| A general methodology for the probabilistic evaluation of landslide hazard is applied,
taking in account both the landslide susceptibility and the instability triggering factors,
mainly rainfall. The method is applied in the Fanhões-Trancão test site (north of Lisbon,
Portugal) where 100 shallow translational slides were mapped and integrated into a GIS
database.
For the landslide susceptibility assessment it is assumed that future landslides can be
predicted by statistical relationships between past landslides and the spatial data set of the
predisposing factors (slope angle, slope aspect, transversal slope profile, lithology,
superficial deposits, geomorphology, and land use). Susceptibility is evaluated using
algorithms based on statistical/probabilistic analysis (Bayesian model) over
unique-condition terrain units in a raster basis. The landslide susceptibility map is
prepared by sorting all pixels according to the pixel susceptibility value in descending order.
In order to validate the results of the susceptibility ana- lysis, the landslide data set is
divided in two parts, using a temporal criterion. The first subset is used for obtaining a
prediction image and the second subset is compared with the prediction results for
validation. The obtained prediction-rate curve is used for the quantitative interpretation of
the initial susceptibility map.
Landslides in the study area are triggered by rainfall. The integration of triggering
information in hazard assessment includes (i) the definition of thresholds of rainfall
(quantity-duration) responsible for past landslide events; (ii) the calculation of the
relevant return periods; (iii) the assumption that the same rainfall patterns
(quantity/duration) which produced slope instability in the past will produce the same
effects in the future (i.e. same types of landslides and same total affected area).
The landslide hazard is present as the probability of each pixel to be affected by a slope
movement, and results from the coupling between the susceptibility map, the
prediction-rate curve, and the return periods of critical rainfall events, on a scenario basis.
Using this methodology, different hazard scenarios were assessed, corresponding to
different rain paths with different return periods. |