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

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International agencies affirm an increment of natural hazard and its consequences,especially in low-middle income countries. What it is changing within the conceptof risk is the boundary conditions which influence the hazard, exposure andvulnerability. In particular the combination of a rapid and unplanned urbandevelopment with a likely future climate change could affect landslide occurrencesmainly in the humid tropics, where high intensity and high duration rainfall eventsare the dominant trigger for landslide risk. In a worldwide view the population isexpected to increase, reaching 8.5 billion in 2030 with the half of the populationvirtually concentrated in urban areas. But the rapid urbanisation is not alwaysassociated with acceptable quality standard of life: in developing countries thepace of the city growth has not given the opportunity to an adaptation from rural tourban system. The initial scarce resources make impossible the necessary publicinvestment and infrastructures to support widespread urban agglomeration,inducing the development of megacity often characterized by spatial inequality andslums. On the other hands the climate change is predicted to influence theextreme precipitations. Warmer temperatures are supposed to increase highintensity rainfall occurrences in the humid regions but even with the same weatherconditions the land susceptibility is enhanced by the developing human activities.Deforestation and cut slopes, typical of the urbanisation process, are known toincrease the incidence of landslides, acting on the hydrological processes andshear-stress distribution. In addition low quality houses, resulting from a rapidurbanisation, combined with low economic possibilities enhance the landslide riskbecause usually located in unstable and unauthorised areas, often associated withobsolete drainage networks. The first aim is to understand the main risk drivers toevaluate their role in the landslide process as topography, vegetation, drainagenetwork and loading added as can be the houses of new contractions. Ranges ofpreparatory factors, triggers mechanisms and aggravating factors have to beevaluated to delimit the possible failing areas. According with these set conditions,different rainfall and urbanisation future scenarios can be superimposed to GISmaps in order to identify high landslide susceptibility areas. Specific geographiccontexts are then analysed more in details with the software CHASM, a physicallybased model which combine soil hydrology and slope stability. The single houseand the single drainage network effects on site hydrology and stability mechanismare evaluated and then implemented in a denser urban and network context. Theresults of the single analysis in CHASMS can modify (and improve) the initialconditions ranges, creating a general-specific cycle able to identify the landslidesusceptibility regions and outline a risk map. At the same time possibleconsequences with prevention practical solutions can be assessed. Since the high frequency landslides negatively affect the economic growth mainly of developingcountries, the risk map aims to be a tool for a decision making process to preventand mitigate landslides occurrences. However the suggested theoretical intentspresent some difficulties to be applied in the field both for economic and socialreasons. A scientific evidence demonstration coupled with cost-benefit evaluationwithin different risk scenarios, including some forms of mitigations, are necessaryto encourage political long-term investments.