Changing debris flow activity after sudden sediment input: a case study from the Swiss Alps

Spatial and temporal evolution of long-term debris flow activity and the dynamic influence of condition factors in the Wenchuan earthquake-affected area, Sichuan, China

Controls on spatiotemporal heterogeneity and evolution of debris flows along a traffic corridor affected by the strong Wenchuan earthquake

Impact of Chi-Chi earthquake on the occurrence of landslides and debris flows: example from the Chenyulan River watershed, Nantou, Taiwan

Debris flow hazard posts a big threat to the main downstream of Jinsha River where a number of huge power stations are under construction. The characteristics of spatial distribution of debris flows and the effect of their sediment yield on the reservoir areas have been studied. An automatic recognition module was developed to extract the geometry of debris flow channels from remote sensing data. Spatial distribution pattern of debris flows is obtained through combining the inventory database and multi-source remote sensing investigation. The distribution of debris flows has high dependency on the various factors including geology, geomorphology, climate, hydrology and human economic activities. The debris flows distributed in the study area are characterized by group and pair distribution, uncompleted deposition fans, highly controlled by faults and tectonic activities, spatial variation between left bank and right bank, and different subdivisions. The sediment yield caused by debris flow activates is evaluated using multi-year observation data from numerous observation stations. Quantitative studies have been performed on the relationship between the sediment yield and the debris flow area. A relatively fix ratio of 2.6 (×104 t/km2) has been found in different subdivisions of main downstream area which shows that the source of sediment discharged into Jinsha River primarily come from debris flow activities. Another ratio is evaluated to represent the transforming possibility of debris flow materials to bed-sediment load and suspended-sediment load in the river. Based on these findings, the potential effect of sediment yield caused by debris flows on reservoir areas is discussed. The zonation map shows the different effect of debris flow sediment on different dam site area which shows a good agreement with variation of debris flow spatial distribution.

<p>Debris flows are a common hazard in Alpine headwater catchments during intense convective rainstorms. These debris flows are commonly triggered by runoff entraining previously deposited sediment within the catchment. A debris flow will be initiated if rainfall exceeds the given rainfall intensity threshold. We usually define the rainfall intensity threshold as a function of storm duration (rainfall intensity-duration threshold). Above this empirically recorded threshold, the resulting surface runoff can mobilise sediment from the hillslopes and within the channel network. Thresholds are usually defined empirically for a given geographic region via monitoring of debris flow occurrence and the triggering rainfall intensity. However, direct field observations and rainfall data are sparse and noisy, and it is impossible to define rainfall thresholds when historical data are unavailable. An alternative methodology to derive rainfall ID thresholds is to use simplified physics-based model simulations. In this case, a greater understanding of the controlling factors for debris-flow activities could enable better threshold estimation in unmonitored catchments.</p><p>Here we present the initial simulation results of three different monitored catchments in the Dolomite mountains of Northeast Italy. These catchments are dominated by steep dolomite bedrock walls, which can provide large volumes of surface runoff to the catchment during rainfall. To simulate the response to rainfall in these catchments, we use the SWEHR (Shallow Water Equation & Harsine Rose) debris flow model, which we calibrate using a combination of field data and a correlation maximising framework. By focusing on the runoff response to rainfall in the catchments, we identified several key factors in the calibration of the model. The timing and magnitude of the runoff is controlled by the hydrological characteristics of the bedrock, the roughness of the catchment, the availability of sediment in the catchment, and the characteristics of the rainfall. By running multiple rainfall simulations for the catchments, we show how these factors impact rainfall ID thresholds</p>

The Siwalik hill of Nepal lies in between the two major thrusts; Main boundary Thrust and Main Frontal Thrust. Thrusting and continuous erosion of material from surface causes several landslides and debris flow problem in the Siwalik region. Debris flow commonly occurs after landslide during heavy rainfall in the steep slope. This paper described about the topographical and geological controls on debris flow occurring in gullies in the Siwaliks of the Malai River catchment of mid-Western Nepal. The length of gully, length of debris channel, area of debris channel, area of catchment of debris flow, area of gully without debris flow were found using Google earth pro, a free version online database. Similarly, a form factor (F), average gradient of stream, slope area ratio and a topographical factor (T) were calculated. Lithology and geological structure were studied in the field. The relation between each factor was identified.
 Gully having larger debris flow event had T value greater than 0.01 and that having small debris flow event had T value less than 0.01. Gully without debris flow had T value less than 0.001. Gully having debris flow had F more than 0.1 and that free from debris flow had F less than 0.1. Both topographic and form factors were found greater than 0.1 at the hanging wall of the Malai Thrust, where large size debris flows were encountered. The T and F values obtained from the Middle and Upper Siwaliks were greater than 0.1.The number of debris flow events and large debris flows were found high the in the Middle and the Upper Siwaliks, and hanging wall of the Malai Thrust.

Estimating the size and travel distance of Klapperhorn Mountain debris flows for risk analysis along railway, Canada

A physically-based multi-hazard risk assessment platform for regional rainfall-induced slope failures and debris flows

On 27 July 2011, a total of 33 debris flows occurred in Umyeonsan, Seoul, Korea, resulting in 16 fatalities and extensive damage to houses, roads, and other properties. The debris flows originated during an intense rainstorm with a peak intensity of 112.5 mm/h and a cumulative rainfall of 306.5 mm over 16 h. The debris flows were triggered by a combination of two processes: (1) shallow translational landslides in colluvium at the ridge crest along a fault plane associated with the Chugaryeong Fault Zone and (2) the progressive entrainment of loose material due to surface water runoff in the channel. The ensuing debris flows extensively eroded the colluvial deposits overlying the bedrock along their paths and transported large quantities of soil and woody debris. Two watersheds with catastrophic debris flows in the study area were chosen for a forensic investigation to analyze the geomorphological features of the debris flow gullies and to study several factors influencing the debris flows. The debris flow activity was found to be primarily related to rainfall, as well as to the loose, compressible colluvium overlying the gneiss bedrock, the watershed topography with >30° slopes, and the vegetation.

Debris flows are one of the most hazardous and frequent landslides in mountainous regions. In this research, we apply dendrogeomorphological techniques to analyze the debris flow that occurred in February 2010 in the Monarch Butterfly Biosphere Reserve in central Mexico. Firstly, analyzed precipitation data from two nearby meteorological stations has indicated that an extraordinary precipitation event accumulated 236.4 mm of rainfall over a 5-day period, from January 31st to February 4th, 2010. In addition, we generated a geomorphological cartography of the area at 1:600 scale by interpreting the images obtained using an Unmanned Aerial Vehicle. Subsequently, we carried out dendrochronological sampling on 98 Abies religiosa trees (125 cores); the cores were dated using cross-dating and the ring width was measured for generating a chronology, in which the years with growth disturbances as a result of the debris flow could be identified and characterized. We identified three types of growth anomalies in the tree ring series: (a) traumatic resin ducts, (b) growth suppression, and (c) growth releases, related to the 2010 debris flow event. We also estimated the debris flow intensity through the scar heights recorded in the trees, which allowed us to identify that the intensity of the debris flow was greater in the transport zone. These results highlight the benefit of the dendrogeomorphological technique used in the collection of spatio-temporal information on debris flow activity in intertropical montane environments, especially where information about mass movement processes is limited. Therefore, we demonstrate that dendrogeomorphological techniques, in combination with other parameters such as scar height on the trees, as well as a detailed geomorphological survey, allows for an understanding of the dynamics of debris flows, which, in the case of Mexico have not been previously evaluated using this approach.

Rock avalanches events pose significant concerns in mountainous regions characterised by deep and narrow valleys. This has not deterred the ongoing development in these areas, where population settlements and infrastructure continue to expand, becoming increasingly susceptible to these risks. Cima del Simano instability in the Swiss Alps, located in the narrow Blenio Valley, is a deep-seated rockslide which could trigger such events in the future. A previous work outlined several scenarios for the rockslide failure defined by a specific area, volume (ranging from 2.30x105 m3 to 4.30x106 m3), and susceptibility to happen. Given the proximity of a major road and several villages on both sides of the Valley, there is a real need to evaluate the potential runout distance in the event of rupture and propagation of the different failure scenarios.  Literature often presents two distinct approaches for estimating the runout distance and the impacted area, both based on the retrospective analysis of historical landslide occurrence. The first approach is through empirical equations linking volumes of failure V and Fahrböschung angles f (tanf=aV^(-b), with a and b two empirical parameters to determine). The second approach consists in numerically simulating the flow propagation by means of dedicated software and by applying specific rheological models.  This present work suggests applying both those techniques to evaluate the area that would be affected in the case of a rock avalanche at Cima del Simano, triggered by one of the suggested scenarios. We evaluated the runout distance for different angles f estimated based on the empirical relationship, and Dan3D for simulating the propagation applying the Voellmy rheology. Four simulations were conducted by varying the friction coefficient μ [-] and the parameter of turbulence ξ [m.s-2] in order to assess the minimal and maximal possible propagation in terms of runout distance L and lateral spreading based on domain of validity of those parameters according to literature.  The distances L obtained empirically are longer than the ones from the simulations. This can be explained by the frontal confinement of the flow slowing down the propagation. The study is completed by an evaluation for each scenario of the probability of exceeding a certain distance L using existing statistical models for f variations.  Additionally, the numerical simulations highlight the areas in gullies where debris are deposited during the flow propagation. Those areas can be sources for subsequent debris flow events. In a second step, we conducted an analysis of areas susceptible to debris flow with Flow-R and compared them with former debris flow events for validation.  This study aligns with risk management to assist in making informed decisions regarding the evacuation plan in the event of a rupture and propagation of an important volume at Cima del Simano. 

Taiwan is highly susceptible to frequent debris flow disasters due to its steep terrain, fragile geological structures, and intense rainfall from typhoons. To mitigate these risks, the Agency of Rural Development and Soil and Water Conservation (ARDSWC) under the Ministry of Agriculture (MOA) established a rainfall-based debris flow warning system in 2005, adopting the critical rainfall model proposed by Jan (2004). This system utilizes Effective Accumulated Rainfall (EAR)—combining current event rainfall and 7-day antecedent rainfall—as the primary indicator for debris-flow warning. Through statistical analysis of historical rainfall events, specific warning criteria have been set for townships prone to debris flows across Taiwan, classified into 9 levels ranging from 200 to 600 mm at 50-mm intervals. Consequently, warning issuance is triggered when the EAR exceeds these designated criteria. Based on a dataset of 252 officially recorded debris flow events and their associated warning records spanning from 2005 to 2025, this study assesses the effectiveness of warning operations using four performance indices: Capture Rate of Warning Issuance (C1, reflecting operational effectiveness), Capture Rate of Warning Criteria (C2, evaluating the appropriateness of criteria setting), Disaster Occurrence Rate within Issued Warnings (C3, indicating occurrence probability in warned areas), and Disaster Warning Coverage (C4, evaluating if events occurred within designated potential zones). The results show that both C1 and C2 achieved 67.5%, C3 was 8.4%, and C4 reached 90.8%. Overall, the debris flow warning system has proven to be operationally mature and highly effective in disaster mitigation. Approximately 70% of debris flow events were preceded by warnings that facilitated preemptive resident evacuation. Since the implementation of the system, casualties have been significantly reduced, notably with zero debris flow-related fatalities recorded over the past decade. Furthermore, the majority of events occurred within identified potential risk zones. Nevertheless, facing climate-change-induced extreme rainfall and post-seismic slope instability, continuous optimization of warning protocols and criteria remains essential to ensure robust early warning capabilities.Keywords: debris flow, early warning system, performance evaluation

Evidence for enhanced debris-flow activity in the Northern Calcareous Alps since the 1980s (Plansee, Austria)

Different factors influence the disposition of a watershed for initiation of debris flows, including meteorological trigger conditions as well as the hydrologic and geomorphic disposition. The latter includes slowly changing factors like relief energy or sediment availability, whereas the hydrologic state of a watershed may vary over short time scales. This contribution summarizes the outcomes of a long term project to quantify meteorological and hydrological trigger conditions leading to debris flows at different temporal and spatial scales in the Austrian Alps. The analysis employs a database of more than 4,500 debris flows over the last 100+ years, which is the period for which systematic rainfall data is available. A Bayesian analysis was carried out for determining occurrence probabilities for all Austria. For selected regions, hydrological trigger conditions were assessed using a semi-distributed, conceptual rainfall-runoff model, which was calibrated to measured runoff data. As expected we find increasing trigger probabilities with increasing rainfall amounts and intensities. However, the additional information of regional hydrological parameters as well as their temporal evolution over days prior to a debris-flow event, enables to capture different trigger conditions, including short duration rainstorms, long lasting rainfall events, and snow melt. We also find that a trigger-type resolved prediction of debris-flow susceptibility based on the hydro-meteorological catchment information is superior to simple rainfall-only approaches. The results of this analysis shall improve our understanding of long-term trigger conditions and trends of extreme mass wasting processes in the Alps and aim to become a valuable tool in engineering hazard assessment.

Preliminary study of the effect of earthquakes on the rainfall threshold of debris flows