
- A disaster is realised risk. Return period T relates to annual probability P = 1/T; the probability of at least one event in n years is 1 − (1 − 1/T)^n.
- Magnitude–frequency: large events are rare, small ones frequent.
Earthquakes
- Focus versus epicentre; magnitude (energy, moment magnitude) versus intensity (EMS-98 damage scale). Shallow, moderate earthquakes can be more damaging than deep, large ones.
- Secondary effects: liquefaction, landslides, fires, tsunamis. Spanish seismicity is moderate: Torrevieja 1829, Lorca 2011, tied to the Betic shear zone.
Volcanic and mass-movement hazards
- Volcanic: lava, pyroclastic flows, ash fall, lahars, gas, sector collapse.
- Mass movements classified by material and velocity: falls, slides, flows, creep. Triggers: rainfall, slope steepening, earthquakes, vegetation loss; stability depends on the balance of driving and resisting stresses.
Floods in the Valencian Community
- Mediterranean flash floods from autumn cold drops (DANA), steep catchments and impermeable surfaces. The Turia flood of 14 October 1957 led to the Plan Sur diversion; the Júcar flood of 1982 destroyed the Tous dam; the DANA of 29 October 2024 produced extreme rainfall and loss of life.
- Mitigation: structural (dams, dykes, channel works, check dams) and non-structural (flood mapping such as PATRICOVA and SNCZI, land-use zoning, insurance, early warning, education).
Worked example
A 50-year flood has annual probability 0.02. Over a 30-year period the chance of at least one occurrence is 1 − (0.98)^30 ≈ 0.45, i.e. about 45 %. A 100-year flood therefore has a 26 % chance in the same period and can still occur twice in a decade - the return period is not a waiting time.
Traps
- Confusing magnitude with intensity.
- Treating hazard and risk as identical.
- Assuming a levee removes risk (the levee effect can increase exposure).
- Reading a return period as a guarantee.