Frequency Of Debris Flows Research Articles

Projected effects of climate change on shallow landslides, North Shore Mountains, Vancouver, Canada

Previous research in southwest British Columbia (BC) suggested that projected climate change would result in in increases in the frequency of debris flows on the order of a 10% by the end of the century compared to the last approximately 30 years (Jakob and Lambert, 2009). Advances in climate modeling now allow higher resolution simulation of precipitation compared to the work completed in 2009. Regional climate models have become better able to resolve convective activity at a finer temporal scale. This paper quantifies and projects changes in the frequency and magnitude of shallow landslides and resulting debris flows because of the impacts of climate change for the North Shore Mountains of Vancouver, BC. Analysis of a landslide database curated by Metro Vancouver shows a doubling in the frequency of landslides since 1981, prior to which recording was sporadic. How much of the increase since 1981 is attributable to recording improvements cannot be quantified which implies that it is too early to attribute frequency changes unambiguously to climate change. Prior to 2004, the average volume of shallow landslides was 1800 m3, whereas after 2004, this volume increased 7-fold to 13,000 m3. In particular, landslides with volumes exceeding 10,000 m3 appear to have increased abruptly since in the early 2000s. Using a Poisson model, we project an increase of up to 300% in the frequency of shallow landslides until the end of this century. A multivariate regression analysis suggests that this increase in landslide frequency will be accompanied by a potential increase in the average landslide volume of up to 50% for Relative Concentration Path (RCP) 8.5. Such changes may challenge watershed management attributable to increases in sediment transport rates, widening channels and in some cases may transition from single thread to braiding or anastomosing. The likelihood of landslides impacting and impounding water courses is projected to increase as is the turbidity in reservoirs because of the influx of fine-grained sediment. Outside of the Vancouver watersheds, infrastructures will increasingly be tested by more frequent and sometimes higher magnitude landslides.

Landslide Inventory along a National Highway Corridor in the Hissar-Allay Mountains, Central Tajikistan

An increasing amount of landslides leading to significant human and economic consequences is a primary concern for the government of Tajikistan and local authorities. Based on the Committee on Emergency Situations data, from 1996 to 2018, there were 3460 emergencies and more than 1000 fatalities because of earthquake-triggered and rainfall-induced landslides in the region. In addition, landslides caused severe damage to houses and infrastructure facilities due to the population’s lack of landslide hazard knowledge. Therefore, current research focuses on developing a regional-scale landslide inventory map in the Hissar–Allay region, central Tajikistan, where the population density is much higher than at other mountainous territories. In recent decades, the enhancements in geographic information systems, the open access to high-resolution remote sensing data, and an extensive field survey allowed us to identify 922 landslides possible along the highway corridor in the Hissar–Allay region. Based on Varnes’s system, these landslides are classified into four categories: debris flows, rockfalls, shallow landslides, and complex (deep-seated) landslides, considering landslides morphology, geology, deformation of slopes, degree and aspect of slopes, and weathered and disintegrated zones on slopes in the study area. The results show that 8.24% of the total study area is affected by landslides. Along the highway corridor in the Hissar–Allay region there are 96 bodies of deep-seated landslides and 216 rockfall catchments, 273 debris flow catchments, and 313 shallow landslides. Thus, shallow landslides are the most frequent type of movement. In addition, landslide frequency-area distribution analysis shows that shallow landslides are frequent with an area of 1.88E+04 m2; most frequent debris flow channels have a place of 5.58E+05 m2; rockfalls, for its part, are rife with an area of 1.50E+05 m2, and frequent complex landslides have an area of 4.70E+06 m2. Furthermore, it was found out that slopes consist of Silurian formation comprise shales, pebbles, sands, loams, and limestones, metamorphic clays are exposed to landslides more than other geological formations because of the layered structure and their broad spatial distribution in the study area. As the first applied research to compile a landslide inventory map in the Hissar–Allay region on the regional scale, our study provides a sound basis for future explorations of landslide susceptibility, hazard, and risk assessment for this region.

Effects of terrain on the occurrence of debris flows after forest harvesting

ABSTRACT Forest harvesting and subsequent forest regeneration represent widespread changes in land cover in mountain regions. Although impacts of forest harvesting on landslide initiation has been widely reported, the effects of forest harvesting on the occurrence of debris flows remains unclear. We propose that forest harvesting will differentially affect the susceptibility of debris flows amongst catchments with different terrain characteristics. This hypothesis was assessed in the Sanko catchment, Japan, where comprehensive forest harvest records date back to 1913. The frequency of debris flows directly originating from landslides occurred in similar timeframes as the occurrence of landslides. Landslides that reached channels and continued downstream as debris flows were more prevalent in steep channel reaches with small hillslope-channel junction angles. In addition to the increase in the frequency of landslides, especially within 10 years after forest harvesting, debris flows caused by mass movement of channel deposits in steep reaches increased during this period. These relationships between the occurrence of debris flows and channel topography indicate a high susceptibility of debris flow occurrence after forest harvesting in first and zero-order mountain streams. Sediment previously routed into channel networks by landslides is likely a more important factor for in-channel debris flow initiation in the lower channel reaches, while instability of areas proximate to riparian zones, including stream banks and geomorphic hollows, possibly accelerate occurrence of debris flows in upper channel reaches. Consequently, catchment topography should be considered in evaluating debris flow risk after forest harvesting.

Multi‐decadal changes in the relationships between rainfall characteristics and debris‐flow occurrences in response to gully evolution after the 1990–1995 Mount Unzen eruptions

AbstractExplosive volcanic eruptions can cause long‐term landscape change, leading to increased sediment discharge that continues after the cessation of the eruptions. During the period 1990–1995, eruptions of Mount Unzen, Japan, generated large amounts of pyroclastic material, resulting in 57 debris‐flow events during 1991–2018. To investigate changes in the relationships between rainfall characteristics and debris‐flow occurrence, we conducted the following: geometric analysis of two gullies (i.e., debris‐flow initiation zones) using LiDAR (light detection and ranging)‐generated 1 m DEMs (digital elevation models); rainfall analysis, based on the relationship between rainfall duration and mean intensity (i.e., considering the intensity–duration, or ID, threshold); and debris‐flow monitoring during 2016–2018. Since 1991, rainfall runoff has caused erosion of the supplied pyroclastic material, generating a channel network consisting of incised gullies. With sufficient rainfall, debris flows formed, accompanied by further gully erosion; this resulted in both vertical and lateral adjustments of the cross‐sectional geometry. In the two decades since the eruptions ceased, readily mobilized pyroclastic material has become scarce as the gullies have adjusted to local hydrographic conditions. At the same time, the infiltration capacity of the volcanic flank has increased, reducing the capacity for overland flow. As a result, since 2000, rainfall events with intensities above the ID threshold have occurred; however, the lack of sediment supplied by the gullies appears to have hindered the occurrence and development of debris flows. This suggests that debris flows in volcanically perturbed landscapes may occur at lower rainfall thresholds as long as the corresponding upland channels are evolving as a result of intense overland flow. However, as such channels evolve towards equilibrium geometries, the frequency of debris flows decreases in response to the reduction in sediment availability.

Risk Assessment of Highway in the Upper Reaches of Minjiang River under the Stress of Debris Flow

The southwest mountainous area is a frequent debris flow disaster area in China, which poses a serious threat to the regional roads and greatly affects the normal traffic operation and the safety of residents’ lives and property. The debris flow risk assessment of highway can quantify the threat degree of debris flow to the roads. In this paper, from the perspective of villages and towns, taking the upper reaches of Minjiang River as the research area, four factors including road network density, highway disaster resistance capacity, population density and highway cost are selected, and the weight is obtained by using entropy weight method, and the debris flow vulnerability evaluation results of highway are obtained by weighted calculation. Four indexes of debris flow density, shape factor, relative height difference and annual rainfall are selected to evaluate the debris flow hazard of highway by using the information method. Based on the vulnerability and hazard evaluation results, the risk of highway debris flow is evaluated, and the results are classified and discussed. The results show that: the risk of debris flow on the upper reaches of Minjiang River is relatively low, the overall spatial distribution shows a trend of high in the East and low in the west, and the overall risk of township roads in Wenchuan county is the highest; The risk of debris flow is generally high within -10 to 30 km from the central fault zone of Longmenshan.

Talus slope geomorphology investigated at multiple time scales from high‐resolution topographic surveys and historical aerial photographs (Sanetsch Pass, Switzerland)

AbstractTalus slopes are common places for debris storage in high‐mountain environments and form an important step in the alpine sediment cascade. To understand slope instabilities and sediment transfers, detailed investigations of talus slope geomorphology are needed. Therefore, this study presents a detailed analysis of a talus slope on Col du Sanetsch (Swiss Alps), which is investigated at multiple time scales using high‐resolution topographic (HRT) surveys and historical aerial photographs. HRT surveys were collected during three consecutive summers (2017–2019), using uncrewed aerial vehicle (UAV) and terrestrial laser scanning (TLS) measurements. To date, very few studies exist that use HRT methods on talus slopes, especially to the extent of our study area (2 km2). Data acquisition from ground control and in situ field observations is challenging on a talus slope due to the steep terrain (30–37°) and high surface roughness. This results in a poor spatial distribution of ground control points (GCPs), causing unwanted deformation of up to 2 m in the gathered UAV‐derived HRT data. The co‐alignment of UAV imagery from different survey dates improved this deformation significantly, as validated by the TLS data. Sediment transfer is dominated by small‐scale but widespread snow push processes. Pre‐existing debris flow channels are prone to erosion and redeposition of material within the channel. A debris flow event of high magnitude occurred in the summer of 2019, as a result of several convective thunderstorms. While low‐magnitude (<5,000 m3) debris flow events are frequent throughout the historical record with a return period of 10–20 years, this 2019 event exceeded all historical debris flow events since 1946 in both extent and volume. Future climate predictions show an increase of such intense precipitation events in the region, potentially altering the frequency of debris flows in the study area and changing the dominant geomorphic process which are active on such talus slopes. © 2020 John Wiley & Sons, Ltd.

Debris Flow Behavioral Characteristic Based on Rheological Properties: A Case Study on Mt. Hwangnyeong and Mt. Umyeon

Debris flow, one of the major natural disasters in Korea, usually occurs because of heavy rainfall during the rainy season. Recently in Korea, rainfall, as well as the frequency of debris flow, has continually increased as a result of climate change. Therefore, it is necessary to study the characteristics of the debris flow behavior for hazard mitigation and damage assessment. In this study, vane-type rheometer tests were conducted to estimate the rheological properties (viscosity, yield stress) of two soil samples collected from Mt. Hwangnyeong and Mt. Umyeon Several series of small-scale flume experiments were also performed to evaluate the characteristics of the debris flow behaviors (flow velocity, runout distance, and deposition volume) of these two sites. The results of the experiments show that front velocity, runout distance, and deposition volume of the debris flow gradually decrease with increase in viscosity and yield stress. Especially in the case of the Mt. Hwangnyeong sample, which has a high fine content, experimental results showed that the rate of increase in front velocity, runout distance, and deposition volume tended to high as viscosity and yield stress decreased compared to Mt. Umyeon.

Physical Interpretation of Rainfall Thresholds for Runoff‐Generated Debris Flows

AbstractRainfall thresholds for the occurrence of debris flows are commonly defined by Intensity‐Duration curves (IDthresholds). Interestingly, many empiricalIDthresholds show up as straight lines in a log‐log plot and therefore can be expressed by power‐law functions of the formI = α ⋅ Dβ, whereαis the scaling coefficient andβis the exponent of the power function. The different values ofαandβreflect the variability of geological and hydrological conditions in the different areas. In most cases, however, field conditions are so complex that a quantitative interpretation of the empirical rainfall threshold is impossible and the physical meaning ofαandβremains obscure. In this work, we provide a physical interpretation of the rainfall thresholds that characterize an active debris flow catchment in the Eastern Italian Alps (the Dimai Basin, Belluno Province). The catchment is affected by frequent debris flows generated by surface‐water runoff and has been monitored since 2010 to investigate the initiation process. Monitoring data allowed for the detection of two rainfall thresholds: a lower one that identifies the arrival of water in the initiation area (Catchment Outflow Threshold) and an upper one that identifies the initiation of debris flows by channel runoff (Debris Flow Threshold). We demonstrate that these two thresholds can be satisfactorily reproduced by a simple physically based model in which the excess rainfall is routed over the catchment using a kinematic‐wave scheme. This simple analysis provides a sound explanation of the observed thresholds and can be used to develop physically based thresholds for runoff‐generated debris flows.

Activity patterns and controlling factors of debris flows in the Upper Salween Alpine Valley

The frequency of debris flows in mountainous areas has increased in recent years, threatening sustainable development of mountainous communities. The Upper Salween Alpine Valley (USAV) experiences the most serious debris flow disasters in China. However, activity patterns and formation mechanisms of debris flow in this area are still unclear. We analyzed debris flow activity patterns and identified their key controlling factors in the USAV based on 66-year historical data, field records, remote sensing images, and geology, gravity, and precipitation characteristics. The results showed that (1) 200 contemporary debris flow gullies were distributed with a linear density of 0.64 gully/km. Twenty-four large-scale debris flow disasters occurred in the past 66 years; (2) the activity patterns of debris flow in the USAV were characterized by double rain season, nocturnal, high concealment, low frequency, and strong destructive force; (3) the USAV was a debris-flow-prone area with strong precipitation, strong gravity, and moderate geology driving forces. It belongs to loose material-controlled debris flow region. Our findings could contribute to the understanding of formation mechanisms of debris flows, and further improve prevention and mitigation of debris flow disasters in the USAV.

Comparison of Different Machine Learning Methods for Debris Flow Susceptibility Mapping: A Case Study in the Sichuan Province, China

Debris flow susceptibility mapping is considered to be useful for hazard prevention and mitigation. As a frequent debris flow area, many hazardous events have occurred annually and caused a lot of damage in the Sichuan Province, China. Therefore, this study attempted to evaluate and compare the performance of four state-of-the-art machine-learning methods, namely Logistic Regression (LR), Support Vector Machines (SVM), Random Forest (RF), and Boosted Regression Trees (BRT), for debris flow susceptibility mapping in this region. Four models were constructed based on the debris flow inventory and a range of causal factors. A variety of datasets was obtained through the combined application of remote sensing (RS) and geographic information system (GIS). The mean altitude, altitude difference, aridity index, and groove gradient played the most important role in the assessment. The performance of these modes was evaluated using predictive accuracy (ACC) and the area under the receiver operating characteristic curve (AUC). The results of this study showed that all four models were capable of producing accurate and robust debris flow susceptibility maps (ACC and AUC values were well above 0.75 and 0.80 separately). With an excellent spatial prediction capability and strong robustness, the BRT model (ACC = 0.781, AUC = 0.852) outperformed other models and was the ideal choice. Our results also exhibited the importance of selecting suitable mapping units and optimal predictors. Furthermore, the debris flow susceptibility maps of the Sichuan Province were produced, which can provide helpful data for assessing and mitigating debris flow hazards.

Chapter 5 Debris flows

Abstract Fast-moving, rainfall-induced debris-flow events are relatively common in the mountainous areas of the UK. Their impacts are largely, although by no means exclusively, economic and social. They often sever (or delay) access to and from relatively remote communities for services and markets for goods; employment, health and educational opportunities; and social activities. Specific forms of economic impact are described and their extent is defined by the vulnerability shadow. The mechanisms of rainfall-induced, fast-moving debris flows are considered to bridge between slow mass movements and flood phenomena. The occurrence of debris flows is largely restricted to mountainous areas and a series of case studies from Scotland is briefly described. Hazard and risk assessment are briefly considered and a strategic approach to risk reduction is described. The latter allows a clear focus on that overall goal before concentrating on the desired outcomes and the generic approach to achieving those outcomes. The effects of climate change on debris-flow hazard and risk are also considered and it is concluded that, in Scotland, increases in debris-flow frequency and/or magnitude are most likely and that increases in the risks associated with debris flows are also likely.

Impact characteristics of gustiness debris flow on check dam

The gustiness is a common motion form of debris flow, especially viscous debris flow. However, few researches have considered the gustiness into the impact characteristics of debris flow, which needs further research. In this study, the check dam was used as the load receptor to study the impact characteristics of gustiness debris flow. The results show that, under the same general flow depth, the dam stress considering the impact of gustiness debris flow is far less than that of disposable continuous debris flow. The impact force increases with the increase of the times of gustiness debris flow, and the impact torque increases linearly. Under the impact of gustiness debris flow, the displacement and stress of the dam vary in the same rule. The safety factors of anti-sliding and anti-overturning of the dam under the effect of gustiness debris flow are larger than those of disposable continuous debris flow, and the decrease rate of the safety factors increases with the increase of the times of gustiness debris flow. And under the condition of the same total flow depth, the increase of the debris flow depth can significantly decrease the stability of dam. The conclusions can provide reference for the design of blocking structures of gustiness and frequent debris flow.

Multi-temporal analysis of the role of check dams in a debris-flow channel: Linking structural and functional connectivity

Check dams represent one of the most common hydraulic engineering solution to manage debris-flow risk. However, the design of these structures only rarely considers their impact on sediment connectivity and sediment fluxes, both at the channel reach and catchment scale. In the last two decades, High-Resolution Topography (HRT) provided fundamental data to analyse the morphology evolution of landscapes through the realization of sequential DTMs (Digital Terrain Models) and DTMs of Difference (DoDs). In the context of mountain catchments, HRT offers essential information on the evolution of erosion and deposition patterns and allows the application of multi-temporal geomorphometric indexes that can enrich the DoDs analysis with important data as the assessment of sediment connectivity. In this research, multi-temporal LiDAR-derived DTMs were used to derive DoDs and to perform a multi-temporal analysis of an index of sediment connectivity (IC) in three reaches of a debris-flow channel (Moscardo torrent, eastern Italian Alps) where several check dams have been built over time. The research aims at outlining the effects of torrent control works on sediment dynamics in steep channels affected by frequent debris flows. The integration of the difference of IC with DoD maps allowed quantifying the impact of newly built check dams on changes in sediment pathways and related erosion processes, highlighting a close relationship between structural and functional connectivity: in areas where erosive processes prevailed, there were increases in connectivity, while deposition areas showed a decrease of IC. The geomorphometric analyses show that the new check dams have affected the sediment transfer along the main channel but also the hillslope-to-channel connectivity by altering sediment pathways. In the Moscardo torrent, however, the strong impact of these structures on sediment transfer is not matched by long-term effectiveness in debris-flow control.

Respective influence of geomorphologic and climate conditions on debris-flow occurrence in the Northern French Alps

We investigate the influence of geomorphic and climate conditions on the spatiotemporal variability in debris flow occurrence using a dedicated hierarchical Bayesian model. The case study includes 104 catchments from the French Alps with debris flow occurrences since 1970. We identify from a list of 21 variables two geomorphic and two climate variables that explain 72% of the total variance. Lithology is identified as the primary driver (42%) of the spatiotemporal variability of debris flow activity. To analyze the effect of the duration of the observation on inferred relations, we run the model using debris flow occurrence data and climate variables from the last 12 and 23 years of the study period, respectively. The model is significant as soon as at least 23 years of observations are considered. To detect the influence of spatial variability in debris flow frequency, we partition our data set into two subsamples composed of catchments with at least or less than 4 debris flows over the period. This reveals that precipitation during the event and the 2 days before exceeding 20 mm in total are the most important variables for both subsamples. It explains at least 57% of the total variance and 73% of its climate component. We conclude that difference in debris flow occurrences from one catchment to another is mostly due to geomorphologic conditions, whereas homogeneous groups of catchments react strongly to climatic conditions in a rather similar way.

Predicting debris-flow clusters under extreme rainstorms: a case study on Hong Kong Island

Debris flows can cause severe loss of human lives and damage to property, especially on densely populated hilly terrains. In the changing climate, the frequency of debris flows is on a rising trend. Therefore, it is important to forecast possible scenarios of debris flows under extreme weather conditions. Previous numerical studies often deal with one individual debris flow in one analysis. Yet a large number of debris flows can occur in a large storm. This paper presents a physically based model to predict likely debris flow clusters on Hong Kong Island with an area of approximately 80 km2 considering the influence of the changing climate. Firstly, a slope stability analysis is conducted, and unstable cells and landslide deposition areas are predicted. Then clusters of debris flows initiating from these landslides are simulated considering hillslope erosion. The models are validated with historical debris flows triggered by a rainstorm in 2008. Finally, debris flow clusters under three reference extreme rainstorms (i.e. 44%, 65% and 85% of the 24-h probable maximum precipitation, PMP) are predicted. With the increase of rainstorm magnitude, numerous debris flows can occur simultaneously and merge, posing much greater threat to society. The consequences of debris flows grow dramatically when the magnitude reaches a certain extent, i.e. 65% of the 24-h PMP.

Landslides and dammed lakes triggered by the 2017 Ms6.9 Milin earthquake in the Tsangpo gorge

An Ms6.9 earthquake in the Yarlung Tsangpo gorge on November 18, 2017 caused more than 700 landslides concentrating along the gorge’s great bend, and three dammed lakes. The location and area of the landslides are obtained by interpreting high-resolution satellite images before and after the earthquake. About 31.45 Mm3 of sediment from the landslides are distributed in seven catchments that are highly susceptible to debris flows. In 2018, at least three large-scale glacier-debris flows happened at Sedongpu, one of the seven catchments, and blocked temporarily the main river twice. The subsequent inundation and outburst floods resulted in infrastructure and property losses extensively beyond the high-seismic-intensity zone. Increase in the frequency of debris flows is attributed to the instability of glaciers and sediment supply caused by the earthquake. Glacier-debris flows will remain active for a long period due to massive glacial till, abundant rainfall, and meltwater. Geomorphic hazard chain triggered by such a mid-strong earthquake demonstrates that although coseismic landslides only occur in a limited area, the space and duration of the quake-induced hazards could extend largely under special geographical conditions.

Modelling the role of material depletion, grain coarsening and revegetation in debris flow occurrences after the 2008 Wenchuan earthquake

A large amount of debris was generated by the co-seismic mass wasting associated with the 2008 Mw 7.9 Wenchuan earthquake. The abundance of this loose material along the slopes caused more frequent debris flows, triggered by less intense and/or shorter rainfalls. However, both the triggering rainfall and the debris flow frequency seem to have normalised progressively during the past decade. Although changes of rainfall thresholds for post-seismic debris flows were recorded after several major earthquakes, the factors controlling these changes remain poorly constrained. With the aid of a virtual experiment, we investigate the roles of material depletion, grain coarsening and revegetation of the co-seismic debris on the propagation and deposition of debris flows initiated by runoff, as well as their influence on the triggering rainfall thresholds. We employ a Geographic Information System (GIS)-based simulation of debris flow initiation by runoff erosion, which we first calibrate on the 14th August 2010 Hongchun gully event that occurred near the Wenchuan earthquake epicentre. We obtain, by investigating each of the aforementioned processes, changing critical rainfall intensity-duration thresholds for given debris flow runout distances. Grain coarsening appears to play a major role, which is consistent with published laboratory experiments, while material depletion and revegetation do not seem able to account alone for the actual quick decay of debris flow frequency. While the virtual experiment has proven useful in identifying the first-order controls on this decay, model improvements and verification over multiple catchments are needed to make the results useful in hazard assessments.

Sediment Conditions and Frequency of Debris Flows in and around Asaminami-ku and Asakita-ku, Hiroshima City

Sediment Conditions and Frequency of Debris Flows in and around Asaminami-ku and Asakita-ku, Hiroshima City

Study on transient dynamics of piers under the impact of debris flow

Because of the climate warming and the serious ecological damage of the valley slope, the frequency of debris flow in the valley is increasing frequently. The solid deposits in the debris flow are, transported at high speed under the action of water flow and self-gravity, have a great impact on the piers along the line, which greatly destroys the integrity and stability of the pier. Through the theoretical analysis and numerical simulation of debris flow, the transient dynamic response of the pier under the impact of debris flow is analyzed. It is concluded that the pier is subjected to the punching and shearing form load, and the impact damage of the coarse solid particles in the debris flow to the pier is more obvious. The pier displacement reaches the maximum at the second impact of the debris flow; the maximum displacement is 10.9mm, which appears at the 5m above the bottom of the pier. A tensile stress concentration zone appears at the bottom of the pier, and the maximum tensile stress is 1.84MPa, which exceeds the concrete strength of the pier and so we can take corresponding measures to guard against this problem so as to ensure the safety of bridge structure.

Landslides and increased debris‐flow activity: A systematic comparison of six catchments in Switzerland

AbstractAn increase in debris‐flow frequency is expected in steep Alpine catchments after the occurrence of a large landslide, such as a rock avalanche. Herein we describe changes in debris‐flow activity following increases in sediment availability due to landslides, or accelerated rock‐glacier movement, for five catchments in the Swiss Alps, the Spreitgraben, Schipfenbach, Bondasca, Riascio, and Dorfbach catchments. Documentation on debris‐flow activity is available from both before and after the landslide that generated the new sediment deposits. Data from nearby meteorological stations were used to explore possible changes in rainfall activity, and how the intensity and duration of rainfall events may have changed. In all cases there was a considerable increase in debris‐flows frequency for one to eight years following the landslide. The annual number of days with debris‐flow activity following the landslide was similar to that observed for the Illgraben catchment, where many such landslides occur annually. No clear change in precipitation totals preceding debris flows was apparent for the Riascio catchment, suggesting that the increase in frequency of debris flows is related to the increase in the amount of sediment that can be readily mobilized. In the two cases where rainfall data were available on an hourly basis, no systematic changes in the intensity or duration of rainfall related to debris‐flow triggering were apparent, as shown by the close‐clustering of storms on the intensity‐duration plots. Following the sediment‐generating event, an initial and sudden increase of the sediment yield was observed, followed by a decrease over time towards pre‐disturbance values. The response of the catchments appears to be related to the amount of debris‐flow activity prior to the landslide: sediment yield from catchments with frequent debris flows prior to the landslide activity did not increase as dramatically as in catchments where debris‐flow activity was less common prior to the landslide. © 2018 John Wiley & Sons, Ltd.