Geomorphic effects of large debris flows on channel morphology at North Fork Mountain,eastern West Virginia,USA

Extreme rainfall in June 1949 and November 1985 triggered numerous large debris flows on the steep slopes of North Fork Mountain, eastern West Virginia. Detailed mapping at four sites and field observations of several others indicate that the debris flows began in steep hillslope hollows, propagated downslope through the channel system, eroded channel sediment, produced complex distributions of deposits in lower gradient channels, and delivered sediment to floodwaters beyond the debris-flow termini. Based on the distribution of deposits and eroded surfaces, up to four zones were identified with each debris flow: an upper failure zone, a middle transport/erosion zone, a lower deposition zone, and a sediment-laden floodwater zone immediately downstream from the debris-flow terminus. Geomorphic effects of the debris flows in these zones are spatially variable. The initiation of debris flows in the failure zones and passage through the transport/erosion zones are characterized by degradation; 2300 to 17 000 m³ of sediment was eroded from these zones. The total volume of channel erosion in the transport/erosion zones was 1·3 to 1·5 times greater than the total volume of sediment that initially failed, indicating that the debris flows were effective erosion agents as they travelled through the transport/erosion zones. The overall response in the deposition zones was aggradation. However, up to 43 per cent of the sediment delivered to these zones was eroded by floodwaters from joining tributaries immediately after debris-flow deposition. This sediment was incorporated into floodwaters downstream from the debris-flow termini causing considerable erosion and deposition in these channels. © 1998 John Wiley & Sons, Ltd.     

Thresholds for post-wildfire debris flows: Insights from the Pinal Fire,Arizona, USA

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h⁻¹ lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd.     

Controls on debris-flow initiation on burned and unburned hillslopes during an exceptional rainstorm in southern New Mexico,USA

AbstractUsing observations from 688 debris flows, we analyse the hydrologic and landscape characteristics that influenced debris-flow initiation mechanisms and locations in a watershed that had been partially burned by the 2012 Whitewater-Baldy Complex Fire in the Gila Mountains, southern New Mexico. Debris flows can initiate due to different processes. Slopes can fail as discrete landslides and then become fluidized and move downstream as debris flows (landslide initiated) or progressive bulking of sediment from a distributed area can become channelized and concentrated as it moves downslope (runoff generated). In this study, we have an unusual opportunity to investigate both types of debris-flow initiation mechanisms in our observations of debris flows, triggered by an exceptional rainstorm in the autumn of 2013. Additionally, we compare our observations with those of a dataset of 1138 debris flows in the Colorado Front Range, triggered during the same weather system. We found that runoff-generated debris flows dominated in burn areas, and runoff required to start these flows could be well characterized by the Shields stress. Landslide-initiated debris flows were dominant in unburned areas. Debris-flow densities were tied to total rainfall and precipitation intensities. Like the observations in the Colorado Front Range, debris-flow initiation locations were found primarily in areas of relatively sparse vegetation on south-facing slopes between 25 and 40°, and with upslope contributing areas less than 1000 m². In terms of preferential locations for debris-flow initiations, 2013 vegetation coverage, approximated by Green–Red Vegetation Index metrics, proved to be more influential than the 2012 burn-severity designation. The uniformity of observations between our study area and those in the Colorado Front Range indicate that the underlying hydrologic and landscape patterns of the debris-flow initiation locations documented in these studies could be applicable to the wider southwest and Rocky Mountain regions. © 2019 John Wiley & Sons, Ltd.     

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

An 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.     

Geomorphological characteristics of small debris flows on Junior's Kop,Marion Island,maritime sub ‐ Antarctic

The geomorphological characteristics of small debris flows in a maritime sub‐Antarctic environment are described. The morphological and sedimentological characteristics of the debris flows are comparable to debris flows documented for other parts of the world; their initiation appears closely linked to the unusual environment in which they are found. Sediment supply is generated by diurnal frost heave of loamy sediment associated with Azorella selago. The debris flows are triggered by sediment mobilization upon saturation of the frost‐heaved surface gravel and overland flow over the low‐permeability and frost‐susceptible slope materials. Morphological effects of the flows are short‐lived due to obliteration by subsequent frost heave activity. Copyright © 2000 John Wiley & Sons, Ltd.     

Debris-flow volumes in northeastern Italy: Relationship with drainage area and size probability

A dataset of 809 debris flows that occurred in 537 basins in mountainous areas of northeastern Italy between the mid-19th century and 2015 is collected and analyzed. A remarkable increase in the number of events is observed in the last four decades and is mainly ascribed to more systematic data collection. The correlation between debris-flow volume and drainage basin area is obtained assuming a power-law relationship. The exponent of the power-law curve at the 50th percentile (0.67 ± 0.02) indicates negative allometry, which means that basin area grows out of proportion to debris-flow volume. In contrast, the exponents at the 98th and 99th percentiles are close to one, implying that debris-flow volumes grow linearly with basin areas. The isometric relationship between the largest debris flows and the corresponding basin areas is due to the enhanced debris supply provided by the activation of widespread sediment sources, the extent of which is proportional to the basin size. The probability density function of debris-flow volume for a subset of the collected dataset is calculated using the kernel density estimation function, which permits estimating the probability of the occurrence of debris flows that exceed a given threshold volume. The comparison with debris flows in other hydroclimatic regions shows that, although debris-flow volumes in northeastern Italy may attain high values, they are often exceeded by debris flows that occur in seismically active regions and/or are triggered by more intense rainstorms. © 2018 John Wiley & Sons, Ltd.     

Spatial and temporal diversity for debris‐flow meteorological control in subarctic oceanic periglacial environments in Iceland

Slopes in fjord environments of Iceland are prone to debris‐flow initiation, responding to a wide variety of meteorological triggering factors, such as rain on snow, rapid snowmelt, long‐lasting rainfall or intense rainfall. If all fjord regions have similar debris flows with regards to their magnitude, their meteorological control is diverse both in space and in time. Debris flows in Northwest Iceland are triggered mostly by rain‐on‐snow and long‐lasting rainfall, while snowmelt is more characteristic in North Iceland, and rainfall has a clear impact in East Iceland. Most debris‐flow events occur on a single slope, and only a few are recorded at the same time in different regions. Observations of the threshold values underline the diversity of debris‐flow initiation, occurring with huge amounts of sudden water supply as well as with very moderate ones. Copyright © 2007 John Wiley & Sons, Ltd.     

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

Explosive 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.     

Deciphering controls for debris-flow erosion derived from a LiDAR-recorded extreme event and a calibrated numerical model (Roßbichelbach,Germany)

Debris flows are among the most destructive and hazardous mass movements on steep mountains. An understanding of debris-flow erosion, entrainment and resulting volumes is a key requirement for modelling debris-flow propagation and impact, as well as analysing the associated risks. As quantitative controls of erosion and entrainment are not well understood, total volume, runout and impact energies of debris flows are often significantly underestimated. Here, we present an analysis of geomorphic change induced by an erosive debris-flow event in the German Alps in June 2015. More than 50 terrestrial laser scans of a 1.2 km long mountain torrent recorded geomorphic change in comparison to an airborne laser scan performed in 2007. Errors were calculated using a spatial variable threshold based on the point density of airborne laser scanning and terrestrial laser scanning and the slope of the digital elevation models. Highest erosion rates approach 5.0 m³/m² (mean 0.6 m³/m²). During the event 9550 ± 1550 m³ was eroded whereas only 650 ± 150 m³ was deposited in the channel. Velocity, flow pressure, momentum and shear stress were calculated using a carefully calibrated RAMMS Debris Flow model including material entrainment. Here we present a linear regression model relating debris-flow erosion rates to momentum and shear stress with an R² up to 68%. Channel transitions from bedrock to loose debris sections cause excessive erosion up to 1 m³/m² due to previously unreleased random kinetic energy now available for erosion. © 2019 John Wiley & Sons, Ltd.     

The triggering of debris flow due to channel‐bed failure in some alpine headwater basins of the Dolomites: analyses of critical runoff

The debris deposits at the bottom of very steep natural channels and streams in high mountain areas can be mobilized by runoff, triggering a water–sediment mixture flow known as debris flow. The routing of debris flow through human settlements can cause damage to civil structures and loss of human lives. The prediction of such an event, or the runoff discharge that triggers it, assumes an interest in risk analyses and the planning of defence measures. The object of this study is to find a method to determine the critical runoff value that triggers debris flow as a result of channel‐bed failure. Historical and rainfall data on 30 debris flows that occurred in six watersheds of the Dolomites (north‐eastern Italian Alps) were collected from different sources. Field investigations at the six sites, together with the hydrologic response to the rainfalls that triggered the events, were performed to obtain a realistic scenario of the formation of the debris flow there occurred. Field observations include a survey along the channel of the triggering reach of debris flow, with measurements of the channel slope and cross‐section and sampling of debris deposits for grain size distribution. Simulated runoff discharge values based on the rainfall recorded by pluviometers were then compared with values obtained through experimental criteria on the initiation and formation of debris flow by bed failure. The results are discussed to provide a plausible physical‐based method for the prediction of the triggering of debris flow by channel‐bed failure. Copyright © 2007 John Wiley & Sons, Ltd.     

Deciphering seismic and normal-force fluctuation signatures of debris flows: An experimental assessment of effects of flow composition and dynamics

Debris flows are gravity-driven mass movements that are common natural hazards in mountain regions worldwide. Previous work has shown that measurements of ground vibrations are capable of detecting the timing, speed, and location of debris flows. A remaining question is to what extent additional flow properties, such as grain-size distribution and flow depth can be inferred reliably from seismic data. Here, we experimentally explore the relation of seismic vibrations and normal-force fluctuations with debris-flow composition and dynamics. We use a 5.4 m long and 0.3 m wide channel inclined at 20°, equipped with a geophone plate and force plate. We show that seismic vibrations and normal-force fluctuations induced by debris flows are strongly correlated, and that both are affected by debris-flow composition. We find that the effects of the large-particle distribution on seismic vibrations and normal-force fluctuations are substantially more pronounced than the effects of water fraction, clay fraction, and flow volume, especially when normalized by flow depth. We further show that for flows with similar coarse-particle distributions seismic vibrations and normal-force fluctuations can be reasonably well related to flow depth, even if total flow volume, water fraction, and the size distribution of fines varies. Our experimental results shed light on how changes in large-particle, clay, and water fractions affect the seismic and force-fluctuation signatures of debris flows, and provide important guidelines for their interpretation.     

Volcaniclastic debris-flow occurrences in the Campania region (Southern Italy) and their relation to Holocene–Late Pleistocene pyroclastic fall deposits: implications for large-scale hazard mapping

The Campania Region (southern Italy) is characterized by the frequent occurrence of volcaniclastic debris flows that damage property and loss of life (more than 170 deaths between 1996 and 1999). Historical investigation allowed the identification of more than 500 events during the last four centuries; in particular, more than half of these occurred in the last 100 years, causing hundreds of deaths. The aim of this paper is to quantify debris-flow hazard potential in the Campania Region. To this end, we compared several elements such as the thickness distribution of pyroclastic fall deposits from the last 18 ka of the Vesuvius and Phlegrean Fields volcanoes, the slopes of relieves, and the historical record of volcaniclastic debris flows from A.D. 1500 to the present. Results show that flow occurrence is not only a function of the cumulative thickness of past pyroclastic fall deposits but also depends on the age of emplacement. Deposits younger than 10 ka (Holocene eruptions) apparently increase the risk of debris flows, while those older than 10 ka (Late Pleistocene eruptions) seem to play a less prominent role, which is probably due to different climatic conditions, and therefore different rates of erosion of pyroclastic falls between the Holocene and the Late Pleistocene. Based on the above considerations, we compiled a large-scale debris-flow hazard map of the study area in which five main hazard zones are identified: very low, low, moderate, high, and very high.     

Laboratory investigations of the role of biofilms on stream-bed surfaces in debris-flow runout

Debris-flow runout is a fascinating process to understand due to its implications for downstream alluvial fans. Based on the propagation-deposition behaviors of the Dongyuege (DYG) debris flow, in Yunnan, the effect of biofilms on channel surfaces on debris-flow runout is investigated in laboratory flumes with two different internal surfaces: surfaces are lined with granite slabs (Model I) and gravel (Model II), respectively. Our results show that biofilms can significantly reduce frictional resistance to flows. They increase flow velocities, slow down the deceleration of the snouts, prolong runout distances, and subsequently extend the areas covered with resulting deposits, thus greatly assisting the propagation of experimental debris flows. Slippery biofilms consisting mainly of diatoms and their extracellular mucus (ECM) reduce the contact friction between the flume-beds and the overlying fluids, and greatly promote the propagation of tested flows. Well-developed biofilms are found on the underwater channel surfaces of the DYG Creek. Acting as lubricating layers, they likely played a key role in the DYG debris-flow runout. Most of the debris transported during the DYG event was deposited on overbanks, and the sediment that caused the disaster was transported to the populated fan region through the stream-bed clad in the thick biofilms. Owing to their impacts on the development and width of the temporary debris dam breach, the stream-bed covered with biofilms became a direct contributor to the debris-flow hazard. Because of the ubiquitous presence of biofilms on mountain stream-bed surfaces, the development of perennial streamflows can be viewed as an indicator of gully susceptibility to debris flows threatening creek fans. The underwater areas of pre-event channel cross-sections should be regarded as slip or low-friction boundaries, and the parts above stream-levels can be viewed as no-slip boundaries. © 2019 John Wiley & Sons, Ltd.     

Physical modelling of rainfall‐ and snowmelt‐induced erosion of stony slope underlain by permafrost

Physical modelling experiments have been carried out in a cold room to test on a small scale, the effects of water supply during the thaw of an experimental slope with permafrost. Permafrost was maintained at depth and a thin active layer was frozen and thawed from the surface. Data from the experiments relate to two different conditions, first with moderate rainfall, and second with heavy rainfall during the thaw period. When moderate rainfall is applied during thaw phases, the experimental slope is slightly degraded. At the scale of the experiment, erosion processes involve frost jacking of the coarse blocks, frost creep and gelifluction that induce slow and gradual down slope displacements of the active layer, but also small landslides leading to large but slow mass movements with short displacements. Changes in experimental slope morphology are marked by the initiation of a small‐scale drainage network and the development of a little crest line which shows a progressive upslope migration. With such boundary conditions, there is not enough water supply to evacuate downslope the whole of the eroded material and a topographic smoothing is observed. When heavy rainfall is applied during thaw periods, rapid mass wasting (small mud‐flows and debris flows) become prominent. Slope failures are largely controlled by the water saturation of the active layer and by the occurrence of steeper slopes. At the scale of the experiment, rates of erosion and maximum incision increase by about 100% leading to significant slope degradation with marked and specific scars comparable to gullying. These morphological changes are dependant on both the size and the frequency of catastrophic events. These experiments provide detailed data that could improve the knowledge of the physical parameters that control the initiation, at a small‐scale, of erosion processes on periglacial slopes with a thin active layer and/or with thin cover of mobilizable slope deposits. Copyright © 2010 John Wiley & Sons, Ltd.     

The structure and sedimentology of relict talus,Trotternish, northern Skye,Scotland

Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP , and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd.     

Investigating the impact of the Chi‐Chi earthquake on the occurrence of debris flows using artificial neural networks

Debris flows have caused enormous losses of property and human life in Taiwan during the last two decades. An efficient and reliable method for predicting the occurrence of debris flows is required. The major goal of this study is to explore the impact of the Chi‐Chi earthquake on the occurrence of debris flows by applying the artificial neural network (ANN) that takes both hydrological and geomorphologic influences into account. The Chen‐Yu‐Lan River watershed, which is located in central Taiwan, is chosen for evaluating the critical rainfall triggering debris flows. A total of 1151 data sets were collected for calibrating model parameters with two training strategies. Significant differences before and after the earthquake have been found: (1) The size of landslide area is proportioned to the occurrence of debris flows; (2) the amount of critical rainfall required for triggering debris flows has reduced significantly, about half of the original critical rainfall in the study case; and (3) the frequency of the occurrence of debris flows is largely increased. The overall accuracy of model prediction in testing phase has reached 96·5%; moreover, the accuracy of occurrence prediction is largely increased from 24 to 80% as the network trained with data from before the Chi‐Chi earthquake sets and with data from the lumped before and after the earthquake sets. The results demonstrated that the ANN is capable of learning the complex mechanism of debris flows and producing satisfactory predictions. Copyright © 2009 John Wiley & Sons, Ltd.     

Assessment and mapping of debris-flow risk in a small catchment in eastern Sicily through integrated numerical simulations and GIS

This paper describes the application of a methodology for the evaluation of debris-flow risk in alluvial fans by incorporating numerical simulations with Geographical Information Systems to identify potential debris-flow hazard areas. The methodology was applied to a small catchment located in the north-eastern part of Sicily, Italy where an extreme debris flow event occurred in October 2007. The adopted approach integrates a slope stability model that identifies the areas of potential shallow landslides under different meteorological conditions using a two-dimensional finite-element model based on the De Saint Venant equation for the debris-flow propagation. The mechanical properties of the debris were defined using both laboratory and in situ test results. The risk classification of the area under study was derived using total hydrodynamic force per unit width (impact pressure) as an indicator for event intensity. Based on the simulation results, a potential risk zone was identified and mapped.     

CLUSTERING ANALYSIS OF DEBRIS-FLOW STREAMS

The Chi-Chi earthquake in 1999 caused disastrous landslides, which triggered numerous debris flows and killed hundreds of people, A critical rainfall intensity line for each debris-flow stream is studied to prevent such a disaster. However, setting rainfall lines from incomplete data is difficult, so this study considered eight critical factors to group streams, such that streams within a cluster have similar rainfall lines. A genetic algorithm is applied to group 377 debris-flow streams selected from the center of an area affected by the Chi-Chi earthquake, These streams are grouped into seven clusters with different characteristics, The results reveal that the proposed method effectively groups debris-flow streams.     

Pronounced increase in slope instability linked to global warming: A case study from the eastern European Alps

In recent decades, slope instability in high-mountain regions has often been linked to increase in temperature and the associated permafrost degradation and/or the increase in frequency/intensity of rainstorm events. In this context we analyzed the spatiotemporal evolution and potential controlling mechanisms of small- to medium-sized mass movements in a high-elevation catchment of the Italian Alps (Sulden/Solda basin). We found that slope-failure events (mostly in the form of rockfalls) have increased since the 2000s, whereas the occurrence of debris flows has increased only since 2010. The current climate-warming trend registered in the study area apparently increases the elevation of rockfall-detachment areas by approximately 300 m, mostly controlled by the combined effects of frost-cracking and permafrost thawing. In contrast, the occurrence of debris flows does not exhibit such an altitudinal shift, as it is primarily driven by extreme precipitation events exceeding the 75th percentile of the intensity-duration rainfall distribution. Potential debris-flow events in this environment may additionally be influenced by the accumulation of unconsolidated debris over time, which is then released during extreme rainfall events. Overall, there is evidence that the upper Sulden/Solda basin (above ca. 2500 m above sea level [a.s.l.]), and especially the areas in the proximity of glaciers, have experienced a significant decrease in slope stability since the 2000s, and that an increase in rockfalls and debris flows during spring and summer can be inferred. Our study thus confirms that “forward-looking” hazard mapping should be undertaken in these increasingly frequented, high-elevation areas of the Alps, as environmental change has elevated the overall hazard level in these regions.     

BOULDERS ON THE MOVE: GEOMORPHIC HAZARDS FROM FLOODS AND DEBRIS FLOWS ALONG MOUNTAIN RIVERS

IMOUNTAINE~ON-MENTSANDSEDIMENTGeologistsandgeomorphologistsareabletomakeatleasttWoimportantcontributionstomitigatinghazardsassociatedwithsedimentprocessessuchasfloodsordebrisflows.First,geomorphologistscaninterpretgeologicrecordsofthehiStoryofseddrientprocesses.Theserecordsmayprovideinsightintothemagnitude,frequency,andlocationofsedimentsourcesandtransport,aswellashillslopeandchannelresponsestosedimentprocesses.Informationonpastsedimentproductionandmovement,andchannelresponse,mayb'…