北京市泥石流灾害临界雨量研究

降雨是泥石流灾害的主要诱发因素。文章根据北京市历史上泥石流灾害发生时的前期雨量与当日激发雨量,建立了临界雨量判别模型。通过对北京地区泥石流灾害与降雨频率的分析,计算了不同时段的临界雨量;经验证明,计算结果是可信的。基于灾害与降雨频率分析来确定北京地区泥石流发生的临界雨量是一种新的尝试。该方法可用于计算不同泥石流沟道发生泥石流的临界雨量。     

山洪灾害临界雨量研究综述

临界雨量是一个关键的山洪灾害预警指标。按其技术原理将临界雨量推求方法划分为数据驱动的统计归纳法和基于灾变物理机制的水文水力学法分别进行评述,并介绍了临界雨量指标的两个拓展:动态临界雨量和暴雨临界曲线,综述了临界雨量不确定性分析的研究进展。通过综述发现:中国目前主要应用的是较简单的统计归纳法;临界雨量推求主要考虑前期降雨量（前期土壤饱和度）和时段累积降雨量两个因素的影响或仅后者一个;临界雨量指标难以反映山洪灾害的规模;考虑临界雨量不确定性有助于提高预警质量,但如何充分考虑其影响仍然是一个挑战。     

基于Fisher判别算法的山洪灾害临界雨量指标计算

根据临界雨量指标的概念,将模式识别算法引入临界雨量计算中,提出了基于Fisher判别算法的临界雨量计算方法.首先确定特征时段,基于长系列山洪灾情记录将降雨数据分成成灾和非成灾两类,并分别统计特征时段雨量、计算其前期有效降雨,构建山洪灾害各特征时段雨量与前期有效降雨状态空间;建立基于Fisher判别算法的不同特征时段雨量...     

浙江省小流域山洪灾害临界雨量确定方法分析

小流域山洪灾害具有突发性,预测预警难度较大.本文结合浙江省小流域山洪灾害防御的实践经验,研究提出了以水位反推法计算临界雨量,简单实用,具有较好的实践价值和推广意义.     

湖北省山洪(泥石流)灾害气象条件分析及其预报研究

根据湖北省气象部门收集的1954年至2003年7月份的226个山洪(泥石流)灾害个例和同期气象资料，分析发现：(1)湖北省山洪(泥石流)灾害3～10月均有发生，6～8月最为集中．占总数84％；空间分布广而不均，鄂东南、鄂西南最多，灾害的时空分布与降雨的时空分布较一致。(2)68％的灾害发生在系统性降雨或集中降雨天气形势下，把握好系统性降雨和集中降雨的预测是有效预测山洪(泥石流)灾害的关键。(3)82．3％的灾害与当日暴雨以上强降雨密切相关。最后用点聚图方法确定了各区致灾的临界雨量，建立了灾害气象预报指标．该指标已投入试用。     

Exploiting historical rainfall and landslide data in a spatial database for the derivation of critical rainfall thresholds

Critical rainfall thresholds for landslides are powerful tools for preventing landslide hazard. The thresholds are commonly estimated empirically starting from rainfall events that triggered landslides in the past. The creation of the appropriate rainfall–landslide database is one of the main efforts in this approach. In fact, an accurate agreement between the landslide and rainfall information, in terms of location and timing, is essential in order to correctly estimate the rainfall–landslide relationships. A further issue is taking into account the average moisture conditions prior the triggering event, which reasonably may be crucial in determining the sufficient amount of precipitation. In this context, the aim of this paper is exploiting historical landslide and rainfall data in a spatial database for the derivation of critical rainfall thresholds for landslide occurrence in Sicily, southern Italy. The hourly rainfall events that caused landslides occurred in the twentieth century were specifically identified and reconstructed. A procedure was proposed to automatically convert rain guages charts recorded on paper tape into digital format and then to provide the cumulative rainfall hyetograph in digital format. This procedure is based on a segmentation followed by signal recognition techniques which allow to digitalize and to recognize the hyetograph automatically. The role of rainfall prior to the landslide events was taken into account by including in the analysis the rainfall occurred 5, 15 and 30 days before each landslide. Finally, cumulated rainfall duration thresholds for different exceedance probability levels were determined. The obtained thresholds resulted in agreement with the regional curves proposed by other authors for the same area; antecedent rainfall turned out to be particularly important in triggering landslides.     

An integrated model to assess critical rainfall thresholds for run-out distances of debris flows

A dramatic increase in debris flows occurred in the years after the 2008 Wenchuan earthquake in SW China due to the deposition of loose co-seismic landslide material. This paper proposes a preliminary integrated model, which describes the relationship between rain input and debris flow run-out in order to establish critical rain thresholds for mobilizing enough debris volume to reach the basin outlet. The model integrates in a simple way rainfall, surface runoff, and concentrated erosion of the loose material deposited in channels, propagation, and deposition of flow material. The model could be calibrated on total volumes of debris flow materials deposited at the outlet of the Shuida catchment during two successive rain events which occurred in August 2011. The calibrated model was used to construct critical rainfall intensity-duration graphs defining thresholds for a run-out distance until the outlet of the catchment. Model simulations show that threshold values increase after successive rain events due to a decrease in erodible material. The constructed rainfall intensity-duration threshold graphs for the Shuida catchment based on the current situation appeared to have basically the same exponential value as a threshold graph for debris flow occurrences, constructed for the Wenjia catchment on the basis of 5 observed triggering rain events. This may indicate that the triggering mechanism by intensive run-off erosion in channels in this catchment is the same. The model did not account for a supply of extra loose material by landslips transforming into debris flow or reaching the channels for transportation by run-off. In August 2012, two severe rain events were measured in the Shuida catchment, which did not produce debris flows. This could be confirmed by the threshold diagram constructed by the model.     

Cost-sensitive rainfall thresholds for shallow landslides

The risk management of rainfall-induced landslides requires reliable rainfall thresholds to issue early warning alerts. The practical application of these thresholds often leads to misclassifications, either false negative or false positive, which induce costs for the society. Since missed-alarm (false negative) and false-alarm (false positive) cost may be significantly different, it is necessary to find an optimal threshold that accounts for and minimises such costs, tuning the false-alarm and missed-alarm rates. In this paper, we propose a new methodology to develop cost-sensitive rainfall thresholds, and we also analyse several factors that produce uncertainty, such as the accuracy of rainfall intensity values at landslide location, the time of occurrence, the minimum rainfall amount to define the non-triggering event, and the variability of cost scenarios. Starting from a detailed mapping of landslides that occurred during five large-scale rainfall events in the Italian Central Alps, we first developed rainfall threshold curves with a ROC-based approach by using both rain gauge and bias-adjusted weather radar data. Then, based on a reference cost scenario in which we quantified several cost items for both missed alarms and false alarms, we developed cost-sensitive rainfall threshold curves by using cost-curve approach (Drummond and Holte 2000). Finally, we studied the sensitivity of cost items. The study confirms how important is the information regarding rainfall intensity at the landslide site for the development of rainfall thresholds. Although the use of bias-corrected radar strongly improves these values, a large uncertainty related to the exact time of landslide occurrence still remains, negatively affecting the analysis. Accounting for the different missed-alarm and false-alarm misclassification costs is important because different combinations of these costs make an increase or decrease of the rainfall thresholds convenient. In our reference cost scenario, the most convenient threshold is lower than ROC-based thresholds because it seeks to minimise the number of missed alarms, whereas the missed-alarm costs are almost seven times greater than false-alarm costs. However, for different cost scenarios, threshold may vary significantly, as much as half an order of magnitude.

     

Updating EWS rainfall thresholds for the triggering of landslides

In this paper, the updating of rainfall thresholds for landslide early warning systems (EWSs) is presented. Rainfall thresholds are widely used in regional-scale landslide EWSs, but the efficiency of those systems can decrease during the time, so a periodically updating should be required to keep their functionality. The updating of 12 of the 25 thresholds used in the EWS of Tuscany region (central Italy) is presented, and a comparison between performances of new and previous thresholds has been made to highlight the need of their periodical update. The updating has been carried out by collecting ca. 1200 new landslide reports (from 2010 to March 2013) and their respective rainfall data, collected by 332 rain gauges. The comparison has been made by the use of several statistical indexes and showed a marked increasing in the performances of the new thresholds with respect to previous ones.     

Spatially distributed rainfall thresholds for the initiation of shallow landslides

The evaluation of the combined influence of rainfall patterns (in terms of mean intensity and duration) and the geomorphological and mechanical characteristics of hillslopes on their stability conditions is a major goal in the assessment of the shallow landslide triggering processes. Geographic Information Systems (GIS) represent an important tool to develop models that combine hydrological and geomechanical analyses for the evaluation of slope stability, as they allow to combine information concerning rainfall characteristics with topographic and mechanical properties of the slopes over wide areas. In this paper, a GIS-based code is developed to determine physically based intensity/duration rainfall thresholds at the local scale. Given the rainfall duration and the local geometric, hydrological and mechanical characteristics of the slopes, the code evaluates the spatial distribution of the minimum rainfall intensity that triggers shallow landslides and debris flows over a given area. The key feature of the code is the capability of evaluating the time t _p required to reach the peak pore pressure head on the failure surface and computing the corresponding critical intensity/duration thresholds based on post-event peak pore pressures. The reliability of the model is tested using a set of one-dimensional analyses, comparing the physically based thresholds obtained for three different slopes with some empirical rainfall thresholds. In a log–log scale, the thresholds provided by the model decrease linearly with increased rainfall duration and they are bracketed by the empirical thresholds considered. Finally, an example of application to a study area of the Umbria region in central Italy is presented, describing the capability of the model of providing site-specific thresholds for different rainfall scenarios.     

浙江宁波市地质灾害的雨量阈值及预报分析

山体滑坡、崩塌、泥石流等地质灾害主要由强降水或连续性降水引发产生,根据最近七年来宁波市国土资源局和宁波市气象台联合制作的地质灾害气象预报预警和地质灾害实况资料分析,结果发现:在最近七年里出现的159处地质灾害中,由热带气旋强降水引发的地质灾害有142处,约占89%;宁波市绝大多数地质灾害是由强降水引发,而强降水中又以热带气旋强降水为主要诱因,引发宁波市地质灾害的有效雨量大多数在100mm以上,因此,100mm有效雨量可作为预报引发宁波市地质灾害的雨量阈值,而200mm有效雨量可引发多处地质灾害产生。统计结果为以后地质灾害气象预报预警及强降水的临近监测提供参考依据。     

Accounting for mudflow genesis in preliminary assessment of the maximum volume of solid mudflow sediments in the North Caucasus

The area of investigation was subject to multifactor analysis of the relationship between the maximum amount of mudflow solid sediments (W) and parameters such as the mudflow basin area (S), average channel slope (α), and mudflow channel length (L). They were used to obtain analytical expressions in order to approximate the W(S, L, α) relation based on the mudflow genesis and source height. Statistical data on mudflow manifestations in different basins in the North Caucasus covering more than fifty years were used to obtain the analytical expressions in order to assess the maximum volume of mudflow solid sediments.