Temporal variability of influence factors weights and rainfall thresholds of geological hazards in Ningbo City
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摘要: 降雨是诱发突发地质灾害的关键要素,持续降雨与斜坡各地质环境要素耦合后会发生变形破坏,进而引起地质灾害各影响因子的权重体系发生变化。基于该理论,在降雨阈值研究过程中充分考虑了地质灾害各影响因子权重的时变性,这对精准预警、预报地质灾害具有重要意义。文章针对宁波市4种地质灾害(滑坡、崩塌、坡面泥石流和沟谷泥石流),选取9类影响因子(岩性、坡度等),分析了10个降雨时段(1 h、3 h、6 h等)的权重体系变化特征,提出了不同地质灾害风险防范区红、橙、黄三类预警的预报值。结果表明:4类地质灾害、9类影响因子的权重均在灾害发生前24 h出现明显变化,其中滑坡中坡度和覆盖层厚度、崩塌中坡度和高差、坡面泥石流中坡度和岩性、沟谷泥石流中坡度和高程6类影响因子权重在持续降雨过程中的变化较明显,坡度均呈正向变化,其余影响因子坡度呈反向变化。通过将时变权重与I-D对数法计算的雨量阈值进行对比,发现运用时变权重来计算雨量阈值能较真实地反映预警值,提高了预警的准确率,减少了过度预警的发生。Abstract: Rainfall is one of the key inducements of geological hazards. Continuous rainfall coupled with the geological environment elements will lead to the deformation and failure of the slope, which in turn causes the continuous changes in the weighting system of the various influence factors of geological hazards. Therefore, it is of great significance for accurate hazard warning and forecast to fully consider the temporal variability of the influence factors' weights of geological hazards in the research of rainfall threshold. This paper selected nine influence factors(lithology, slope, etc.)for four types of geological hazards including landslides, collapses, slope debris flows and gully debris flows to analyze the changes in the weighting system of ten rainfall periods(1 h,3 h,6 h etc.)in Ningbo, and finally put forward three levels (red, orange and yellow) of warning and forecasting value for different geo-hazard risk prevention zones. The results show that the weights of the nine influence factors of the four types of geological hazards change apparently within 24 hours after rainfall. The weights of the six influence factors, including the slope and soil layer thickness of landslides, the slope and elevation difference of collapses, the slope and lithology of slope debris flows, and the slope and elevation of the gully debris flows, change more obviously in the process of rainfall. The weights of slopes are generally positively correlated with time, while the weights of the other factors are negatively correlated with time. Comparing the rainfall thresholds calculated by the method considering the time-varying weights and the I-D logarithmic method, the result indicates that the rainfall thresholds calculated by the former method can reflect more real warning values.
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