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

国际机构确认自然灾害及其后果有所增加，特别是在中低收入国家。在风险概念中变化的是影响危害、暴露和脆弱性的边界条件。特别是，快速和无计划的城市发展与未来可能发生的气候变化相结合，可能会影响主要发生在潮湿热带地区的滑坡，在那里，高强度和长时间的降雨事件是滑坡风险的主要触发因素。从世界范围来看，人口预计将增加，到2030年达到85亿，其中一半人口实际上集中在城市地区。但是，快速的城市化并不总是与可接受的生活质量标准相关联：在发展中国家，城市增长的速度没有给农村城市系统提供适应的机会。最初的稀缺资源使必要的公共投资和基础设施不可能支持广泛的城市群，从而导致超大城市的发展，往往以空间不平等和贫民窟为特征。另一方面，气候变化预计会影响极端降水。气温升高应该会增加潮湿地区高强度降雨的发生，但即使在相同的天气条件下，土地的易感性也会因人类活动的发展而增强。众所周知，城市化过程中典型的森林砍伐和削坡会增加山体滑坡的发生率，从而影响水文过程和剪切应力分布。此外，由于快速城市化导致的低质量房屋，加上低经济可能性，增加了山体滑坡的风险，因为它们通常位于不稳定和未经批准的地区，通常与陈旧的排水网络有关。第一个目标是了解主要的风险驱动因素，以评估其在滑坡过程中的作用，如地形、植被、排水网络和加载增加，可能是新的收缩的房屋。必须评估预备因素、触发机制和加重因素的范围，以划定可能的失败区域。根据这些设定条件，可以将不同的降雨和城市化未来情景叠加到GISmaps上，以识别高滑坡易感性区域。然后用CHASM软件对特定的地理环境进行更详细的分析，CHASM是一种结合了土壤水文和边坡稳定性的基于物理的模型。单一住宅和单一排水网络对场地水文和稳定机制的影响进行了评估，然后在更密集的城市和网络环境中实施。CHASMS的单一分析结果可以修改（和改进）初始条件范围，创建一个能够识别滑坡易感区域和概述风险图的一般特定周期。与此同时，可以评估预防实际解决办法可能产生的后果。由于高频率的山体滑坡主要对发展中国家的经济增长产生负面影响，因此风险地图旨在成为决策过程的工具，以预防和减轻山体滑坡的发生。然而，由于经济和社会原因，建议的理论意图在实际应用中存在一些困难。为了鼓励政治上的长期投资，有必要进行科学证据论证，并在不同风险情景下进行成本效益评估，包括采取某些缓解措施。

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.