The effects of increasing urbanisation on landslide risk: quantitative analysis and possible solutions

International agencies affirm an increment of natural hazard and its consequences,
especially in low-middle income countries. What it is changing within the concept
of risk is the boundary conditions which influence the hazard, exposure and
vulnerability. In particular the combination of a rapid and unplanned urban
development with a likely future climate change could affect landslide occurrences
mainly in the humid tropics, where high intensity and high duration rainfall events
are the dominant trigger for landslide risk. In a worldwide view the population is
expected to increase, reaching 8.5 billion in 2030 with the half of the population
virtually concentrated in urban areas. But the rapid urbanisation is not always
associated with acceptable quality standard of life: in developing countries the
pace of the city growth has not given the opportunity to an adaptation from rural to
urban system. The initial scarce resources make impossible the necessary public
investment and infrastructures to support widespread urban agglomeration,
inducing the development of megacity often characterized by spatial inequality and
slums. On the other hands the climate change is predicted to influence the
extreme precipitations. Warmer temperatures are supposed to increase high
intensity rainfall occurrences in the humid regions but even with the same weather
conditions the land susceptibility is enhanced by the developing human activities.
Deforestation and cut slopes, typical of the urbanisation process, are known to
increase the incidence of landslides, acting on the hydrological processes and
shear-stress distribution. In addition low quality houses, resulting from a rapid
urbanisation, combined with low economic possibilities enhance the landslide risk
because usually located in unstable and unauthorised areas, often associated with
obsolete drainage networks. The first aim is to understand the main risk drivers to
evaluate their role in the landslide process as topography, vegetation, drainage
network and loading added as can be the houses of new contractions. Ranges of
preparatory factors, triggers mechanisms and aggravating factors have to be
evaluated to delimit the possible failing areas. According with these set conditions,
different rainfall and urbanisation future scenarios can be superimposed to GIS
maps in order to identify high landslide susceptibility areas. Specific geographic
contexts are then analysed more in details with the software CHASM, a physically
based model which combine soil hydrology and slope stability. The single house
and the single drainage network effects on site hydrology and stability mechanism
are evaluated and then implemented in a denser urban and network context. The
results of the single analysis in CHASMS can modify (and improve) the initial
conditions ranges, creating a general-specific cycle able to identify the landslide
susceptibility regions and outline a risk map. At the same time possible
consequences with prevention practical solutions can be assessed. Since the high
frequency landslides negatively affect the economic growth mainly of developing
countries, the risk map aims to be a tool for a decision making process to prevent
and mitigate landslides occurrences. However the suggested theoretical intents
present some difficulties to be applied in the field both for economic and social
reasons. A scientific evidence demonstration coupled with cost-benefit evaluation
within different risk scenarios, including some forms of mitigations, are necessary
to encourage political long-term investments.