Shallow landslide hazard assessment using a physically based model and digital elevation data

 A model for the analysis of topographic influence on shallow landslide initiation is applied to an experimental mountain basin where high-resolution digital elevation data are available: the Cordon catchment (5 km²) located in northern Italy. The model delineates those areas most prone to shallow landsliding due to surface topographic effects on hydrologic response. The model is composed of two parts: a steady-state model for shallow sub-surface runoff and an infinite-slope Coulomb failure model which assumes that the soil is cohesionless at failure. An inventory of landslide scars is used to document sites of instability and to provide a test of model performance by comparing observed landslide locations with model predictions. The model reproduces the observed distribution of landslide locations in a consistent way, although spatial variations in soil strength and transmissivity, which are not accounted for in the model, influence specific distribution of landslide areas within regions of similar topographic control. Received: 15 October 1996 · Accepted: 25 June 1997     

Evaluation of approaches to calculate debris-flow parameters for hazard assessment

Many different runout prediction methods can be applied to estimate the mobility of future debris flows during hazard assessment. The present article reviews the empirical, analytical, simple flow routing and numerical techniques. All these techniques were applied to back-calculate a debris flow, which occurred in 1982 at La Guingueta catchment, in the Eastern Pyrenees. A sensitivity analysis of input parameters was carried out, while special attention was paid to the influence of rheological parameters. We used the Voellmy fluid rheology for our analytical and numerical modelling, since this flow resistance law coincided best with field observations. The simulation results indicated that the “basal” friction coefficients rather affect the runout distance, while the “turbulence” terms mainly influence flow velocity. A comparison of the velocity computed on the fan showed that the analytical model calculated values similar to the numerical ones. The values of our rheological parameters calibrated at La Guingueta agree with data back-calculated for other debris flows. Empirical relationships represent another method to estimate total runout distance. The results confirmed that they contain an important uncertainty and they are strictly valid only for the conditions, which were the basis for their development. With regards to the simple flow routing algorithm, this methods could satisfactorily simulate the total area affected by the 1982 debris flow, but it was not able to directly calculate total runout distance and velocity. Finally, a suggestion on how different runout prediction methods can be applied to generate debris-flow hazard maps is presented. Taking into account the definition of hazard and intensity, the best choice would be to divide the resulting hazard maps into two types: “final hazard maps” and “preliminary hazard maps”. Only the use of numerical models provided final hazard maps, because they could incorporate different event magnitudes and they supplied output-values for intensity calculation. In contrast, empirical relationships and flow routing algorithms, or a combination of both, could be applied to create preliminary hazard maps. The present study only focussed on runout prediction methods. Other necessary tasks to complete the hazard assessment can be looked up in the “Guidelines for landslide susceptibility, hazard and risk zoning” included in this Special Issue.     

Debris flow hazard assessment with numerical simulation

Debris flow disasters are usually accompanied by serious loss of lives and properties. However, debris flows are also part of earth’s natural phenomenon, and so what is the reasonable budget to be spent on mitigation measures becomes an important issue for the budget allocation processes. This article utilizes economic concepts to propose a reasonable estimation of the hazard damage and the cost of proposed mitigation measures. The proposed method is composed of four steps, namely, delineating the area of the disaster with different return periods, itemizing the land use within those areas, calculating the hazard loss using official values, and computing the expected probable maximum loss with a probability distribution. The comparison between the assessment of hazard and the economic gains of any proposed mitigation measures can be used as a reference for future decision-making process.     

Zoning for flowslide and debris flow in pyroclastic soils of Campania Region based on “infinite slope” analysis

Campania Region (Italy), one of the most densely populated areas in Europe, is probably the one with the highest risk of landslide. A large part of the region is covered by unsaturated cohesionless pyroclastic deposits subjected to rainfall-induced landslides. According to experience, these can display different features and magnitude. The most catastrophic landslides are liquefied debris flows which periodically occur on steep slopes, causing death and destruction in areas located downslope. Therefore, zoning of those areas which can be the source of liquefied debris flow is necessary. The paper reports some useful elements for zoning based on infinite slope analysis, accounting for the results of recent research on the mechanics of rainfall-induced landslides in pyroclastic soils.     

Assessing debris-flow hazard in a watershed in Taiwan

This paper presents the results of a pilot study for assessing debris-flow hazards using geographic information system (GIS) techniques. The watershed of the Chen-Yu-Lan River is investigated in this pilot study. Factors that are believed to be critical to the occurrence of debris flow are identified and considered in the assessment of debris-flow hazards. Using the spatial analysis feature of GIS, the impact of these factors, expressed in terms of debris-flow hazard (DH) index, is calculated. By taking a simple summation of all DH indexes according to each factor, the overall debris-flow hazard at a particular watershed may be assessed. The applicability of the proposed approach for analyzing the watershed of the Chen-Yu-Lan River has been confirmed with the field observations in a recent typhoon event.     

Neural network modeling for regional hazard assessment of debris flow in Lake Qionghai Watershed, China

This paper presents a neural network (NN) based model to assess the regional hazard degree of debris flows in Lake Qionghai Watershed, China. The NN model was used as an alternative for the more conventional linear model MFCAM (multi-factor composite assessment model) in order to effectively handle the nonlinearity and uncertainty inherent in the debris flow hazard analysis. The NN model was configured using a three layer structure with eight input nodes and one output node, and the number of nodes in the hidden layer was determined through an iterative process of varying the number of nodes in the hidden layer until an optimal performance was achieved. The eight variables used to represent the eight input nodes include density of debris flow gully, degree of weathering of rocks, active fault density, area percentage of slope land greater than 25° of the total land (APL25), frequency of flooding hazards, average covariance of monthly precipitation by 10 years (ACMP10), average days with rainfall >25 mm by 10 years (25D10Y), and percentage of cultivated land with slope land greater than 25° of the total cultivated land (PCL25). The output node represents the hazard-degree ranks (HDR). The model was trained with the 35 sets of data obtained from previous researches reported in literatures, and an explicit uncertainty analysis was undertaken to address the uncertainty in model training and prediction. Before the NN model is extrapolated to Lake Qionghai Watershed, a validation case, different from the above data, is conducted. In addition, the performances of the NN model and the MFCAM were compared. The NN model predicted that the HDRs of the five sub-watersheds in the Lake Qionghai Watershed were IV, IV, III, III, and IV–V, indicating that the study area covers normal hazard and severe hazard areas. Based on the NN model results, debris flow management and economic development strategies in the study are proposed for each sub-watershed.     

A GIS-based methodology for hazard mapping of small volume pyroclastic density currents

We present here a methodology implemented within a geographical information system (GIS) for hazard mapping of small volume pyroclastic density currents (PDCs). This technique is implemented as a set of macros written in Visual Basic for Applications (VBA) that run within GIS-software (i.e. ArcGIS). Based on the energy line concept, we calibrated an equation that relates the volume (V) and the mobility (ΔH/L) of single PDCs using data from Soufrière Hills volcano (Montserrat) and Arenal volcano (Costa Rica). Maximum potential run-outs can be predicted with an associated uncertainty of about 30%. Also based on the energy line concept and with data from Soufrière Hills volcano and Mt. St. Helens (USA), we were able to calibrate an equation that predicts the flow velocity as a function of the vertical distance between the energy line and the ground surface (Δh). Velocities derived in this way have an associated uncertainty of 3 m s⁻¹. We wrote code to implement these equations and allow the automatic mapping of run-out and velocity with the inputs being (i) the height and location of the vent (ii) the flow volume and (iii) a digital elevation model (DEM) of the volcano. Dynamic pressure can also be estimated and mapped by incorporating the density of the pyroclastic density current (PDC). This computer application allows the incorporation of uncertainties in the location of the vent and of statistical uncertainties expressed by the 95% confidence limits of the regression model. We were able to verify predictions by the proposed methodology with data from Unzen volcano (Japan) and Mayon volcano (The Philippines). The consistencies observed highlight the applicability of this approach for hazard mitigation and real-time emergency management.     

贡献权重模型在区域滑坡危险性评价中的应用

以攀枝花市米易县为研究区域,采用GIS技术,选取坡度、坡向、坡形、地层岩性和相对高差等5个指标作为评价因子,利用贡献权重模型对研究区的滑坡地质灾害进行危险性评价。评价结果经过检验符合实际情况:高危险度区占全区面积31.9%;中危险度区占全区面积52.6%;低危险度区占全区面积15.5%;高、中危险度区所包含的滑坡个数占到全区滑坡总数的95.9%。评价结果表明,GIS技术和贡献权重模型结合能够很好地为滑坡灾害危险性评价服务,可以解决过去危险性评价中效率低、精度差等问题,从而实现滑坡灾害评价的信息化和科学化。     

Rain induced shallow landslide hazard assessment for ungauged catchments

A physically based distributed slope stability model derived from the integration of hydrologic analysis and slope stability analysis is presented for GIS based modeling on a catchment scale. The catchment is represented by a mesh of squares with vertical columns. The hydrologic model calculates subsurface flow and resulting pore-water pressure during rainfall in each cell for every time step. The slope stability module then calculates the stability of each cell for the changing water pressure scenario. There are very few input data requirements, which makes the model applicable to ungauged catchments, also where the record of the past landslides and associated rainfall are not available. The model was tested in Garjuwa catchment, Nepal where all landslide scars were documented. The model reproduces the observed distribution of landslide locations in a consistent way. Several model runs were conducted, increasing the intensity of rainfall to see corresponding increase in instability. The results show that there is a critical rainfall intensity for the catchment, beyond which increase in rainfall intensity does not increase instability significantly.     

Empirical modelling of the May 1998 small debris flows in Sarno (Italy) using LAHARZ

The catastrophic events that occurred in May 1998 in the area of Sarno (Southern Italy) highlight the destructive potential of debris flows, even when they are of relatively low magnitude. More than 130 people were killed and severe property damage took place when volcaniclastic debris flows triggered by heavy rainfall inundated various towns located in piedmont areas. This work investigates the suitability of LAHARZ, a GIS-assisted method for the automatic delineation of lahar inundation areas, for reproducing the May 1998 flows at Sarno. It was found that recalibration of the empirical relationship employed by LAHARZ is required in order to realistically hind-cast the inundation areas of considered events. The potential for further improvements in prediction outputs for this type of geomorphic setting is discussed, taking into account the observed lower mobility of these small volcaniclastic debris flows as compared to lahars of similar size.     

A procedure for making objective preliminary assessments of outburst flood hazard from moraine-dammed lakes in southwestern British Columbia

Existing methods of evaluating the hazard posed by moraine-dammed lakes are unsystematic, subjective, and depend on the expertise and biases of the geoscientist. In this paper, we provide a framework for making objective preliminary assessments of outburst flood hazard in southwestern British Columbia. Our procedure relies on remote sensing methods and requires only limited knowledge of glacial processes so that evaluations of outburst flood hazard can be incorporated into routine hazard assessments of glaciated regions. We describe objective approaches, which incorporate existing empirical relations applicable to the study region, for estimating outburst peak discharge, maximum volume, maximum travel distance, maximum area of inundation, and probability. Outburst flood hazard is greatest for moderately large lakes that are impounded by large, narrow, ice-free moraine dams composed of sedimentary rock debris and drain into steep, sediment-filled gullies above major river valleys. We demonstrate the application of the procedure using three case studies and show that flood hazard varies, especially with major changes in lake level. Our assessment scheme yields reproducible results and enables engineers and geoscientists to prioritize potentially hazardous lakes for more detailed field investigation.     

Performance-based assessment of earthquake-induced catastrophic landslide hazard in liquefiable soils

This paper outlines a methodology for evaluating the likelihood of catastrophic landslide occurrence on gentle slopes in liquefiable soils during earthquake. The approach is based on a modified Newmark sliding block model of assessing the earthquake-induced undrained landslide displacements for conditions of no shear stress reversals on the sliding surface. By employing the shear resistance-displacement relationship from undrained monotonic ring shear tests, the simulation model incorporates the sensitivity of computed displacements to variations in yield acceleration. The proposed approach involves an examination of undrained seismic slope performance under various horizontal seismic waveforms scaled to different specific values of the peak earthquake acceleration. An example problem illustrates how the proposed methodology may be used to demarcate, based on the magnitude of permanent seismic displacement, the levels of low, moderate and high risk of catastrophic landslide on a gentle slope in a saturated cohesionless soil susceptible to liquefaction during earthquake.     

A time-space based approach for mapping rainfall-induced shallow landslide hazard

A rainfall-induced shallow landslide is a major hazard in mountainous terrain, but a time-space based approach is still an unsettled issue for mapping rainfall-induced shallow landslide hazards. Rain induces a rise of the groundwater level and an increase in pore water pressure that results in slope failures. In this study, an integrated infinite slope analysis model has been developed to evaluate the influence of infiltration on surficial stability of slopes by the limit equilibrium method. Based on this new integrated infinite slope analysis model, a time-space based approach has been implemented to map the distributed landslide hazard in a GIS (Geographic Information Systems) and to evaluate the shallow slope failure induced by a particular rainfall event that accounts for the rainfall intensity and duration. The case study results in a comprehensive time-space landslide hazard map that illustrates the change of the safety factor and the depth of the wetting front over time.     

Morphological methods and dynamic modelling in landslide hazard assessment of the Campania Apennine carbonate slope

Landslide risk of the Campanian carbonate slopes covered by pyroclastic deposits is mainly connected with the occurrence of high-velocity debris avalanches and debris flows. Analyses show that flows initiate as small translational slides in the pyroclastics. The failure process is controlled by the interaction of both natural and human-induced factors. Geomorphological settings play a decisive role in locating the source failures. Therefore, the crucial aspects in landslide hazard and risk assessment are: (a) recognise the geomorphological control factors, (b) determine parameters defining landslide intensity (velocity, volume, depth of deposit) and (c) predict landslide runout distance. An approach combining geomorphology and numerical analysis has been adopted in the work reported here. Potential future landslide intensity scenarios are simulated predicting the runout behaviour of potential instabilities by using a dynamic model previously calibrated by back-analysing observed events of similar scale and type. The selected area is a sector of the Avella Mountains having the same geomorphological environment as the 1998 Sarno landslides (Campania, Southern Italy).     

Mapping of hazard from rainfall-triggered landslides in developing countries: Examples from Honduras and Micronesia

Loss of life and property caused by landslides triggered by extreme rainfall events demonstrates the need for landslide-hazard assessment in developing countries where recovery from such events often exceeds the country's resources. Mapping landslide hazards in developing countries where the need for landslide-hazard mitigation is great but the resources are few is a challenging, but not intractable problem. The minimum requirements for constructing a physically based landslide-hazard map from a landslide-triggering storm, using the simple methods we discuss, are: (1) an accurate mapped landslide inventory, (2) a slope map derived from a digital elevation model (DEM) or topographic map, and (3) material strength properties of the slopes involved. Provided that the landslide distribution from a triggering event can be documented and mapped, it is often possible to glean enough topographic and geologic information from existing databases to produce a reliable map that depicts landslide hazards from an extreme event. Most areas of the world have enough topographic information to provide digital elevation models from which to construct slope maps. In the likely event that engineering properties of slope materials are not available, reasonable estimates can be made with detailed field examination by engineering geologists or geotechnical engineers. Resulting landslide hazard maps can be used as tools to guide relocation and redevelopment, or, more likely, temporary relocation efforts during severe storm events such as hurricanes/typhoons to minimize loss of life and property. We illustrate these methods in two case studies of lethal landslides in developing countries: Tegucigalpa, Honduras (during Hurricane Mitch in 1998) and the Chuuk Islands, Micronesia (during Typhoon Chata'an in 2002).     

Qualitative hazard and risk assessment of landslides: a practical framework for a case study in China

Natural Hazards - Landslides are presented in various types; some of which are unique or completely different from those in other countries due to geological conditions in China. Baoji City in...     

基于强度折减概念的滑坡稳定性三维分析方法(I)：滑带土抗剪强度参数反演分析

滑带土抗剪强度是滑坡稳定性分析和防治工程设计中十分重要而又难于确定的参数之一。因此,基于临界状态假定的二维反分析方法得到了广泛应用。但是,自然界发生的滑坡基本上呈三维形态,其主滑方向有时变化也较大,使得有必要研究一种滑带土抗剪强度的三维反分析方法和滑坡稳定性的三维评价方法。笔者建议了一种基于强度折减概念的滑带土抗剪强度反分析方法,即通过逐步折减滑动面的强度参数,使滑动面的塑性区完全贯通,此时,塌滑体处于极限状态,所用强度参数即为滑带土的平均抗剪强度参数。从洪家渡水电站1#塌滑体计算成果来看,反演的滑带土摩擦角较二维反演值低4.1?,反映了滑坡体的三维效应,验证了所建议方法的可行性。     

Spatial analysis and hazard assessment of mercury in soil around the coal-fired power plant: a case study from the city of Baoji, China

Based on systematic sampling of soil around the coal-fired power plant (CFPP), the content of Hg was determined, using atomic fluorescence spectrometry. The result shows that the content of Hg in soil is different horizontally and vertically, ranges from 0.137 to 2.105 mg/kg (the average value is 0.606 mg/kg) and is more than the average content of Hg in Shaanxi, Chinese and world soil. In this study, spatial distribution and hazard assessment of mercury in soils around a CFPP were investigated using statistics, geostatistics and geographic information system (GIS) techniques. Ordinary kriging was carried out to map the spatial patterns of mercury and disjunctive kriging was used to quantify the probability of the Hg concentration higher than the threshold. The maps show that the spatial variability of the Hg concentration in soils was apparent. These results of this study could provide valuable information for risk assessment of environmental Hg pollution and decision support. An erratum to this article can be found at     

The influence of the geological and geomorphological settings on shallow landslides. An example in a temperate climate environment: the June 19, 1996 event in northwestern Tuscany (Italy)

On June 19, 1996, an extremely heavy rainstorm hit a restricted area in the Apuan Alps (northwestern Tuscany, Italy). Its max intensity concentrated over an area of about 150 km² astride the Apuan chain, where 474 mm was recorded in about 12 h (21% of the mean annual precipitation, with an intensity up to 158 mm/h). The storm caused floods and hundreds of landslides and debris flows, which produced huge damage (hundreds of millions of Euros), partially destroyed villages and killed 14 people. This paper reports the results obtained from a detailed field survey and aerial view interpretation. In the most severely involved area, 647 main landslides were investigated, mapped and related to the geologic, geomorphic and vegetational factors of the source areas. This was in order to define the influence of these factors and contribute to an evaluation of the landslide hazard in the study area. An assessment was also made of the total area and volume of material mobilised by landsliding. The study area, about 46 km² wide, includes three typically mountainous basins, characterised by narrow, deep cut valleys and steep slopes, where many rock types outcrop. Most of the landslides were shallow and linear, referable to complex, earth and debris translational slide, which quickly developed into flow (soil slip–debris flow). Usually, they involved colluvium and started in hollows underlain by metamorphic rock (metasandstone and phyllite), often dipping downslope. Therefore, bedrock lithology and impermeability appeared to be important factors in the localisation of the landslide phenomena. The investigation of the geomorphic and land use features in the source areas also frequently highlighted a rectilinear profile of the slope, a high slope gradient (31–45°) and dense chestnut wood cover. In the area, about 985,000 m² (2.1% of 46 km²) was affected by landsliding and about 700,000 m² of this area was covered by chestnut forest. The landslides removed about 7000 trees. The volume of mobilised material was about 1,360,000 m³; about 220,000 m³ remained on the slopes, while the rest poured into the streams. In addition, about 945,000 m³ was mobilised by the torrential erosion in the riverbeds.     

Test of a distributed modelling approach to predict flood flows in the karst Suoimuoi catchment in Vietnam

The major obstacles for modelling flood processes in karst areas are a lack of understanding and model representations of the distinctive features and processes associated with runoff generation and often a paucity of field data. In this study, a distributed flood-modelling approach, WetSpa, is modified and applied to simulate the hydrological features and processes in the karst Suoimuoi catchment in northwest Vietnam. With input of topography, land use and soil types in a GIS format, the model is calibrated based on 15 months of hourly meteorological and hydrological data, and is used to simulate both fast surface and conduit flows, and groundwater discharges from karst and non-karst aquifers. Considerable variability in the simulation accuracy is found among storm events and within the catchment. The simulation results show that the model is able to represent reasonably well the stormflows generated by rainfall events in the study catchment.