Debris Flow Occurrence Research Articles

B40 肥薩型土石流発生のメカニズム : 2003 年 7 月 20 日水俣市宝川内土石流災害

B40 肥薩型土石流発生のメカニズム : 2003 年 7 月 20 日水俣市宝川内土石流災害

2015年公路G314群发性水毁灾害类型特征及成因分析

2015年7月30日至8月8日，新疆G314奥布段发生群发性水毁灾害，致使公路多处阻断且严重毁损。公路水毁灾害具有类型多、群发性、规模大、破坏力强等特点，主要类型包括路基冲毁破坏、泥石流掩埋破坏、挡墙冲毁破坏、桥涵冲毁破坏、防护工程损坏等。持续高温下冰川融水和局地短历时强降雨径流的叠加，促使盖孜河及支流暴发群发性洪水泥石流，公路设计欠合理和施工不恰当导致公路发生水毁灾害。研究成果可为公路设计养护和管理部门提供参考，为公路水毁防治提供指导。 Massive waste washout disasters occurred in the Aobu section of highway G314 in Xinjiang, which led to severe traffic jam and serious road damage during 30th July to 8th August in 2015.The types of washout disasters includes the subgrade water logging, buried damage from debris flow and destruction of the bridge, retaining wall and protection engineering, etc., accompanied with the features of various types, group-occuring, large scale, and strong destructive power, and so on. The glacier melt-water with continuous high temperature and the local heavy rainfall with short duration together promote the occurrence of floods and debris flows along the Gaizi River and its tributaries. More important, due to unreasonable designing and inappropriate construction of the highway, the washout disasters along the highway have happened and caused heavy damage. The research results can provide reference for highway design maintenance and management, and provide the guidance for highway washout prevention.

Regional Scale Mapping of Debris-Flow Susceptibility in the Southern French Alps

This article describes a regional methodology of debris-flow susceptibility mapping. The implementation of this approach is based on statistical and GIS methods and makes it possible to identify catchments in the Southern French Alps, according to their morphometric (Melton index and local slope) and sediment supply conditions (active erosion zones in headwater catchments), that are prone to debris flows because of hydrometeorological forcing. This methodology has been validated on 50 sites with very high scores; the geomorphic response was correctly predicted for 80% of the sites. The map of susceptibility to the occurrence of debris flow has been integrated into the RhyTMME project’s Web platform. This platform also offers a visualisation of the rainfall’s spatial distribution (from meteorological radar data), enabling automatic real-time detection of the triggering of debris flow at the regional scale.

Slope failure and subsequent debris flow occurrence on May 2017 at Iiyama, Nagano, Japan

Slope failure and subsequent debris flow occurrence on May 2017 at Iiyama, Nagano, Japan

Debris flows in the central sector of the northern slopes of the Main Caucasus Ridge: peculiarities of the current situation

This paper provides examples of real events of debris flows in the 21st century. The analysis of the debris flows reveals that there is a variety of manifestation of debris flow processes and triggering factors. It also demonstrates the lack of prospects for the modern trend of creating a single model for forecasting the occurrence, development, impacts and parameters of debris flows that would be equally suitable for other phenomena of gravitational nature such as avalanches, landslides or rock falls. Moreover, it shows that the monitoring is unrepresentative, which is unacceptable, and demonstrates the often lack of hydro-meteorological data on mountainous areas.

Spatial estimation of debris flows-triggering rainfall and its dependence on rainfall return period

Forecasting the occurrence of debris flows is fundamental for issuing hazard warnings, and often focuses on rainfall as a triggering agent and on the use of empirical rainfall thresholds based on rain gauge observations. A recognized component of the uncertainty associated with the use of rainfall thresholds is related to the sampling of strongly varying rainfall variability with sparse rain gauge networks. In this work we examine the spatial distribution of rainfall depth in areas up to 10km from the debris flow initiation points as a function of return period, and we exploit this information to analyze the errors expected in the estimation of debris flow triggering rainfall when rain gauge data are used. In particular, we investigate the impact of rain gauge density and of the use of different interpolation methods. High-resolution, adjusted radar rainfall estimates, representing the best available spatially-distributed rainfall estimates at the debris flows initiation point and in the surrounding area, are sampled by stochastically generated rain gauge networks characterized by varying densities. Debris flow triggering rainfall is estimated by means of three rainfall interpolation methods: nearest neighbor, inverse distance weighting and ordinary kriging. On average, triggering rainfall shows a local peak corresponding to the debris flow initiation point, with a decay of rainfall with distance which increases with the return period of the triggering rainfall. Interpolation of the stochastically generated rain gauge measurements leads to an underestimation of the triggering rainfall that, irrespective of the interpolation methods, increases with the return period and decreases with the rain gauge density. For small return period events and high rain gauge density, the differences among the methods are minor. With increasing the return period and decreasing the rain gauge density, the nearest neighbor method is less biased, because it makes use only of the closest rain gauge to the debris flow initiation point. On the contrary, inverse distance weighting and ordinary kriging, which are using rain gauges located farther from the debris flows in addition to the closest one, exhibit negative biases that increase with return period. The standard deviation of the interpolated values is larger when the nearest neighbor is used with respect to inverse distance weighting and ordinary kriging. For large return period and low rain gauge density, the differences among the methods are minor.

Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the western United States

Early warning of post-fire debris-flow occurrence during intense rainfall has traditionally relied upon a library of regionally specific empirical rainfall intensity–duration thresholds. Development of this library and the calculation of rainfall intensity-duration thresholds often require several years of monitoring local rainfall and hydrologic response to rainstorms, a time-consuming approach where results are often only applicable to the specific region where data were collected. Here, we present a new, fully predictive approach that utilizes rainfall, hydrologic response, and readily available geospatial data to predict rainfall intensity–duration thresholds for debris-flow generation in recently burned locations in the western United States. Unlike the traditional approach to defining regional thresholds from historical data, the proposed methodology permits the direct calculation of rainfall intensity–duration thresholds for areas where no such data exist. The thresholds calculated by this method are demonstrated to provide predictions that are of similar accuracy, and in some cases outperform, previously published regional intensity–duration thresholds. The method also provides improved predictions of debris-flow likelihood, which can be incorporated into existing approaches for post-fire debris-flow hazard assessment. Our results also provide guidance for the operational expansion of post-fire debris-flow early warning systems in areas where empirically defined regional rainfall intensity–duration thresholds do not currently exist.

Runoff of small rocky headwater catchments: Field observations and hydrological modeling

In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼0.032 km2, average slope ∼320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp-crested weir. Six runoff events were recorded in the period 2011–2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve-Number (SCS-CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape, including peak value and timing, is obtained only by combining the SCS-CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins.

Assessment of the debris-flow susceptibility in tropical mountains using clast distribution patterns

Channel morphometric parameters and clast distribution patterns in selected basins of the Ferriferous Quadrangle tropical mountains, Brazil, were analyzed in order to assess susceptibility to debris flows. Median bed surface clast size (D50) in the main stream channel of these basins shows a coarsening downstream trend with drainage areas of up to 6km2, which is attributed to debris flow dominated-channels by some authors. The composition and roundness of the bed load, clast sand, and the presence of allochthonous large boulders throughout the channels also suggest the occurrence of past debris flow in the region. Luminescence Optically Stimulated (LOE) dating points out that debris flow could have occurred as a consequence of climate changes in the Late Pleistocene and Holocene and it can now be triggered by deforestation or extreme rainfall events. There has not been any record of past debris flow in the study area, or in other mountainous regions of Brazil where debris flows have recently occurred. Thus, the adopted approach can be useful to assess debris flow susceptibility in this and other similar areas.

The changing ability of Norway spruce (P. abies) to record hydro-geomorphic processes based on the age and diameter of the tree stem – A dendrogeomorphic approach

Dendrogeomorphic reconstructions of debris flows and related hydro-geomorphic processes (i.e. debris floods and flash floods) are widely used in territories with incomplete chronologies and potential future hazards. In recent years, the tree-ring analysis of disturbed trees affected by dangerous geomorphic processes has developed into a discipline with sophisticated methodical approaches and with annual up to monthly accuracy of dating. Field sampling strategies are based on various ranges of sample depth (i.e., different amounts of sampled trees of different age structures). The age and diameter of trees influence the ability of trees to respond to geomorphic processes; therefore, we investigate age-dependent and diameter-dependent tree sensitivity. Our study, for the first time, addresses the sensitivity to hydro-geomorphic processes in detail. We tested 462 individuals of Norway spruce (Picea abies (L.) Karst.) – a very common species used in the dendrogeomorphic reconstructions – in the apical part of the medium-high-mountains where there is widespread occurrence of debris flows and debris floods. The tree sensitivity analysis was performed based on the position of growth disturbances within the increment cores of trees (original age and original diameter of the tree before the tree was affected). P. abies shows very high tree sensitivity in the first two decades of the tree lifespan. With increasing age and diameter, trees are less sensitive recorders of debris flows/floods. Moreover, the intensity of growth disturbances significantly decreases with tree age. Mature trees (51–120years) record the geomorphic events mainly through abrupt growth changes (suppression and release) and tangential rows of traumatic resin ducts. In contrast, scars, the onset of reaction wood and traumatic resin ducts are dominant growth disturbances in young, thin trees (11–30years; diameter 0–15cm). Different occurrences of particular growth disturbances as well as the different age- and diameter-dependent sensitivities emphasize the need for the inclusion of new variable that will take into account the amount of sensitive trees for each year. After that, adequate mixture of sampled tree (i.e. mixture of age classes), which is necessary for the completeness of debris flow/flood chronologies, could be balanced regarding their sensitivity.

Debris Flow Occurrence and Sediment Persistence, Upper Colorado River Valley, CO.

Debris flow magnitudes and frequencies are compared across the Upper Colorado River valley to assess influences on debris flow occurrence and to evaluate valley geometry effects on sediment persistence. Dendrochronology, field mapping, and aerial photographic analysis are used to evaluate whether a 19th century earthen, water-conveyance ditch has altered the regime of debris flow occurrence in the Colorado River headwaters. Identifying any shifts in disturbance processes or changes in magnitudes and frequencies of occurrence is fundamental to establishing the historical range of variability (HRV) at the site. We found no substantial difference in frequency of debris flows cataloged at eleven sites of deposition between the east (8) and west (11) sides of the Colorado River valley over the last century, but four of the five largest debris flows originated on the west side of the valley in association with the earthen ditch, while the fifth is on a steep hillslope of hydrothermally altered rock on the east side. These results suggest that the ditch has altered the regime of debris flow activity in the Colorado River headwaters as compared to HRV by increasing the frequency of debris flows large enough to reach the Colorado River valley. Valley confinement is a dominant control on response to debris flows, influencing volumes of aggradation and persistence of debris flow deposits. Large, frequent debris flows, exceeding HRV, create persistent effects due to valley geometry and geomorphic setting conducive to sediment storage that are easily delineated by valley confinement ratios which are useful to land managers.

Rainfall control of debris-flow triggering in the Réal Torrent, Southern French Prealps

This paper investigates the occurrence of debris flow due to rainfall forcing in the Réal Torrent, a very active debris flow-prone catchment in the Southern French Prealps. The study is supported by a 4-year record of flow responses and rainfall events, from three high-frequency monitoring stations equipped with geophones, flow stage sensors, digital cameras, and rain gauges measuring rainfall at 5-min intervals. The classic method of rainfall intensity–duration (ID) threshold was used, and a specific emphasis was placed on the objective identification of rainfall events, as well as on the discrimination of flow responses observed above the ID threshold. The results show that parameters used to identify rainfall events significantly affect the ID threshold and are likely to explain part of the threshold variability reported in the literature. This is especially the case regarding the minimum duration of rain interruption (MDRI) between two distinct rainfall events. In the Réal Torrent, a 3-h MDRI appears to be representative of the local rainfall regime. A systematic increase in the ID threshold with drainage area was also observed from the comparison of the three stations, as well as from the compilation of data from experimental debris-flow catchments. A logistic regression used to separate flow responses above the ID threshold, revealed that the best predictors are the 5-min maximum rainfall intensity, the 48-h antecedent rainfall, the rainfall amount and the number of days elapsed since the end of winter (used as a proxy of sediment supply). This emphasizes the critical role played by short intense rainfall sequences that are only detectable using high time-resolution rainfall records. It also highlights the significant influence of antecedent conditions and the seasonal fluctuations of sediment supply.

Assessing debris flow activity in a changing climate

Future trends in debris flow activity are constructed based on bias-corrected climate change projections using two meteorological proxies: daily precipitation and Convective Available Potential Energy (CAPE) combined with specific humidity for two Alpine areas. Along with a comparison between proxies, future number of days with debris flows are analyzed with respect to different regional and global climate models, Representative Concentration Pathways (RCPs), and area for quantile mapping. Two different base periods are also analyzed, as debris flows were observed on only 6 (17) days between 1950 and 1979, yet on 18 (49) days between 1980 and 2009 for Fella River, NE Italy (Barcelonnette, SE French Alps). For both areas, future climate projections vary between no change up to an increase of 6.0 % per decade in days with debris flow occurrences towards the end of 21st century. In Barcelonnette, the base period and proxy have a bigger impact on the future number of debris flow days than the climate model or RCP used. In Fella River, the base period, RCP, and proxy used define the future range. Therefore the selection of proxy, base period and downscaling technique should be carefully considered for future climate change impact studies concerning debris flow activity and associated fast-moving landslides.

Hyperspectral Soil Dispersion Model for the Source Region of the Zhouqu Debris Flow, Gansu, China

A new hyperspectral soil dispersion model was established by using multivariable linear regression based on experimentally derived soil dispersion data and hyperspectral field spectra of soil components. The coefficient of determination for the model is 0.8084, which shows that the model effectively reproduced soil dispersion. Those bands that were relevant to soil dispersion included 370, 377, 398, 410, 570, 1918, 1933, 2392, 2401, 2444, and 2448 nm, which identified soil components and their adsorbed ions. The correlation between soil spectral bands and soil components indicates that calcite and clay minerals were the main factors that caused soil to be dispersive because they are easily dissolved in water. This result provides a scientific basis for countering soil dispersion and reducing the occurrence of debris flows. The spectral bands determined by this model may also be relevant to air- or satellite-borne hyperspectral remote sensing in the future.

A 10-min rainfall prediction model for debris flows triggered by a runoff induced mechanism

Debris flows are triggered every year in the Jiangjia Gully, Yunnan Province, China. High-intensity short duration rainfall was the main triggering factor for these gully type debris flows which are triggered by a runoff induced mechanism. The 10-min rainfall intensity is considered as the trigger. A revised prediction model with factors related to hydrology (rainfall) is introduced for this kind of gully type debris flows and applied to the Jiangjia Gully. A 10-min rainfall coefficient intensity is proposed based on the observational data of Jiangjia Gully, and a minimum 10-min rainfall intensity for triggering debris flow is presented. The Jiangjia data were used also to assess critical boundaries between three probability classes of debris flow initiation. The model was successfully validated in debris flow gullies with the same initiation mechanism in other areas of southwest China. The generic character of the model is explained by the fact that its factors are partly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new way to predict the occurrence of debris flows initiated by a runoff induced mechanism.

“生命の危険からの自発的回避”のための土砂災害危険情報サービスの構築

A landslide and debris flow disaster information distribution service was developed to raise resident's awareness and enhance safe and quick self-evacuation. The information distribution service provides the information on the occurrence area of landslide and debris flow directly to the users. The landslide and debris flow occurrence is estimated by using the integrated information of the spatial and temporal rainfall distribution measured by the X-band multi parameter Radar, landslide and debris flow hazard expected area and topographic factors such as slope. The landslide and debris flow disaster information service is distributed by using an application software which works on Android and iOS base smartphones and PCs. The applicability of the predicted landslide and debris flow occurrence area was discussed by using the sediment disasters occurred in Tochigi prefecture in September 2015. The predicted alert areas have good agreements with the landslide and debris flow occurred area in Tochigi.

Analysis of a rainfall-triggered landslide at rest and be thankful in Scotland

Weather and climate are key influences upon the triggering of landslides in Scotland. One of the primary factors influencing the landslide and debris flows occurrence is rainwater infiltration into an initially unsaturated slope. This leads to an increase in both degree of saturation and pore-water pressure and, consequently, to a decrease in the shear strength of the soil, which eventually triggers a landslide event. The A83 trunk road at the Rest and Be Thankful in Scotland is an area prone to risk from landslide and, because of climate change, this risk is going to become higher. An increased frequency of heavier rainfall has been observed in recent years and forecasts for the next decades are not more reassuring. In 2014, the biggest landslide event has occurred at the Rest and Be Thankful Pass. More than 2000 tonnes of landslip debris have been removed in efforts to reopen the route A83. Although the route is now open again, still many are the aspects related to the triggering of such event that have not been clarified yet. The aim of this paper is to address the problem of developing a simple numerical model to analyse rainfalltriggered shallow landslides.

Numerical Simulation of the Topographical Change in Korea Mountain Area by Intense Rainfall and Consequential Debris Flow

The objectives of this study are to simulate the topographical changes associated with rainfall and the consequential debris flow using terrestrial LiDAR (Light Detection And Ranging). Three rainfall events between July 9 and July 14, 2009, triggered a number of debris flows at Jecheon County in Korea. Rain fell at a rate of 64 mm/h, producing 400 mm of total accumulation during this period. Tank simulation model for SWI (Soil Water Index) estimated the water stored beneath the ground and debris flow occurrence in study area. For the LiDAR (Light Detection and Ranging) survey, the terrestrial laser scanning system RIEGL LMS-Z390i consists of an accurate and fast 3D scanner, associated RTK GPS system. The DEM derived from LiDAR enabled the debris flow to be mapped and analyzed in great detail. The estimated affected area and erosion/deposition volumes by debris flow were compared with two-dimensional numerical simulation. The simulation results were sufficiently in good agreement with the debris flow track, and a success rate of over 90% was achieved with a simulation time of 300 s. A comparison of the simulated and surveyed results based on deposition volume yields a success rate of over 97% with 350 s of simulation time.

Probabilistic rainfall thresholds for triggering debris flows in a human-modified landscape

In the Carrara Marble Basin (CMB; Apuan Alps, Italy) quarrying has accumulated widespread and thick quarry waste, lying on steep slopes and invading valley bottoms. The Apuan Alps are one of the rainiest areas in Italy and rainstorms often cause landslides and debris flows. The stability conditions of quarry waste are difficult to assess, owing to its textural, geotechnical and hydrogeological variability. Therefore, empirical rainfall thresholds may be effective in forecasting the possible occurrence of debris flows in the CMB. Three types of thresholds were defined for three rain gauges of the CMB and for the whole area: rainfall intensity–rainfall duration (ID), cumulated event rainfall–rainfall duration (ED), and cumulated event rainfall normalized by the mean annual precipitation–rainfall intensity (EMAPI).The rainfall events recorded from 1950 to 2005 was analyzed and compared with the occurrence of debris flows involving the quarry waste. They were classified in events that triggered one or more debris flows and events that did not trigger debris flows. This dataset was fitted using the logistic regression method that allows us to define a set of thresholds, corresponding to different probabilities of failure (from 10% to 90%) and therefore to different warning levels. The performance of the logistic regression in defining probabilistic thresholds was evaluated by means of contingency tables, skill scores and receiver operating characteristic (ROC) analysis. These analyses indicate that the predictive capability of the three types of threshold is acceptable for each rain gauge and for the whole CMB. The best compromise between the number of correct debris flow predictions and the number of wrong predictions is obtained for the 40% probability thresholds. The results obtained can be tested in an experimental debris flows forecasting system based on rainfall thresholds, and could have implications for the debris flow hazard and risk assessment in the CMB.

Biographical sketch of a giant: Deciphering recent debris-flow dynamics from the Ohya landslide body (Japanese Alps)

Debris-flow frequency, discharge, and travel distance are highly catchment dependent and typically controlled by topography, hydrological conditions, and sediment supply. As a consequence, detailed and case-specific investigations are needed to decipher debris-flow histories in order to improve hazard mitigation. This study documents past (ca. 10years) debris-flow occurrences originating from the Ohya landslide, central Japan, by using a large set of methods including field monitoring, repeat airborne LiDAR, orthophoto interpretation, and tree-ring reconstructions. We demonstrate that the different approaches generally agree on the occurrence of debris flows but that mismatches may exist when it comes to the assessment of areas affected by individual events. These differences may even exceed the usual errors in precision inherent to each of the methods used. In the present case, high-resolution orthophoto interpretation tends to underestimate areas affected by debris flows, especially in the vertical direction, in the absence of lateral movement of the channel bed and as a result of shade and areas under trees. On the other hand, we realize that LiDAR data cannot necessarily be used to distinguish local changes in topography from noise. Tree-ring analyses can help to improve the temporal resolution of the analysis, but may have limitations when it comes to the definition of areas affected by an event because of the point-type nature of data. We conclude that the best and most complete results are obtained by combining multiple methods to improve the spatial and temporal resolution of past debris flows and to delimit areas affected by individual events.