Landslide Scars Research Articles

Fire-related debris flows in the Iberian Range, Spain

Debris flows occurred three weeks after a wildfire in August 1986 in the Najerilla River valley in the Iberian Range, northern Spain. The flows were triggered by a brief, intense rainstorm (approximately 25mmh−1 over 15min) in a small area with steep slopes covered by a thick colluvium of quartzite clasts. This storm resulted in the development of several unconfined hillslope debris flows and the formation of an alluvial fan at the mouth of the Pítare stream, which partially blocked the Najerilla River. We analysed the conditions that led to the development of the debris flows, and estimated the rainfall threshold for the debris flows to occur as well as the total volume of mobilised sediment. Four factors contributed to the debris flows: (i) the occurrence of a rainstorm three weeks after a wildfire, which had removed the plant cover from the soil; (ii) the steep slopes in the area (>30°), which were the most affected by debris flows; (iii) the presence of quartzite scarps on the hillslopes, which favoured the development of a ‘firehose effect’ involving channelised surface runoff; and (iv) the low plasticity index values of the fine material of the colluvium (indices of 7 to 8), which enabled rapid liquefaction. Estimates of rainfall intensity derived from the estimated peak flow in the Pítare stream suggests that around 80mm of rainfall fell in approximately 15min, although this is clearly an overestimated value given the high proportion of sediment load transported during the peak flow. Various equations estimated a rainfall-threshold of approximately 25mmh−1 considering a concentration time of 15min. The total sediment transported by the debris flows was 10,500m3 (15,750Mg, 6800Mgkm−2), and the Pítare stream alone transported a minimum of 4000m3 (6000Mg, 2500Mgkm−2). These results suggest that the rainfall threshold for initiating debris flows decreases following a wildfire, such that an ordinary rainstorm is able to trigger a severe erosion and sediment transport event. Given the absence of fresh landslide scars on the hillslopes, the origin of the fire-related debris flows in the Najerilla River valley appears to have been directly linked to increased rates of overland flow having a greater effect than infiltration for triggering debris flows.

Landslide susceptibility assessment in Limbe (SW Cameroon): A field calibrated seed cell and information value method

The dissected volcanic terrains around Limbe, SW Cameroon are frequently affected by small scale but destructive landslides. In this study, a raster-based data driven method involving seed cells is used to build a landslide susceptibility model for the Limbe area. Factors considered to be potential controls of slope failure within this area include slope gradient, rock type, distance from roads, slope orientation, mean annual precipitation, soil type, land cover type, stream density and distance from stream. 63 small to very small translational and rotational landslide scars were identified through extensive field work. Landslide data is randomly divided into a training (75%) and validation set (25%) and seed cells are generated by creating 25 m buffer zones around the head scarp of each scar. The quantitative relationship between landslide seed cells and the above-mentioned factors is established by a data driven approach to obtain weighted factor classes. Summing weighted factor layers, a continuous scale of susceptibility indices is obtained and reclassified into 5 susceptibility classes. Seed cells obtained from the validation data set were used to evaluate the quality of several models involving different controlling factors. Our preferred model combines the weight of 6 factors (i.e. slope gradient, land cover, mean annual precipitation, stream density, proximity to roads and slope orientation). 78% of the validation seed cells are located within the high to very high susceptibility class, which occupy 16.9% of the study area. The obtained susceptibility map is combined with the outline of urban areas and key infrastructures to evaluate zones that are vulnerable to the impact of future slope failures. Such an approach will assist civil protection and urban planning efforts in SW Cameroon. Language: en

Slope stability analysis and landslide susceptibility assessment on the SE foot slopes of Mt Cameroon

Landslides are well known processes that hamper sustainable development efforts, particularly in developing countries where extreme poverty is already a household to societal scale problem. Though a worldwide issue, landslides remain enigmatic especially with regards to their geometry, causative factors, triggering mechanisms, relationship between triggering factors and landslide occurrence, and the susceptibility of any area to mass wasting phenomena. This thesis presents an inventory and thorough analysis of small–scale devastating landslides that occurred in recent years around the city of Limbe at the base of Mt Cameroon. A multidisciplinary approach was adopted in order to understand the complex and multi-phase nature of landslide processes. This involved integrating results of traditional field mapping, rainfall measurements, analyses of geotechnical properties, soil mineralogy and geochemistry to get an idea of the processes and mechanisms involved in landslide initiation and mobilisation. From these results, two conceptual models are proposed to explain the occurrence of landslides affecting areas populated by vulnerable people. The thesis thus provides first-hand field-measured geometric characteristics of landslide scars and systematically documents and quantifies for the first time the overall features of volume-limited devastating slides around Limbe. Hence, it provides constraints for modelling, monitoring and remediation efforts that might be applied in other areas affected by similar small-scale failures. Field observations, geotechnical and mineralogical characterisation suggests that slope failure within the study area is enhanced by human intervention through anarchical construction and slope undercutting whereas natural slope failure is generated as a result of fracture enhanced permeability. This implies that in the absence of external factors the slopes within the study area would generally remain stable for long periods. It also demonstrates that understanding landslide triggering processes and relationships between triggers and slide occurrence could be improved through more accurate recording of landslide events and the corresponding triggering process (intensity, magnitude and duration). This knowlegde can be used in landslide susceptibility assessment which was one of the most significant achievements of this thesis, and in the development of early-warning systems that might enable a reduction in the number of casualties and economic loss from future landslide phenomena. The findings from the present study have generic value and can be used to tackle small-scale slope instability problems elsewhere in the subtropics.

Erosion and channel change as factors of landslides and valley formation in Champlain Sea Clays: The Chacoura River, Quebec, Canada

The Champlain Sea clays of Eastern Canada are incised by numerous rivers. Their slopes have been modified by landslides: on the Chacoura River near Trois-Rivières (Quebec), several large landslide scars, more or less recent, are visible. The role of erosion (channel incision, lateral channel migration and erosion of slopes due to agricultural drainage) as a trigger of these landslides is important. The aim of this study is to understand how erosion and landslides are related to valley development. From a detailed analysis of aerial photographs and DEMs, a map of the phenomena has been drawn by identifying various elements such as landslides, limits of the slope, position of the channel, and the area covered by forest. It is shown that channel change and erosion are strongly linked to landslides by the fact that they change the bank morphology in an unstable way. A slide in itself is a natural way for the slope to achieve stability. But when it occurs in a stream, it creates a disturbance to the stream flow enhancing local erosion which may change the river path and generate more erosion downstream or upstream resulting in more slides. Cross-valley sections and a longitudinal profile show that landslides are a major factor of valley formation. It appears that the upper part of the Chacoura River valley is still unaffected by landslides and has V-shaped sections. The lower part has been subject to intense erosion and many landslide scars can be seen. This shows that the valley morphology is transient, and that future activity is more likely to occur in the upper part of the river. Therefore the identification of areas prone to erosion will help determine the possible location of future large landslides just like the ones that occurred in the lower part.

Eruptive response of oceanic islands to giant landslides: New insights from the geomorphologic evolution of the Teide–Pico Viejo volcanic complex (Tenerife, Canary)

Large sector collapses are a major component of oceanic islands evolution. Here we show that voluminous events such as the Icod landslide on Tenerife (Canary Islands) cause dramatic changes on the magma feeding system and control the subsequent volcanic and geomorphologic evolution of the eruptive complex over a period of more than 150 kyr. Instantaneous unloading by the Icod landslide is marked by the development of a large phonolitic explosive eruption dated at 175 ± 3 ka and interpreted as reflecting the immediate emptying of a shallow pre-existing magma chamber. Geochronological, geomorphological and geochemical analyses, carried out on the post-landslide volcanic succession sampled in a 4.4 km-long underground water-recovery gallery, provide further evidence for an enhanced extrusion of primitive lavas starting in the 10 kyr time interval following the failure. Rapid construction (< 40 kyr) of a thick basaltic volcano in the landslide scar at high eruptive rates (up to 8 km 3 kyr −1) increased the lithostatic pressure which then favored the intermittent storage of basic magma under the edifice. This resulted in more episodic construction evidenced by a significant decrease in output rates and the increasing occurrence of lavas with intermediate composition from 117 ± 7 to 52 ± 7 ka. An apparent volcanic gap is observed between 52 ± 7 and 18 ± 1 ka, after which highly differentiated lavas have been dominantly erupted. We propose that part of the gap can be explained by the individualization of a shallow magma reservoir a few kilometers below the base of the Teide volcano. During recent periods, vertical and lateral extrusions of trachytic and phonolitic viscous bodies from this storage area contributed to increase the slope of the main edifice up to 35°, overall favoring its present-day instability.

Earthquake-induced landslide mapping in the western Himalayas using medium resolution ASTER imagery

On 8 October 2005, a devastating earthquake struck northern Pakistan and several parts of Pakistani- and Indian-controlled Kashmir. The severely hit areas lie in close proximity to the most tectonically active region of the western Himalayas. The earthquake destroyed close to 400 000 houses and over 75 000 people lost their lives. The intensity of the earthquake was such that it triggered widespread landslides, which caused considerable destruction of the area's forests, and blocked the mountain roads and rivers. Satellite imagery-based analysis can be effectively used to provide critical geologic information for determining the causes of earthquakes, mapping of faults and lineaments, as well as for hazards and damage assessment mapping. In this study, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery acquired on 27 October 2005 was analysed to assess earthquake-induced land cover changes in severely hit areas. Image analysis techniques including intensity hue and saturation (IHS), principal component analysis (PCA), Normalized Difference Vegetation Index (NDVI) and Iterative Self-Organizing Data Analysis Technique (ISODATA) were applied on VNIR–SWIR bands of ASTER for the purpose of identification and mapping of landslides triggered by the earthquake in areas of northern Pakistan and Pakistani-controlled Kashmir. The techniques proved effective in identifying freshly exposed surfaces on mountain slopes, landslide scars and debris deposits. Accurate identification and mapping of landslides and slope failures can play an important role in post-earthquake damage assessment and hazards mapping in earthquake-prone mountainous areas.

Germanium‐silicon as a flow path tracer: Application to the Rio Icacos watershed

We use dissolved silicon together with its “geochemical twin” germanium for the first time as a hydrologic tracer to study water delivery to the stream during storm events in the Rio Icacos watershed, Puerto Rico. Ge and Si were measured on base flow, stormflow, springwater, and soil water samples. Compositions of all of these waters appear to reflect varying contributions from three components, which we attribute to solutes released from bedrock weathering (groundwater), from short‐term soil‐water interaction (quick soil water), and longer‐term soil‐water interaction (matrix soil water). Base flow stream waters have high Si and moderate Ge (Ge/Si ratio ∼0.29 μmol/mol), consistent with a predominantly bedrock weathering source as indicated by their similarity with water sampled from springs emerging from the saprolite‐bedrock boundary on a hillslope landslide scar. During storm events there is a shift toward more dilute compositions (but higher Ge/Si ratios) similar to those measured on water samples from temporary depression storage and overland flow (quick soil water). Geochemical mass balance shows that 80%–90% of the stream chemistry can be explained by mixing groundwater with this quick soil water composition, which we infer to reflect new water traveling as shallow throughflow. Stream water δ18O values decrease to more negative values typical of precipitation supporting rapid delivery of rainwater to the stream channel during stormflow. The third component, with a Ge‐rich composition characteristic of soil matrix water sampled by tension lysimeters, is required to explain higher stream water Ge/Si ratios measured during hydrograph recession. We infer from this an additional, slower, and less dominant pathway for delivery of soil water to the stream channel.

Scarp morphology and identification of large-scale mass movements in tropical tablelands: the eastern Araripe basin (Ceará, Brazil)

The article aims to recognize and analyse large-scale mass movement topography in the rim and piedmont of the eastern part of the Chapada (plateau) do Araripe, Ceará. So far ignored in the interior of northeast Brazil, this morphology was identified by the means of field work combined with the analysis of high resolution satellite images and GIS-based geomorphological mapping. The context is that of topographic inversion of a Mesozoic intracratonic basin. Exposed to the trade-winds, the escarpment bears an evergreen or semi-deciduous forest typical of a sub-humid tropical climate. A high sandstone ledge overlooks dissected pediments shaped into softer rocks within wide erosion amphitheatres open to NE. Abundant deposits rich in metre-scale blocks of sandstone are identified down to 6-8 km from the escarpment, 500 m below the summit surface. Between short segments scalloped by sapping processes, longer segments of the escarpment present smooth and concave slopes interpreted as landslide scars. Various types of mass movements are recognized and measured, ranging from slumps to composite types of landslides and debris flows of pluri-kilometric scale. Degraded landslide scars and pediment covers with weathered block deposits also indicate the involvement of large-scale mass movements in older stages of scarp evolution and retreat. The lithological and hydrogeological conditions encountered in the tabular structure of the inverted basin appear as determining factors in the occurrence of these landslides. Although neither historical testimony nor absolute ages are presently available, a relative chronology and possible correlations with the regional paleoclimatic evolution are proposed. Since these phenomena appear as usual processes in scarp evolution and retreat and may occur in a presently populated area, and since some of them involve catastrophic events, a preliminary approach of landslide-related hazards is proposed.

Systematic documentation of landslide events in Limbe area (Mt Cameroon Volcano, SW Cameroon): geometry, controlling, and triggering factors

Limbe town and surrounding areas, on the SE foot slopes of the active Mt Cameroon Volcano, have experienced numerous small-scale shallow landslides within the last 20 years. These resulted in the loss of *30 lives and significant damage to farmland and properties. Landslides and their scars are identified in the field, and their geometry systematically measured to construct a landslide inventory map for the study area. Specific landslides are investigated in detail to identify site-specific controlling and triggering factors. This is to constrain key input parameters and their variability for subsequent susceptibility and risk modeling, for immediate local and regional applications in land-use planning. It will also enable a rapid exploration of remediation strategies that are currently lacking in the SW and NW regions of Cameroon. Typical slides within the study area are small-scale, shallow, translational earth, and debris slides though some rotational earth slides were also documented. The depletion zones have mean widths of 22 m ± 16.7 m and lengths of 25 ± 23 standard deviation. Estimated aerial extents of landslide scars and volume of generated debris range from 10 1 to 10 4 m 2 and 2 to 5 9 10 4 m 3 , respectively. A key finding is that most slope instabilities within the study area are associated with and appear to be exacerbated by man-made factors such as excavation, anarchical construction, and deforestation of steep slopes. High intensity rainfall notably during localized storms is the principal triggering factor identified so far. The findings from this case study have relevance to understanding some key aspects of locally devastating slope instabilities that commonly occur on intensely weathered steep terrains across subtropical Africa and in the subtropics worldwide and affecting an ever denser and most vulnerable population.

Assessment of sediment delivery from successive erosion on stream-coupled hillslopes via a time series of topographic surveys in the central high mountain range of Taiwan

Landslides play an important role in the evolution of mountain topography and are a major source of sediment in tectonically active mountain belts. For the active orogeny of Taiwan, erosion rates have been assessed by various methods. The discrepancies observed between erosion rates integrated over modern and Quaternary time scales may result from various effects related to surface processes.More process-oriented research on the spatio-temporal distribution of hillslope processes and coupling relationships with the fluvial system is needed to reconcile the temporally heterogeneous data in the future. This paper assesses and describes the processes of subsequent erosion on hillslopes via a time series of topographic surveys of hillslopes after the initial failure in order to better understand the temporal and spatial variability of landslide erosion, sediment fluxes, and storage of landslide sediment within channels.The results demonstrate that hillslope-channel coupling is controlled by the relative frequency of erosive flooding events and the magnitude of rainfall-driven hillslope processes. The latter is assumed to be disproportionately high on larger failures in periods with less rainfall.Mass wasting may only affect part of the total failure’s area. Volumes of eroded sediment from gullies on the landslide scar may be as high as the volume estimated for the initial failure. The geometry of the sliding masses eroded after the initial failure appears to be uniform, and their depths average 6.1 ± 2.2 m.

Flash-flood hyperpycnal flows generating shallow-water landslides at Fiumara mouths in Western Messina Strait (Italy)

On October 1, 2009, a small area along the Sicilian coast between the villages of Scaletta Zanclea and Giampilieri was struck by intense and concentrated rainfall causing countless small landslides widespread over the catchment area of steep subaerial creeks, locally known as Fiumara. Dense and quick debris flows were channelized within the Fiumara and destructively hit the villages and entered the sea, where they likely transformed into hyperpycnal flows. The availability of pre- and post-flood high-resolution bathymetry allows us to recognize the main features and the most significant morphological variations related to the impact of the flows on the seafloor. The passage of hyperpycnal flows on the seafloor possibly produced a suite of mass-wasting events, encompassing sheet landslides (i.e. erosive scours), retrogressive slope failure on loose sediment at the canyon headwall and rock-falls on the conglomeratic bedrock along canyon sidewall. The possible causes of these events are discussed on the basis of available morphological evidence and geotechnical considerations. Finally, the widespread occurrence of mass-wasting features (i.e. submarine landslide scars) morphologically similar to those generated by the 2009 flash flood allows us to hypothesize, from one side, a strong correlation between this catastrophic event and the evolution of submarine canyons, and from the other side, the possible use of these features for the assessment of flash-flood occurrence.

Recovery of pasture production and soil properties on soil slip scars in erodible siltstone hill country, Wairarapa, New Zealand

Landslide scars previously dated (1977, 1961 and 1941) and sampled in 1984 for pasture production and topsoil characteristics were re-sampled. Pasture dry matter (DM) production and selected soil properties were re-measured on the same scars and uneroded control sites for 2 years, beginning in 2007. The results show that after a further 25 years of recovery, no significant increase in pasture production had occurred on the 1941 and 1961 slip scars. Average DM pasture production on eroded sites increased from 63 to 78% of pasture production levels on uneroded sites, but improvement was restricted to the youngest 1977 slip scars, where DM production increased from 20 to 80% of uneroded levels. Regression analysis revealed that maximum pasture recovery occurred within about 20 years of landsliding and further recovery beyond 80% of uneroded level was unlikely. The recovery of pasture production on slip scars follows similar recovery to soil physical (e.g. soil depth, particle density, bulk density) and chemical properties (e.g. total C, total N). Topsoil depths on eroded sites were roughly a third of topsoil depths on uneroded sites, indicating reduced profile available water capacity on eroded soils. This research was unable to determine if total C would recover to uneroded levels because of the high variability in total C at eroded sites. However, given that uneroded soils were formed under native forest (over a long period of time) and that new soils are forming under pasture (over a short period of time), it is unlikely that total C will recover to uneroded soil levels. This research verifies the conclusion of the previous work carried out in 1984 that it is unlikely pasture production on slip scars will return to production levels on uneroded sites in human timescales. The sustainability of pastoral agriculture on steeper slopes in soft-rock hill country is increasingly under threat from the progressive reduction of pasture production from cumulative erosion.

ジオエコツーリズムの提唱とジオパークによる地域振興・人材育成

A geopark involves not only a geological heritage but also geographical and ecological elements. Therefore, subjects related to geotourism are not only landforms and geology, but also ecosystems, including vegetation and animal communities, and human landscapes, such as terraced rice paddies and grasslands created by human activity. Because geotourism covers a broad range of elements, from geo- and ecosystems to human activities, it is expected that the further development of related activities will be supported and approved by an increasing number of people. In this manuscript, I introduce some examples of geo-ecotourism as a special type of geotourism, including: vegetation at the summit area of Mount Hakusan, vegetation in a landslide scar on Mount Bandaisan, and the habitat of Dicentra peregrine on Mount Yatsugatake. These were all very popular with the participants of excursions I conducted. Geotours also promote regional development because they benefit residents economically by employing local workers and travel-related businesses, and encourage consumption of local services and products by visitors. A pressing issue concerning geotourism in Japan is the development of human resources such as tour guides. A geotour needs an excellent guide with a riveting interpretation. There are, however, few of such guides and there is a need to rapidly foster more. As a model for fostering guides, I introduce my “Intensive course on the natural history of mountains,” which interprets the natural history of mountains based on geo-ecology. In the long term, however, it is necessary to introduce natural history education into the school system and improve the quality of nature programs broadcast on TV. Many participants of geotours are middle aged and older, and tend to be energetic and slow to age. This may be because the tour participants are required to do physical activity and to use their common sense. Based on this perspective, a geopark approach must gain in importance within social and lifelong education.

Testing space‐scale methodologies for automatic geomorphic feature extraction from lidar in a complex mountainous landscape

The next generation of digital elevation data (≤3 m resolution) calls for the development of new algorithms for the objective extraction of geomorphic features, such as channel networks, channel heads, bank geometry, landslide scars, and service roads. In this work, we test the performance of two newly developed algorithms for the extraction of geomorphic features: the wavelet‐based extraction methodology developed by Lashermes et al. (2007) and the GeoNet nonlinear diffusion and geodesic paths methodology proposed by Passalacqua et al. (2010). The study area is part of the Rio Cordon basin, a headwater alpine catchment located in the Dolomites, a mountainous region in the eastern Italian Alps. The aim of this work is to compare the capability of the two new algorithms in extracting the channel network and capturing channel heads, relevant channel disruptions corresponding to landslides, and representative channel cross sections. The extracted channel networks are also compared to the ones obtained using classical methodologies on the basis of an area threshold and an area‐slope threshold. A high‐resolution digital terrain model of 1 m served as the basis for such analysis. The results suggest that, although the wavelet‐based methodology performs well in the channel network extraction and is able to detect channel heads and channel disruptions, the local nonlinear filter together with the global geodesic optimization used in GeoNet is more robust and computationally efficient while achieving better localization and extraction of features, especially in areas where gentle slopes prevail. We conclude that these new methodologies should be considered as valid alternatives to classical methodologies for channel network extraction from lidar, in addition to offering the potential for calibration‐free channel source identification and also extraction of additional features of interest.

Potential tsunamigenic landslides at Stromboli Volcano (Italy): Insight from marine DEM analysis

Seafloor mapping and morphometric analysis of landslide scars can provide useful insights for marine geo-hazard assessment, as demonstrated by several studies performed on different geological settings. The availability of high-resolution Digital Elevation Models (DEMs) obtained through swath bathymetry allows us to apply this approach to the submarine portions of Stromboli volcano, affected for about 90% of their extension by mass-wasting features. Two main kinds of tsunamigenic landslides have been recognized and analyzed. Large-scale sector collapses are catastrophic events that mobilize up to few km 3 of material, generating huge tsunami waves that may affect Stromboli and propagate in surrounding areas. The related hazard is not very high as these collapses show recurrence periods of some (or more) thousand years. Conversely, medium-scale landslides are more hazardous as they occur at higher frequency with respect to the previous events, i.e. from some hundreds up to a few tens of years. These events are able to generate local but severe tsunamis when occurring in shallow-water, as demonstrated by the recent 2002 tsunamigenic landslides that struck the Stromboli coasts with waves up to 10 m high. Medium-scale landslides mobilize volumes up to some millions of cubic meters and may also evolve landwards through retrogressive processes, thus affecting coastal sectors. This study highlights that the main predisposing factors controlling medium-scale slope failures at Stromboli are high slope gradient and sedimentation rates; eruptive activity also plays a key role, as recently witnessed by the 2002 landslide.

Evolution of ocean-island rifts: The northeast rift zone of Tenerife, Canary Islands

The northeast rift zone of Tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone. Field geology, isotopic dating, and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the NE rift zone, which includes a period of very fast growth toward instability (between ca. 1.1 and 0.83 Ma) followed by three successive large landslides: the Micheque and Guimar collapses, which occurred approximately contemporaneously at ca. 830 ka and on either side of the rift, and the La Orotava landslide (between 690 +/- 10 and 566 +/- 13 ka). Our observations suggest that Canarian rift zones show similar patterns of development, which often includes overgrowth, instability, and lateral collapses. Collapses of the rift flanks disrupt established fissural feeding systems, favoring magma ascent and shallow emplacement, which in turn leads to magma differentiation and intermediate to felsic nested eruptions. Rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes. Conversely, the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars, even if concealed by postcollapse volcanism. Comparative analysis of the main rifts in the Canary Islands outlines this general evolutionary pattern: (1) growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions; (2) flank collapse and catastrophic disruption of the established feeder system of the rift; (3) postcollapse centralized nested volcanism, commonly evolving from initially ultramafic-mafic to terminal felsic compositions (trachytes, phonolites); and (4) progressive decline of nested eruptive activity.

Identification of Landslide Scars in the Region of the Serra do Mar, São Paulo State, Brazil, Using Digital Image Processing and Spatial Analysis Tools

The objective of the present study, developed in a mountainous region in Brazil where many landslides occur, is to present a method for detecting landslide scars that couples image processing techniques with spatial analysis tools. An IKONOS image was initially segmented, and then classified through a Batthacharrya classifier, with an acceptance limit of 99%, resulting in 216 polygons identified with a spectral response similar to landslide scars. After making use of some spatial analysis tools that took into account a susceptibility map, a map of local drainage channels and highways, and the maximum expected size of scars in the study area, some features misinterpreted as scars were excluded. The 43 resulting features were then compared with visually interpreted landslide scars and field observations. The proposed method can be reproduced and enhanced by adding filtering criteria and was able to find new scars on the image, with a final error rate of 2.3%.

Shallow landslide prediction in the Serra do Mar, São Paulo, Brazil

Abstract. Various methods are currently used in order to predict shallow landslides within the catchment scale. Among them, physically based models present advantages associated with the physical description of processes by means of mathematical equations. The main objective of this research is the prediction of shallow landslides using TRIGRS model, in a pilot catchment located at Serra do Mar mountain range, São Paulo State, southeastern Brazil. Susceptibility scenarios have been simulated taking into account different mechanical and hydrological values. These scenarios were analysed based on a landslide scars map from the January 1985 event, upon which two indexes were applied: Scars Concentration (SC – ratio between the number of cells with scars, in each class, and the total number of cells with scars within the catchment) and Landslide Potential (LP – ratio between the number of cells with scars, in each class, and the total number of cells in that same class). The results showed a significant agreement between the simulated scenarios and the scar's map. In unstable areas (SF≤1), the SC values exceeded 50% in all scenarios. Based on the results, the use of this model should be considered an important tool for shallow landslide prediction, especially in areas where mechanical and hydrological properties of the materials are not well known.

An extensive late Cenozoic terrestrial record of multiple glaciations preserved in the Tintina Trench of west-central Yukon: stratigraphy, paleomagnetism, paleosols, and pollenThis is a companion paper to Barendregt et al., also in this issue.Geological Survey of Canada Contribution 20100035.

The Tintina Trench in west-central Yukon is a late Miocene graben formed along the antecedent early Tertiary Tintina fault. Since its formation the trench has served as a sediment trap for alluvial and glacial deposits. An extensive record of preglacial, glacial, and interglacial sediments spanning the late Pliocene to late Pleistocene has been preserved and is exposed today in modern landslide scars. This sedimentary record comprises multiple sequences of tills, outwash, mudflows, loess, and paleosols. The glacial sediments are the product of both local (montane) and regional (Cordilleran) ice advances that channeled into the trench, while loess and well-developed paleosols (brunisols and luvisols) reflect nonglacial and interglacial conditions, respectively. The Tintina Trench exposures provide the most complete record of glaciations for the region. Paleomagnetism, paleosols, and palynology provide age constraints for the geological events. A formal stratigraphic nomenclature is proposed for this region. The name West Tintina Trench Allogroup is assigned to the glacial–interglacial and nonglacial strata that occurs above a major regional Miocene–Pliocene unconformity. The allogroup spans the late Pliocene (3.6 Ma) to middle Pleistocene (0.126 Ma), based on magnetostratigraphy and pollen data. The sequence includes an alluvial deposit at the base, overlain by an extensive sequence of tills and outwash, and capped by loess. Paleosols and weathering horizons occur throughout the sequence. Tintina Trench; Beringia; glacial chronology; magnetostratigraphy; early and middle Pleistocene; Yukon paleoenvironments; Yukon paleosols; Yukon pollen; North American glaciations; West Tintina Trench Allogroup.

Climate forcing of geological and geomorphological hazards

The 12 research papers and two summaries of conference discussion sessions contained in this Theme Issue build upon presentations and dialogue at the Third Johnston–Lavis Colloquium held at University College London in September 2009. The meeting brought together delegates from the UK, Europe and