Engineering evaluation of hazard

Classification of engineering evaluation of hazard

Hazard can be evaluated in two ways.

1. Absolute hazard evaluation: the hazard is given a numerical value of probability (for example, a 1 in 100 year event) or a quantity such as factor of safety, annual probability, expected displacement, magnitude. It is quantitative and used in design.
2. Relative hazard evaluation: areas are ranked as high, moderate or low by comparison with each other, without absolute probability; qualitative or semi-quantitative (hazard zoning maps).

Absolute hazard assessment (how it is done)

1. Collect data: past events (inventory), geology, slope, rainfall, earthquake records, geotechnical parameters.
2. Deterministic method: compute the factor of safety of the slope, for example

$$F = \frac{cA + W\cos\beta\tan\phi}{W\sin\beta}$$

with $F<1$ meaning failure. 3. Probabilistic method: use frequency of past events to calculate return period $T$ and annual probability $P = 1/T$; probability of occurrence in $n$ years is $1-(1-1/T)^n$. 4. Define the design event (design earthquake, 100-year flood) and compute magnitude, runout distance or displacement. 5. Present results as hazard maps with numerical values and use them in design and risk calculation.