Holocene debris-flow deposits and their implications on the climate in the upper Jinsha River valley, China

Major debris-flow deposits occur along the xerothermic valley of the upper Jinsha River. The debris-flow deposits, ranging in thickness from 1 to 20 m, invariably occupy gently inclined piedmont slopes. The sediments are presently deeply dissected by gullies, and the process of mass movement has almost ceased. Detailed textural, stratigraphical, and geochemical studies reveal the formation processes of the debris flows. Seven debris-flow incidents are noted based on the unit combination characteristics of debris-flow deposits. The age estimates of optically stimulated luminescence (OSL) show that the occurrence of debris flows started at around 10.6 kyr BP and weakened until 4.5 kyr BP, corresponding to the obvious strengthened phase of the summer monsoons in the region. The ages of the debris-flow deposits indicate that the occurrence of a mass of debris flows was a response to the intensified summer monsoon in the SE fringe of the Tibetan Plateau since the early Holocene.     

The morphometric and stratigraphic framework for estimates of debris flow incidence in the North Cascades foothills, Washington State, USA

Active debris flow fans in the North Cascade Foothills of Washington State constitute a natural hazard of importance to land managers, private property owners and personal security. In the absence of measurements of the sediment fluxes involved in debris flow events, a morphological-evolutionary systems approach, emphasizing stratigraphy, dating, fan morphology and debris flow basin morphometry, was used. Using the stratigraphic framework and 47 radiocarbon dates, frequency of occurrence and relative magnitudes of debris flow events have been estimated for three spatial scales of debris flow systems: the within-fan site scale (84 observations); the fan meso-scale (six observations) and the lumped fan, regional or macro-scale (one fan average and adjacent lake sediments). In order to characterize the morphometric framework, plots of basin area v. fan area, basin area v. fan gradient and the Melton ruggedness number v. fan gradient for the 12 debris flow basins were compared with those documented for semi-arid and paraglacial fans. Basin area to fan area ratios were generally consistent with the estimated level of debris flow activity during the Holocene as reported below. Terrain analysis of three of the most active debris flow basins revealed the variety of modes of slope failure and sediment production in the region.Micro-scale debris flow event systems indicated a range of recurrence intervals for large debris flows from 106−3645 years. The spatial variation of these rates across the fans was generally consistent with previously mapped hazard zones. At the fan meso-scale, the range of recurrence intervals for large debris flows was 273−1566 years and at the regional scale, the estimated recurrence interval of large debris flows was 874 years (with undetermined error bands) during the past 7290 years. Dated lake sediments from the adjacent Lake Whatcom gave recurrence intervals for large sediment producing events ranging from 481−557 years over the past 3900 years and clearly discernible sedimentation events in the lacustrine sediments had a recurrence interval of 67−78 years over that same period.     

Sequence of instability processes triggered by heavy rainfall in the northern Italy

Northern Italy is a geomorphologically heterogeneous region: high mountains, wide valleys, gentle hills and a large plain form a very varied landscape and influence the temperate climate of the area. The Alps region has harsh winters and moderately warm summers with abundant rainfall. The Po Plain has harsh winters with long periods of subfreezing temperatures and warm sultry summers, with rainfall more common in winter.Geomorphic instability processes are very common. Almost every year, landslides, mud flows and debris flows in the Alpine areas and flooding in the Po flood plain cause severe damage to structures and infrastructure and often claim human lives. Analyses of major events that have struck northern Italy over the last 35 years have provided numerous useful data for the recognition of various rainfall-triggering processes and their sequence of development in relation to the intensity and duration of rainfall. Findings acquired during and after these events emphasise that the quantity and typology of instability processes triggered by rainfall are related not only to an area's morphological and geological characteristics but also to intense rainfall distribution during meteorological disturbances. Moreover, critical rainfall thresholds can vary from place to place in relation to the climatic and geomorphological conditions of the area. Once the threshold has been exceeded, which is about 10% of the local mean annual rainfall (MAR), the instability processes on the slopes and along the hydrographic networks follow a sequence that can be reconstructed in three different phases.In the first phase, the initial instability processes that can usually be observed are soil slips on steep slopes, mud–debris flows in small basins of less than 20 km² in area, while discharge increases substantially in larger stream basins of up to 500 km². In continuous precipitation, in the second phase, first mud–debris flows can be triggered also in basins larger than 20 km² in area. Tributaries swell the main stream, which is already in a critical condition. The violent flow causes severe problems mainly along valley bottoms of rivers with basins up to 2000 km² in area. First bedrock landslides can occur, reaching a considerable area density, with volumes from a few hundred up to about one to two million cubic meters. In continuous precipitation, in the third phase, basins of more than 2000 km² in area reach their first critical stage. River-bed morphology is extensively modified, with erosional and depositional processes which can locally undermine the stability of structures and infrastructures. Waters overflow levees, flooding villages and towns to various widths and depths and sometimes claiming casualties. Some days after an intense rainfall period, large landslides involving the bedrock can still take place. These processes usually cause the movement of very large rock masses. The total duration of rainfall usually has a greater effect on these landslides than does the number of short periods of very intensive precipitation. This sequence cannot be divided into separate phases when the events occur simultaneously because of the presence of intense rainfall pulses and the generation of very diffuse surface runoff. Such situations usually happen during short-lasting heavy summer rainstorms or in late spring, when snow melt combines with intense rainfall. The three-phase sequence has been identified in three severe events that are analysed in this paper: Valtellina (Lombardy) in 1987, Tanaro Valley (Piedmont) in 1994 and Aosta Valley in 2000; but this sequence has also been observed during other events that occurred in northern Italy: in Piedmont in 1968, 1977, 1978, 1993 and 2000; in Lombardy in 1983 and 1992; in the Aosta Valley in 1993.     

Debris Flow Triggered by Rapid Snowmelt: A Case Study in The Glei .arhjalli Area, Northwestern Iceland

Debris flows in the Gleivarhjalli area in northwestern Iceland occurred after a sudden and intensive snowmelt period during 10–12 June, 1999. The area, in the northwestern part of the town of Ísafjörvur, was chosen for a detailed study. Meteorological data and bedrock conditions, triggering mechanisms and geomorphological and human impacts were examined. This paper describes and emphasises the role of rapid snowmelt as a mechanism for the release of debris flows in a subpolar basaltic fjord setting. Post‐event mapping of erosional and depositional landforms showed strong geomorphic impacts of debris flows and their role in mass transfer in a mountainous environment. The estimated denudation rate for the singleevent is 0.29 mm/km². The use of a new lichen growth curve provides relative dating of previous unreported events. Finally, the paper estimates the mean return period for debris‐flow events in the Gleivarhjalli area as 4–5 years, thus constituting a serious threat to the community.     

北川地震重灾区泥石流特征与减灾对策

汶川大地震的同震次生灾害以滑坡、崩塌居多,泥石流相对较少。但地震导致滑坡、崩塌为震后泥石流提供了极为丰富的物质来源,使得地震灾区在一年多的时间里已经多次暴发了大面积的泥石流。以北川地震重灾区的苏宝河和魏家沟流域为研究区域,通过野外实地考察、遥感图像分析、历史资料对比等方法,概括总结出受地震强烈影响区域的泥石流具有成因多样、时间同步、颗粒粗大、多灾种复合、空间近似对称和小沟大灾等特征。并提出了"面上监控为主、点上工程为主、分类防治和开展风险评估"的减灾对策。     

Impacts of Holocene climatic variations on alluvial fan activity below snowpatches in subarctic Québec

In the Lake Guillaume-Delisle area of subarctic Québec, storm-generated alluvial fans have been active sporadically throughout the Holocene. In this study, we propose that the persistence of late-lying snowpatches in fan catchments during Holocene cold episodes promoted alluvial fan activity by lowering the precipitation threshold required to trigger a torrential event. This hypothesis was tested by characterizing the depositional processes responsible for alluvial fan formation below snowpatches, and by reconstructing the Holocene alluvial fan activity. Stratigraphic and sedimentary analyses conducted on seven alluvial fans revealed that they were deposited by torrential activity leading to waterlaid, transient, or hyperconcentrated deposition. The chronology of the storm-generated alluvial fans — based on 22 radiocarbon dates — indicates that torrential activity was enhanced during the cooler Late Holocene (i.e., after ca. 3500 cal. yr BP). Snowier winters and cooler summers were beneficial to nival activity, allowing the persistence of larger snowpatches throughout the summer and fall seasons. Rainfall-induced thaw of such snowpatches during rainstorm events is inferred to have contributed to alluvial fan activity by increasing water availability. Three peaks of alluvial activity occurred during the Late Holocene (2950–2750, 1900–1400, and 800–300 cal. yr BP) and are indicative of increased storminess resulting in higher fan activity. Increased fan activity during cooler episodes was concurrent with increased runoff activity in the immediate pronival area. This stresses the importance of nivation below snowpatches and pinpoints the role of nivation in enhancing geomorphological activity during period of cooler and more humid climate in subarctic environments.     

Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado

We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km² drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)^− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m³/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment.     

Monsoon triggered formation of Quaternary alluvial megafans in the interior of Oman

A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.     

Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California

Debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the Western U.S. Field evidence indicate that unlike landslide-triggered debris flows, these events have no identifiable initiation source and can occur with little or no antecedent moisture. Using rain gage and response data from five fires in Colorado and southern California, we document the rainfall conditions that have triggered post-fire debris flows and develop empirical rainfall intensity–duration thresholds for the occurrence of debris flows and floods following wildfires in these settings. This information can provide guidance for warning systems and planning for emergency response in similar settings.Debris flows were produced from 25 recently burned basins in Colorado in response to 13 short-duration, high-intensity convective storms. Debris flows were triggered after as little as six to 10 min of storm rainfall. About 80% of the storms that generated debris flows lasted less than 3 h, with most of the rain falling in less than 1 h. The storms triggering debris flows ranged in average intensity between 1.0 and 32.0 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south-central, and southwestern, Colorado are defined by: I = 6.5D^− 0.7 and I = 9.5D^− 0.7, respectively, where I = rainfall intensity (in mm/h) and D = duration (in hours).Debris flows were generated from 68 recently burned areas in southern California in response to long-duration frontal storms. The flows occurred after as little as two hours, and up to 16 h, of low-intensity (2–10 mm/h) rainfall. The storms lasted between 5.5 and 33 h, with average intensities between 1.3 and 20.4 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for life- and property-threatening floods and debris flows during the first winter season following fires in Ventura County, and in the San Bernardino, San Gabriel and San Jacinto Mountains of southern California are defined by I = 12.5D^−0.4, and I = 7.2D^−0.4, respectively. A threshold defined for flood and debris-flow conditions following a year of vegetative recovery and sediment removal for the San Bernardino, San Gabriel and San Jacinto Mountains of I = 14.0D^−0.5 is approximately 25 mm/h higher than that developed for the first year following fires.The thresholds defined here are significantly lower than most identified for unburned settings, perhaps because of the difference between extremely rapid, runoff-dominated processes acting in burned areas and longer-term, infiltration-dominated processes on unburned hillslopes.     

Reconstructing spatio-temporal patterns of debris-flow activity using dendrogeomorphological methods

Debris flows are a major threat in many parts of the Alps, where they repeatedly cause severe damage to infrastructure and transportation corridors or even loss of life. Nonetheless, the spatial behavior of past debris-flow activity and the analysis of areas affected during particular events have been widely neglected in reconstructions so far. It was therefore the purpose of this study to reconstruct spatio-temporal patterns of past debris flows on a forested cone in the Swiss Alps (Bruchji torrent, Blatten, Valais). The analysis of past events was based on a detailed geomorphic map (1:1000) of all forms related to debris flows as well as on tree-ring series from 401 heavily affected trees (Larix decidua Mill. and Picea abies (L.) Karst.) growing in or next to deposits. The samples were analyzed and growth disturbances related to debris-flow activity assessed, such as tangential rows of traumatic resin ducts, the onset of reaction wood or abrupt growth suppression or release.In total, 960 growth disturbances were identified in the samples, belonging to 40 different event years between A.D. 1867 and 2005. In addition, the coupling of tree-ring data with the geomorphic map allowed reconstruction of eleven formerly active channels and spatial representation of individual events. Based on our results we believe that before 1935, debris flows preferentially used those channels located in the western part of the cone, whereas the eastern part of the cone remained widely unaffected. The spatial representation of the 40 events also allowed identification of five different spatial patterns for debris flows at the study site.     

Soil slip/debris flow localized by site attributes and wind-driven rain in the San Francisco Bay region storm of January 1982

GIS analysis at 30-m resolution reveals that effectiveness of slope-destabilizing processes in the San Francisco Bay area varies with compass direction. Nearly half the soil slip/debris flows mapped after the catastrophic rainstorm of 3–5 January 1982 occurred on slopes that face S to WSW, whereas fewer than one-quarter have a northerly aspect. Azimuthal analysis of hillside properties for susceptible terrain near the city of Oakland suggests that the skewed aspect of these landslides primarily reflects vegetation type, ridge and valley alignment, and storm–wind direction. Bedrock geology, soil expansivity, and terrain height and gradient also were influential but less so; the role of surface curvature is not wholly resolved. Normalising soil-slip aspect by that of the region's NNW-striking topography shifts the modal azimuth of soil-slip aspect from SW to SE, the direction of origin of winds during the 1982 storm—but opposite that of the prevailing WNW winds. Wind from a constant direction increases rainfall on windward slopes while diminishing it on leeward slopes, generating a modelled difference in hydrologically effective rainfall of up to 2:1 on steep hillsides in the Oakland area. This contrast is consistent with numerical simulations of wind-driven rain and with rainfall thresholds for debris-flow activity. We conclude that storm winds from the SE in January 1982 raised the vulnerability of the Bay region's many S-facing hillsides, most of which are covered in shallow-rooted shrub and grass that offer minimal resistance to soil slip. Wind-driven rainfall also appears to have controlled debris-flow location in a major 1998 storm and probably others. Incorporating this overlooked influence into GIS models of debris-flow likelihood would improve predictions of the hazard in central California and elsewhere.     

Modelling net erosion responses to enviroclimatic changes recorded upon multisecular timescales

Land use change has been recognized throughout the Earth as one of the most important factors influencing the occurrence of rainfall-driven geomorphological processes. However, relating the occurrence of historical soil erosion rates is difficult because of the lack of long-term research projects in river basins. Also, complex models are not adequate to reconstruct erosion rate changes because they require significant input data not always available on long timescales. Given the problems with assessing sediment yield using complex erosion models, the objective of this study is to explore a parsimonious scale-adapted erosion model (ADT) from the original Thornes and Douglas algorithms, which aims at reconstruction of annual net erosion (ANE) upon multisecular timescales. As a test site, the Calore River basin (3015 km² in southern Italy) provides a peculiar and unique opportunity for modelling erosion responses to climate and land cover changes, where input-data generation and interpretation results were also supported by documented hydrogeomorphological events that occurred before and after land deforestation. In this way, ANE_ADT-values were reconstructed for the period 1675–2004 by using precipitation indexes, complemented by recent instrumental records, and by using land cover statistics from documented agrarian sources. Pulses of natural sedimentation in the predeforestation period have been related to Vesuvius volcanic activity and changes in rainstorm frequency. After deforestation, the basin system became unstable with sudden fluctuations in the hydrogeomorphological regime contributing significantly to increased erosion and, in turn, sediment transport sequences via river drainage towards the Tyrrhenian coast.     

Debris flows in Glacier National Park, Montana: geomorphology and hazards

Debris flows are one of the many active slope-forming processes within Glacier National Park, Montana. Most debris flow landforms exhibit classic morphology with a distinct failure scarp, incised channel, channel levees, and toe deposits that often develop a lobate form. The Precambrian metasediments that dominate Glacier National Park's geology weather into angular clasts that range in size from platy gravels to boulders. Classic debris flows occur in areas where the topographic expression provides a debris source from cliff faces and an accumulation of regolith, often in the form of talus slopes. Many of these debris flows have long runout zones and can travel many hundreds of meters. Often they cross hiking trails or roads, including the main east–west highway, Going-to-the-Sun Road. Debris flows impacting the road have resulted in several near fatalities, and hikers have been forced to cross active debris flows to reach safe ground. The magnitude of debris flows varies between high magnitude channel incising events and low magnitude channel filling and/or reworking events. The frequency of debris flow events is irregular and appears to be controlled by the hydrology of triggering storms and antecedent moisture conditions, not by the debris supply. As a result, debris flow magnitude is not a function of frequency, but is more closely related to the characteristics of antecedent conditions and individual storms.     

Landsliding and sediment flux in the Central Swiss Alps: A photogrammetric study of the Schimbrig landslide, Entlebuch

This study explores the effects of hillslope mass failure on the sediment flux in the Waldemme drainage basin, Central Swiss Alps, over decadal time scales. This area is characterized by abundant landslides affecting principally flysch units and is therefore an important sediment source. The analysis concentrates on the Schimbrig landslide that potentially contributes up to 15% to the sediment budget of the Waldemme drainage basin. Volumetric changes are quantified using high-resolution elevation models that were extracted using digital photogrammetric techniques. Sediment discharge data were used to constrain the significance of the landslide for sediment flux in the channel network. The temporal extent of the photogrammetric analysis ranges from 1962 to 1998, including an earth slide event in 1994. The analyses reveal that during periods of low slip rates of the landslide, nearly all of the displaced sediments were eroded and supplied to the channel network. In contrast, during active periods, only a fraction of the displaced landslide mass was exported to the trunk stream. Interestingly, the 1994 earth slide event did not disturb the long-term sediment discharge pattern of the channel network, nor did it influence the sediment flux at a weekly scale. However, suspended sediment pulses correlate with higher-than-average precipitation events. This was especially the case in August 2005 when a storm event (> 100 years return period) triggered several debris flows and earth flows in the whole drainage basin and in the Schimbrig area. This storm did not result in a significant increase in the slip rates of the entire landslide's main body. It is therefore proposed that debris flows and earth flows perform the connectivity between hillslope processes (e.g. landsliding) and the trunk stream during and between phases of landslide activity in this particular setting.     

Geomorphological characteristics and slope processes associated with different basins: Mallorca (Western Mediterranean)

We compare the different geomorphological processes which occurred in Pliocene–Quaternary times on two very similar slopes in Mallorca, one located at the Tramuntana Range and the other in the Llevant Ranges. Both slopes have the same geological structure, the same stratigraphic and lithological levels, and the same altitude and orientation. The different slope processes are due to the relationship between the accommodation space and the sedimentation rate in the adjacent basins: In the Valencia Trough (located N of the Tramuntana Range), the sedimentation rate has not been sufficient to fill the accommodation space, whereas in the Alcudia Basin (adjacent to the Llevant Ranges), the sedimentation rate has been sufficient to fill the accommodation space. This difference has resulted in major landslides on the Tramuntana slope, whereas the Llevant slope is characterized mainly by alluvial fans and debris screes.     

Depositional processes and particle characteristics on fans in the Southern Alps, New Zealand

Characteristics of surface particles on four fans dominated by different depositional processes are investigated in the sedimentary ranges of the Aoraki–Mount Cook area, Southern Alps, New Zealand. Statistical testing shows that on three of the fans all indices of particle size and roundness differ significantly according to the depositional process: snow avalanches produce the largest and most angular particles, streamflows the smallest and most rounded, with debris flows in between. On the fourth fan affected by all of these depositional processes, particle size and roundness indicate that snow avalanches are presently dominant with streamflow playing a secondary role. The results also show that all indices of particle size are strongly correlated with each other and suggest that the principal (a-) or intermediate (b-) axis length may provide a satisfactory substitute for more complex indices of size. Indices of particle shape generally do not differ significantly between the fans, reflecting both the strong control exerted on particle shape by source area lithology and structure and the geologic similarity of the four basins. Changes in particle size and roundness occur with down-fan distance on the snow avalanche, debris flow, and hybrid fans but not on the streamflow fan.     

Contrasting rainfall generated debris flows from adjacent watersheds at Forest Falls, southern California, USA

Debris flows are widespread and common in many steeply sloping areas of southern California. The San Bernardino Mountains community of Forest Falls is probably subject to the most frequently documented debris flows in southern California. Debris flows at Forest Falls are generated during short-duration high-intensity rains that mobilize surface material. Except for debris flows on two consecutive days in November 1965, all the documented historic debris flows have occurred during high-intensity summer rainfall, locally referred to as ‘monsoon’ or ‘cloudburst’ rains. Velocities of the moving debris range from about 5 km/h to about 90 km/h. Velocity of a moving flow appears to be essentially a function of the water content of the flow. Low velocity debris flows are characterized by steep snouts that, when stopped, have only small amounts of water draining from the flow. In marked contrast are high-velocity debris flows whose deposits more resemble fluvial deposits. In the Forest Falls area two adjacent drainage basins, Snow Creek and Rattlesnake Creek, have considerably different histories of debris flows. Snow Creek basin, with an area about three times as large as Rattlesnake Creek basin, has a well developed debris flow channel with broad levees. Most of the debris flows in Snow Creek have greater water content and attain higher velocities than those of Rattlesnake Creek. Most debris flows are in relative equilibrium with the geometry of the channel morphology. Exceptionally high-velocity flows, however, overshoot the channel walls at particularly tight channel curves. After overshooting the channel, the flows degrade the adjacent levee surface and remove trees and structures in the immediate path, before spreading out with decreasing velocity. As the velocity decreases the clasts in the debris flows pulverize the up-slope side of the trees and often imbed clasts in them. Debris flows in Rattlesnake Creek are relatively slow moving and commonly stop in the channel. After the channel is blocked, subsequent debris flows cut a new channel upstream from the blockage that results in the deposition of new debris-flow deposits on the lower part of the fan. Shifting the location of debris flows on the Rattlesnake Creek fan tends to prevent trees from becoming mature. Dense growths of conifer seedlings sprout in the spring on the late summer debris flow deposits. This repeated process results in stands of even-aged trees whose age records the age of the debris flows.     

The environmental significance of the remobilisation of ancient mass movements in the Atbara–Tekeze headwaters, Northern Ethiopia

Old landslides are prominent features in the landscape around Hagere Selam, Tigray Highlands, Ethiopia. The available evidence suggests their Late Pleistocene to Middle Holocene age and conditions of soil humidity. The affected geological layers, often silicified lacustrine deposits prone to sliding, rest upon or above the water holding Amba Aradam sandstone aquifer.Three examples of present-day (remobilisation of old) mass movements are illustrated and discussed. The aims of the study were to unravel the environmental conditions of the present-day remobilisation of ancient flows, as well as those of first-time landslides. The first two mass movements discussed are slumps, located in areas with vigorous regeneration of (grassy) vegetation. Their activation is thought to be the consequence of an increase in infiltration capacity of the soils under regenerating vegetation. One of these slumps had a horizontal movement of the order of 10–20 m in 1 day.The other case is the remobilisation of the May Ntebteb debris flow below the Amba Aradam sandstone cliff. The debris flow presently creeps downslope at a rate of 3–6 cm year⁻¹. Palynological evidence from tufa shows that the reactivation of the flow started 70 years ago. Shear resistance measurements indicate the danger for continuous or prefailure creep. From the soil mechanics point of view, the reactivation of the debris flow is due to the combination of two factors: (1) the reduction of flow confining pressures as a result of gully incision over the last hundred years, and (2) the increase of seepage pressure as a consequence of the cumulative effect of this incision and the increase in infiltration rates on the lobe since grazing and woodcutting have been prohibited 8 years ago. The role of such exclosures as possible landslide triggers is discussed.From the geomorphological point of view, the ancient movements and their present-day reactivation cannot be compared: the ancient movements led to the development of debris flows, whereas the reactivations relate to the dissection of these mass movement deposits.     

Simulation of event-based landslides and debris flows at watershed level

A coupled model has been developed to simulate, at watershed level, landslides and debris flows induced by a severe typhoon (tropical cyclone) in Taiwan. The model comprises a landslide susceptibility model to predict landslide occurrence, an empirical model to select debris-flow initiation points, and a debris flow model to simulate the transport and deposit of failed materials from the identified source areas. In raster format with a 10 m spatial resolution, the model output includes unstable cells, debris-flow initiation cells, debris-flow velocities, runout paths, and deposition zones. The model was first tested and calibrated in a small area, where the damage by landslides had been investigated and recorded. It was then applied to a watershed, and the simulation results were validated by comparing them with a landslide/debris-flow inventory map prepared from satellite images using a multiple change detection technique. Model test and validation results confirm the usefulness of the model in predicting the number and size of affected areas (landslides and runouts combined), runout path, and volume of runout deposits. It is a common practice in Taiwan to separate landslide and debris-flow inventories and to study debris flows only in select drainage basins. This study suggests that landslide and debris flow should be modeled as a sequential process for efficient watershed management.