Regional-scale controls on rockfall occurrence

Rockfalls exert a first-order control on the rate of rock wall retreat on mountain slopes and on coastal rock cliffs. Their occurrence is conditioned by a combination of intrinsic (resisting) and extrinsic (driving) processes, yet determining the exact effects of these processes on rockfall activity and the resulting cliff erosion remains difficult. Although rockfall activity has been monitored extensively in a variety of settings, high-resolution observations of rockfall occurrence on a regional scale are scarce. This is partly owing to difficulties in adequately quantifying the full range of possible rockfall volumes with sufficient accuracy and completeness, and at a scale that exceeds the influence of localised controls on rockfalls. This lack of insight restricts our ability to abstract patterns, to identify long-term changes in behaviour, and to assess how rock slopes respond to changes in both structural and environmental conditions, without resorting to a space for-time substitution. This thesis develops a workflow, from novel data collection to analysis, which is tailored to monitoring rockfall activity and the resulting cliff retreat continuously (in space), in 3D, and over large spatial scales $(> 10^4 m)$. The approach is tested by analysing rockfall activity and the resulting erosion recorded along 20.5 km of near-vertical coastal cliffs, in what is considered as the first multi-temporal detection of rockfalls at a regional-scale and in full 3D. The resulting data are then used to derive a quantitative appraisal of along-coast variations in the geometric properties of exposed discontinuity surfaces, to assess the extent to which these drive patterns in the size and shape of the rockfalls observed. High-resolution field monitoring is then undertaken along a subsection of the coastline $(> 10^2 m)$, where cliff lithology and structure are approximately uniform, in order to quantify spatial variations in wave loading characteristics and to relate these to local morphological conditions, which can act as a proxy for wave loading characteristics. The resulting rockfall inventory is analysed to identify the characteristics of rock slope change that only become apparent when assessed at this scale, placing bounds on data previously collected more locally $(< 10^2 m)$. The data show that spatial consistencies in the distribution of rockfall shape and volume through time approximately follow the geological setting of the coastline, but that variations in the strength of these consistencies are likely to be conditioned by differences in local processes and morphological controls between sites. These results are used to examine the relationships between key metrics of erosion, structural, and morphological controls, which ultimately permits the identification of areas where patterns of erosion are dominated by either intrinsic or extrinsic processes, or a mixture of both. Uniquely, the methodologies and data presented here mark a step-change in our ability to understand the competing effects of different processes in determining the magnitude and frequency of rockfall activity, and the resulting cliff erosion. The findings of this research hold considerable implications for our understanding of rockfalls, and for monitoring, modelling, and managing actively failing rock slopes

The effects of differential uplift and sediment supply on major Himalayan river systems at the mountain front

It is well documented that in tectonically active regions, fluvial morphology responds to changes in base level. Vertical incision rates are adjusted through changes in channel morphology to balance imposed rates of rock uplift. It is common for responses in channel width, slope, grain size distribution and stream power to reflect spatial and temporal changes in rock uplift rates. Channels within tectonically active orogenic belts, such as the Himalayas, may be influenced by changes in discharge or sediment supply in addition to tectonic controls. Identifying the causes of morphological response in such areas is therefore of first order importance to enhance our understanding of landscape evolution. The Nepalese Himalayan foreland presents the ideal location to undertake this investigation, with its distinct tectonic frameworks heavily influenced by the style of foreland basin development. Thin-skinned thrust faulting over the past 1.6 Ma, has facilitated the recycling of foreland basin fill into hanging wall deposits of the frontal thrust (HFT), producing topographic entities now recognised as the Siwalik Hills. Above weak basal decollements, Dun valleys separate the frontal Siwalik Hills, and have rapidly filled with erosional detritus from the rising Himalaya. Poorly consolidated lithologies within the Siwalik Hills and Dun valleys are now being remobilised by modern incision of Himalayan River systems. It is unknown whether patterns of sediment storage and release within the foreland affect river morphology. To understand the controls behind Himalayan river morphology, longitudinal profiles and channel slope were extracted from 90 m digital elevation models along the Gandak and Kosi Rivers about the Himalayan mountain front. Remotely sensed channel width measurements have also been made, and further supplemented with grain size data derived in the field and analysed using photo sieving techniques. Short-lived increases in channel slope are noted at the Main Boundary Thrust and Main Dun Thrust (MDT) of both rivers, in addition to a decrease in slope upstream of the HFT and Kosi Main Central Thrust (MCT). Increases in channel width upstream of the HFT and MCT (Kosi) are also consistent with morphological response to tectonic uplift. Where characteristic responses in morphology are absent at identified tectonic structures, it is likely that changes in lithology or anthropogenic modification of flow have overwhelmed tectonic influences. No increase in grain size upstream of recognized fault locations was noted on the Gandak, and it is proposed that an alternative mechanism dictates grain size patterns at the mountain front. On passing downstream of the MDT, an absence of direct hill slope inputs and a greater proportion of seasonal tributary inputs is reflected by a narrowing of grain size distributions, loss of Greater Himalayan lithologies, and decrease in D84 (by 50 mm). These differences between geometry and grain size of the Gandak and Kosi Rivers are interpreted in terms of the style of foreland basin evolution. A lack of foreland accommodation above the strong basal decollement of the Kosi River facilitates continuous exportation of erosional detritus out of the mountain front. Widely spread D84 grain size distributions along the Kosi are dominated by regular inputs of coarse hill slope material from unstable relief produced by exceptional rates of uplift, above closely spaced frontal tectonic structures. It is interpreted that the weak basal decollement characterising the Gandak region has produced more stable hillslopes and accommodation for sediment within the Chitwan Dun. The fine grained and well sorted grain-size distributions of the Gandak noted between the MDT and HFT reflect an absence of direct hill slope inputs and a presence of seasonal tributary derived material. This study concludes that active tectonic structures strongly influence channel geometry at the mountain front. Grain size patterns are believed independent to differential uplift, and are considered a function of lateral sediment inputs, the nature of which reflects the structural evolution and tectonic history of the foreland basin

Geomorphometry 2020. Conference Proceedings

Geomorphometry is the science of quantitative land surface analysis. It gathers various mathematical, statistical and image processing techniques to quantify morphological, hydrological, ecological and other aspects of a land surface. Common synonyms for geomorphometry are geomorphological analysis, terrain morphometry or terrain analysis and land surface analysis. The typical input to geomorphometric analysis is a square-grid representation of the land surface: a digital elevation (or land surface) model. The first Geomorphometry conference dates back to 2009 and it took place in Zürich, Switzerland. Subsequent events were in Redlands (California), Nánjīng (China), Poznan (Poland) and Boulder (Colorado), at about two years intervals. The International Society for Geomorphometry (ISG) and the Organizing Committee scheduled the sixth Geomorphometry conference in Perugia, Italy, June 2020. Worldwide safety measures dictated the event could not be held in presence, and we excluded the possibility to hold the conference remotely. Thus, we postponed the event by one year - it will be organized in June 2021, in Perugia, hosted by the Research Institute for Geo-Hydrological Protection of the Italian National Research Council (CNR IRPI) and the Department of Physics and Geology of the University of Perugia. One of the reasons why we postponed the conference, instead of canceling, was the encouraging number of submitted abstracts. Abstracts are actually short papers consisting of four pages, including figures and references, and they were peer-reviewed by the Scientific Committee of the conference. This book is a collection of the contributions revised by the authors after peer review. We grouped them in seven classes, as follows: • Data and methods (13 abstracts) • Geoheritage (6 abstracts) • Glacial processes (4 abstracts) • LIDAR and high resolution data (8 abstracts) • Morphotectonics (8 abstracts) • Natural hazards (12 abstracts) • Soil erosion and fluvial processes (16 abstracts) The 67 abstracts represent 80% of the initial contributions. The remaining ones were either not accepted after peer review or withdrawn by their Authors. Most of the contributions contain original material, and an extended version of a subset of them will be included in a special issue of a regular journal publication

Debris-flow erosion and deposition dynamics

Debris flows are a major natural hazard in mountains world wide, because of their destructive potential. Prediction of occurrence, magnitude and travel distance is still a scientific challenge, and thus research into the mechanics of debris flows is still needed. Poor understanding of the processes of erosion and deposition are partly responsible for the difficulties in predicting debrisflow magnitude and travel distance. Even less is known about the long-term evolution of debrisflow fans because the sequential effects of debris-flow erosion and deposition in thousands of flows are poorly documented and hence models to simulate debris-flow fans do not exist. Here I address the specific issues of the dynamics of erosion and deposition in single flows and over multiple flows on debris-flow fans by terrain analysis, channel monitoring and fan evolution modeling. I documented erosion and deposition dynamics of debris flows at fan scale using the Illgraben debris-flow fan, Switzerland, as an example. Debris flow activity over the past three millenia in the Illgraben catchment in south-western Switzerland was documented by geomorphic mapping, radiocarbon dating of wood and cosmogenic exposure dating of deposits. In this specific case I also documented the disturbance induced by two rock avalanches in the catchment resulting in distinct patterns of deposition on the fan surface. Implications of human intervention and the significance of autogenic forcing of the fan system are also discussed. Quantification and understanding of erosion and deposition dynamics in debris flows at channel scale hinges on the ability to detect surface change. But change detection is a fundamental task in geomorphology in general. Terrestrial laser scanners are increasingly used for monitoring down to centimeter scale of surface change resulting from a variety of geomorphic processes, as they allow the rapid generation of high resolution digital elevation models. In this thesis procedures were developed to measure surface change in complex topography such as a debris-flow channel. From this data high-resolution digital elevation models were generated. But data from laser scanning contains ambiguous elevation information originating from point cloud matching, surface roughness and erroneous measurments. This affects the ability to detect change, and results in spatially variable uncertainties. I hence developed techniques to visualize and quantify these uncertainties for the specific application of change detection. I demonstrated that use of data filters (e.g. minimum height filter) on laser scanner data introduces systematic bias in change detection. Measurement of debris-flow erosion and deposition in single events was performed at Illgraben, where multiple debris flows are recorded every year. I applied terrestrial laser scanning and flow hydrograph analysis to quantify erosion and deposition in a series of debris flows. Flow depth was identified as an important control on the pattern and magnitude of erosion, whereas deposition is governed more by the geometry of flow margins. The relationship between flow depth and erosion is visible both at the reach scale and at the scale of the entire fan. Maximum flow depth is a function of debris flow front discharge and pre-flow channel cross section geometry, and this dual control gives rise to complex interactions with implications for long-term channel stability, the use of fan stratigraphy for reconstruction of past debris flow regimes, and the predictability of debris flow hazards. Debris-flow fan evolution on time scales of decades up to ten thousands of years is poorly understood because the cumulative effects of erosion and deposition in subsequent events are rarely well documented and suitable numerical models are lacking. Enhancing this understanding is crucial to assess the role of autogenic (internal) and allogenic (external) forcing mechanisms on building debris-flow fans over long time scales. On short time scales understanding fan evolution is important for debris-flow hazard assessment. I propose a 2D reduced-complexity model to assess debris-flow fan evolution. The model is built on a broad range of qualitative and empirical observations on debris-flow behaviour as well as on monitoring data acquired at Illgraben as part of this thesis. I have formulated a framework of rules that govern debris-flow behaviour, and that allows efficient implementation in a numerical simulation. The model is shown to replicate the general behaviour of alluvial fans in nature and in flume experiments. In three applications it is demonstrated how fan evolution modeling may improve understanding of inundation patterns, surface age distribution and surface morphology

Semi-automated geomorphological mapping applied to landslide hazard analysis

Computer-assisted three-dimensional (3D) mapping using stereo and multi-image (“softcopy”) photogrammetry is shown to enhance the visual interpretation of geomorphology in steep terrain with the direct benefit of greater locational accuracy than traditional manual mapping. This would benefit multi-parameter correlations between terrain attributes and landslide distribution in both direct and indirect forms of landslide hazard assessment. Case studies involve synthetic models of a landslide, and field studies of a rock slope and steep undeveloped hillsides with both recently formed and partly degraded, old landslide scars. Diagnostic 3D morphology was generated semi-automatically both using a terrain-following cursor under stereo-viewing and from high resolution digital elevation models created using area-based image correlation, further processed with curvature algorithms. Laboratory-based studies quantify limitations of area-based image correlation for measurement of 3D points on planar surfaces with varying camera orientations. The accuracy of point measurement is shown to be non-linear with limiting conditions created by both narrow and wide camera angles and moderate obliquity of the target plane. Analysis of the results with the planar surface highlighted problems with the controlling parameters of the area-based image correlation process when used for generating DEMs from images obtained with a low-cost digital camera. Although the specific cause of the phase-wrapped image artefacts identified was not found, the procedure would form a suitable method for testing image correlation software, as these artefacts may not be obvious in DEMs of non-planar surfaces.Modelling of synthetic landslides shows that Fast Fourier Transforms are an efficient method for removing noise, as produced by errors in measurement of individual DEM points, enabling diagnostic morphological terrain elements to be extracted. Component landforms within landslides are complex entities and conversion of the automatically-defined morphology into geomorphology was only achieved with manual interpretation; however, this interpretation was facilitated by softcopy-driven stereo viewing of the morphological entities across the hillsides.In the final case study of a large landslide within a man-made slope, landslide displacements were measured using a photogrammetric model consisting of 79 images captured with a helicopter-borne, hand-held, small format digital camera. Displacement vectors and a thematic geomorphological map were superimposed over an animated, 3D photo-textured model to aid non-stereo visualisation and communication of results

Photogrammetric techniques for across-scale soil erosion assessment: Developing methods to integrate multi-temporal high resolution topography data at field plots

Soil erosion is a complex geomorphological process with varying influences of different impacts at different spatio-temporal scales. To date, measurement of soil erosion is predominantly realisable at specific scales, thereby detecting separate processes, e.g. interrill erosion contrary to rill erosion. It is difficult to survey soil surface changes at larger areal coverage such as field scale with high spatial resolution. Either net changes at the system outlet or remaining traces after the erosional event are usually measured. Thus, either quasi-point measurements are extrapolated to the corresponding area without knowing the actual sediment source as well as sediment storage behaviour on the plot or erosion rates are estimated disrupting the area of investigation during the data acquisition impeding multi-temporal assessment. Furthermore, established methods of soil erosion detection and quantification are typically only reliable for large event magnitudes, very labour and time intense, or inflexible. To better observe soil erosion processes at field scale and under natural conditions, the development of a method is necessary, which identifies and quantifies sediment sources and sinks at the hillslope with high spatial resolution and captures single precipitation events as well as allows for longer observation periods. Therefore, an approach is introduced, which measures soil surface changes for multi-spatio-temporal scales without disturbing the area of interest. Recent advances regarding techniques to capture high resolution topography (HiRT) data led to several promising tools for soil erosion measurement with corresponding advantages but also disadvantages. The necessity exists to evaluate those methods because they have been rarely utilised in soil surface studies. On the one hand, there is terrestrial laser scanning (TLS), which comprises high error reliability and retrieves 3D information directly. And on the other hand, there is unmanned aerial vehicle (UAV) technology in combination with structure from motion (SfM) algorithms resulting in UAV photogrammetry, which is very flexible in the field and depicts a beneficial perspective. Evaluation of the TLS feasibility reveals that this method implies a systematic error that is distance-related and temporal constant for the investigated device and can be corrected transferring calibration values retrieved from an estimated lookup table. However, TLS still reaches its application limits quickly due to an unfavourable (almost horizontal) scanning view at the soil surface resulting in a fast decrease of point density and increase of noise with increasing distance from the device. UAV photogrammetry allows for a better perspective (birds-eye view) onto the area of interest, but possesses more complex error behaviour, especially in regard to the systematic error of a DEM dome, which depends on the method for 3D reconstruction from 2D images (i.e. options for additional implementation of observations) and on the image network configuration (i.e. parallel-axes and control point configuration). Therefore, a procedure is developed that enables flexible usage of different cameras and software tools without the need of additional information or specific camera orientations and yet avoiding this dome error. Furthermore, the accuracy potential of UAV photogrammetry describing rough soil surfaces is assessed because so far corresponding data is missing. Both HiRT methods are used for multi-temporal measurement of soil erosion processes resulting in surface changes of low magnitudes, i.e. rill and especially interrill erosion. Thus, a reference with high accuracy and stability is a requirement. A local reference system with sub-cm and at its best 1 mm accuracy is setup and confirmed by control surveys. TLS and UAV photogrammetry data registration with these targets ensures that errors due to referencing are of minimal impact. Analysis of the multi-temporal performance of both HiRT methods affirms TLS to be suitable for the detection of erosion forms of larger magnitudes because of a level of detection (LoD) of 1.5 cm. UAV photogrammetry enables the quantification of even lower magnitude changes (LoD of 1 cm) and a reliable observation of the change of surface roughness, which is important for runoff processes, at field plots due to high spatial resolution (1 cm²). Synergetic data fusion as a subsequent post-processing step is necessary to exploit the advantages of both HiRT methods and potentially further increase the LoD. The unprecedented high level of information entails the need for automatic geomorphic feature extraction due to the large amount of novel content. Therefore, a method is developed, which allows for accurate rill extraction and rill parameter calculation with high resolution enabling new perspectives onto rill erosion that has not been possible before due to labour and area access limits. Erosion volume and cross sections are calculated for each rill revealing a dominant rill deepening. Furthermore, rill shifting in dependence of the rill orientation towards the dominant wind direction is revealed. Two field plots are installed at erosion prone positions in the Mediterranean (1,000 m²) and in the European loess belt (600 m²) to ensure the detection of surface changes, permitting the evaluation of the feasibility, potential and limits of TLS and UAV photogrammetry in soil erosion studies. Observations are made regarding sediment connectivity at the hillslope scale. Both HiRT methods enable the identification of local sediment sources and sinks, but still exhibiting some degree of uncertainty due to the comparable high LoD in regard to laminar accumulation and interrill erosion processes. At both field sites wheel tracks and erosion rills increase hydrological and sedimentological connectivity. However, at the Mediterranean field plot especially dis-connectivity is obvious. At the European loess belt case study a triggering event could be captured, which led to high erosion rates due to high soil moisture contents and yet further erosion increase due to rill amplification after rill incision. Estimated soil erosion rates range between 2.6 tha-1 and 121.5 tha-1 for single precipitation events and illustrate a large variability due to very different site specifications, although both case studies are located in fragile landscapes. However, the susceptibility to soil erosion has different primary causes, i.e. torrential precipitation at the Mediterranean site and high soil erodibility at the European loess belt site. The future capability of the HiRT methods is their potential to be applicable at yet larger scales. Hence, investigations of the importance of gullys for sediment connectivity between hillslopes and channels are possible as well as the possible explanation of different erosion rates observed at hillslope and at catchment scales because local sediment sink and sources can be quantified. In addition, HiRT data can be a great tool for calibrating, validating and enhancing soil erosion models due to the unprecedented level of detail and the flexible multi-spatio-temporal application

The Hidden Hazard Of Melting Ground Ice In Northern Iceland

This thesis explores the morphology, dynamics and causes of landslides and debris flows in mountainous regions of northern Iceland. The primary objectives are to define the initiation and evolution of Icelandic landslides and debris flows, and to understand the link between ground-ice thaw and rapid mass movements. Slopes are predicted to react more intensely to global warming, so improving our knowledge of rapid mass movements in cold environments, which are even more sensitive to climate change, is crucial, as they could pose at risk local population in Iceland and other mountainous periglacial areas. I first perform a detailed study of debris flows in north-western Iceland, distinguishing through quantitative geomorphological methods the different mechanisms of debris-flow initiation and the associated geomorphic features. The approach of this study is easily applicable to similar settings, and its results could help in anticipating new potentially destructive events. Secondly, I describe and quantify the morphometric characteristics of two landslides in northern Iceland, whose source materials comprised ground ice-cemented deposits. This study reveals different dynamic landslide processes and the crucial role of thawing ground ice in landslide emplacement. I then analyse meteorological and seismic data near these two landslides. I define and distinguish precipitation, seismic activity and permafrost degradation as the preparatory and triggering factors for the failures. Finally, through a geomorphic approach I analyse molards, conical mounds of debris that I found in both landslides deposits. I show conclusive evidence that molards form from thawing of blocks of ice-rich sediments that degrade into cones of debris. I demonstrate that molards are the ‘fingerprint” of permafrost degradation, and their different morphology and distribution can reveal different types of landslide processes in periglacial terrains. This thesis widens our knowledge of the conditions and processes controlling rapid mass movements in cold environments, which is crucial in the perspective of hazard assessment, and opens up new avenues for the study of potentially hazardous geomorphic responses of cold landscapes to changing climate conditions

Multi-scale assessment of shore platform erosion

The morphology and erosion of shore platforms is a pivotal component of rocky coast evolution as these features control both wave transformation and sediment dynamics. Models that predict coastline evolution and efforts to reconstruct past cliff retreat rates from cosmogenic isotope concentrations are forced to simplify platform morphology and commonly treat erosion only implicitly. The lack of an explicit incorporation of platform dynamics into such models reflects a poor understanding of erosion processes that have conventionally been considered to operate at one of two scales: fine scale abrasion captured by sub-mm precision point measurements of vertical change, and step back-wearing and block removal at metre-scale. Neither approach is well suited to informing a generalised model of foreshore erosion that bridges these two scales or that can be applied more widely. As a result without understanding mechanisms of foreshore erosion models which use these data are limited in their utility to address future coastal change under changing sea level and storminess. To address this a multi-scale study was undertaken along the North Yorkshire coast (UK) using high-resolution and high-precision monitoring data collected at the spatial and temporal scales relevant to the processes in action. A novel method was developed to monitor mm-scale platform erosion using Structure-from-Motion (SfM) photogrammetry. The average platform down-wearing rate of 0.528 mm yr-1 was calculated from 15 individual 0.5×0.5 m sites. The volume frequency and 3D-shape distributions of the detachments suggest that erosion occurs predominantly via detachment of fabric-defined platelets. The erosion rate is faster closer to the cliff toe and at those locations where the tide cycles more frequently. Erosion rates calculated from the 2.6 years of data from 22 km of shore platform using high-resolution airborne LiDAR was 3.45 mm yr-1 when derived from individual detachments, or 0.01 mm yr-1 when spatially averaged across the platform. Average lowering of the platform sections containing steps was 0.04 mm yr-1, while in areas with no steps 0.01 mm yr-1. Whilst erosion rate cannot be predicted with confidence for any discrete point on the foreshore, systematic trends in across-shore erosion can be shown, with a peak in rate at 10-18 m from the cliff toe, with erosion intensity gradually decreasing seawards. This new understanding of foreshore erosion has then been used to predict exposure ages from cosmogenic 10Be concentrations at the Hartle Loup platform. This analysis shows that the cliff has been retreating at the steady rate of 0.05 m yr-1 cutting the 300 m wide shore platform in the last 6 kyr. This derives rates of retreat comparable to contemporary erosion monitoring. Platform morphology has been shown not to adjust to an equilibrium shape, but it is rather actively modified depending on the interplay between present morphology, sea level and tidal regime. Importantly, this study provides methods to monitor foreshore erosion, enhances our understanding of mechanisms and controls upon it, whilst the results can be used in models to predict rocky coast evolution by providing an empirically-based assessment of foreshore erosion

Quantification of landscape evolution on multiple time-scales

Essential information about the activity or even the mechanics of tectonic and erosional processes can be extracted from their surface expression. For this purpose, it is necessary to appropriately constrain the temporal as well as the spatial framework, in which to consider a specific process. While recently developed dating techniques, such as thermochronology or radiocarbon dating, allow to assess the age of landforms and therefore rates of tectonic and erosional processes, detailed spatial information is also required to assess these rates correctly. Due to a lack of appropriate topographic data in the past it was sometimes challenging to reliably approximate the spatial framework, because the size of a particular landform can often cover a wide range of spatial scales. Recently available, conventional topographic data, such as those of the Shuttle Radar Topography Mission, substantially improved the definition of an appropriate spatial framework due to their spatial coverage and resolution of down to less than 1 m. However, to constrain this framework at a detail beyond the resolution of several decimeter terrestrial laser scanning provides a highly efficient approach. This technique permits the rapid acquisition (within minutes) of tremendous amounts of topographic data with both, a high resolution of a few centimeters and a high accuracy of a few millimeters. High-resolution topographic maps of a certain area of the surface of the Earth are derived from individual laser-scanner measurements, that in turn allow to characterize the in-situ geomorphic setting at great detail. Moreover, repeated measurements of this area allow to quantify morphological changes thereby supporting the survey of surface processes on short-term scales ranging from days up to several years. The former approach is best suited for tectono- and the latter one for fluvial-geomorphic studies, and we present results from two case studies that are either based on single or repeated laser-scanner measurements. In the first case, we combined field mapping and high-resolution digital elevation model (DEM) analysis to evaluate the detailed meter- to hundred meter-scale structure and surface expression of one flank of the Rex Hills pressure ridge in the western United States. Based on terrestrial laser scanning (Riegl LMS-Z420i) we derived a DEM with cm-scale resolution and extracted high-resolution topographic cross-sections. This enabled us to identify fault scarps and determine their relative ages and geometry. In the second case, we carried out a detailed field mapping of erosion and sedimentation patterns in the Alp Valley, central Switzerland, to assess its Holocene evolution. Simultaneously, we conducted repeated high-resolution (less than 1 cm locally) laser-scanning surveys (Topcon TLS-1000) along two tributaries, the Erlenbach and Vogelbach, to determine channel-morphology changes and the nature of shortest-term sediment transport by comparing the individual DEMs derived from these measurements, as well as to evaluate the context to the longer-term evolution of the Alp Valley. Both case studies, however, highlight the potential of medium-range laser scanners with measurement distances of up to hundreds of meters. Such scanners are most appropriate to efficiently analyze closely-spaced fault scarps across a broad range of spatial scales, and to document complex morphologic changes in small mountainous torrents due to sediment transport. Moreover, terrestrial laser scanning is a key tool to monitor surface processes, but the insights gained from this method are generally evaluated best in the context of further data sets including geochronological, structural, subsurface, or climate data. Surface processes, in particular erosion, sediment transport, and deposition in sedimentary basins are intermittent in space and time challenging both, the appropriate definition of a spatiotemporal framework addressed above and a comprehensive process understanding. A major objective of this thesis is to contribute to a better understanding of scale linkage concerning these processes. We therefore first carried out a comprehensive comparison of short- to long-term erosion measurements from the Alps based on an approach originally established to evaluate the significance of geologic and geodetic measurements along intra-continental faults on time scales of millions to tens of years. In a second step, we re-assessed the sediment budget of the Alps, a data set that is usually considered to be an appropriate measure of long-term erosion in the Alps. The two major results of both studies indicate that: short- and medium-term erosion in the Alps over years to ten thousands of years is dominantly influenced by climate and weather variability, e.g., due to seasonal differences in the amount of precipitation; whereas long-term erosion over millions of years is controlled by tectonic processes

Budgeting rockfall and modeling sedimentary delivery in torrent systems

This thesis is a compilation of projects to study sediment processes recharging debris flow channels. These works, conducted during my stay at the University of Lausanne, focus in the geological and morphological implications of torrent catchments to characterize debris supply, a fundamental element to predict debris flows. Other aspects of sediment dynamics are considered, e.g. the coupling headwaters - torrent, as well as the development of a modeling software that simulates sediment transfer in torrent systems. The sediment activity at Manival, an active torrent system of the northern French Alps, was investigated using terrestrial laser scanning and supplemented with geostructural investigations and a survey of sediment transferred in the main torrent. A full year of sediment flux could be observed, which coincided with two debris flows and several bedload transport events. This study revealed that both debris flows generated in the torrent and were preceded in time by recharge of material from the headwaters. Debris production occurred mostly during winter - early spring time and was caused by large slope failures. Sediment transfers were more puzzling, occurring almost exclusively in early spring subordinated to runoffconditions and in autumn during long rainfall. Intense rainstorms in summer did not affect debris storage that seems to rely on the stability of debris deposits. The morpho-geological implication in debris supply was evaluated using DEM and field surveys. A slope angle-based classification of topography could characterize the mode of debris production and transfer. A slope stability analysis derived from the structures in rock mass could assess susceptibility to failure. The modeled rockfall source areas included more than 97% of the recorded events and the sediment budgets appeared to be correlated to the density of potential slope failure. This work showed that the analysis of process-related terrain morphology and of susceptibility to slope failure document the sediment dynamics to quantitatively assess erosion zones leading to debris flow activity. The development of erosional landforms was evaluated by analyzing their geometry with the orientations of potential rock slope failure and with the direction of the maximum joint frequency. Structure in rock mass, but in particular wedge failure and the dominant discontinuities, appear as a first-order control of erosional mechanisms affecting bedrock- dominated catchment. They represent some weaknesses that are exploited primarily by mass wasting processes and erosion, promoting not only the initiation of rock couloirs and gullies, but also their propagation. Incorporating the geological control in geomorphic processes contributes to better understand the landscape evolution of active catchments. A sediment flux algorithm was implemented in a sediment cascade model that discretizes the torrent catchment in channel reaches and individual process-response systems. Each conceptual element includes in simple manner geomorphological and sediment flux information derived from GIS complemented with field mapping. This tool enables to simulate sediment transfers in channels considering evolving debris supply and conveyance, and helps reducing the uncertainty inherent to sediment budget prediction in torrent systems. Cette thèse est un recueil de projets d'études des processus de recharges sédimentaires des chenaux torrentiels. Ces travaux, réalisés lorsque j'étais employé à l'Université de Lausanne, se concentrent sur les implications géologiques et morphologiques des bassins dans l'apport de sédiments, élément fondamental dans la prédiction de laves torrentielles. D'autres aspects de dynamique sédimentaire ont été abordés, p. ex. le couplage torrent - bassin, ainsi qu'un modèle de simulation du transfert sédimentaire en milieu torrentiel. L'activité sédimentaire du Manival, un système torrentiel actif des Alpes françaises, a été étudiée par relevés au laser scanner terrestre et complétée par une étude géostructurale ainsi qu'un suivi du transfert en sédiments du torrent. Une année de flux sédimentaire a pu être observée, coïncidant avec deux laves torrentielles et plusieurs phénomènes de charriages. Cette étude a révélé que les laves s'étaient générées dans le torrent et étaient précédées par une recharge de débris depuis les versants. La production de débris s'est passée principalement en l'hiver - début du printemps, causée par de grandes ruptures de pentes. Le transfert était plus étrange, se produisant presque exclusivement au début du printemps subordonné aux conditions d'écoulement et en automne lors de longues pluies. Les orages d'été n'affectèrent guère les dépôts, qui semblent dépendre de leur stabilité. Les implications morpho-géologiques dans l'apport sédimentaire ont été évaluées à l'aide de MNT et études de terrain. Une classification de la topographie basée sur la pente a permis de charactériser le mode de production et transfert. Une analyse de stabilité de pente à partir des structures de roches a permis d'estimer la susceptibilité à la rupture. Les zones sources modélisées comprennent plus de 97% des chutes de blocs observées et les bilans sédimentaires sont corrélés à la densité de ruptures potentielles. Ce travail d'analyses des morphologies du terrain et de susceptibilité à la rupture documente la dynamique sédimentaire pour l'estimation quantitative des zones érosives induisant l'activité torrentielle. Le développement des formes d'érosion a été évalué par l'analyse de leur géométrie avec celle des ruptures potentielles et avec la direction de la fréquence maximale des joints. Les structures de roches, mais en particulier les dièdres et les discontinuités dominantes, semblent être très influents dans les mécanismes d'érosion affectant les bassins rocheux. Ils représentent des zones de faiblesse exploitées en priorité par les processus de démantèlement et d'érosion, encourageant l'initiation de ravines et couloirs, mais aussi leur propagation. L'incorporation du control géologique dans les processus de surface contribue à une meilleure compréhension de l'évolution topographique de bassins actifs. Un algorithme de flux sédimentaire a été implémenté dans un modèle en cascade, lequel divise le bassin en biefs et en systèmes individuels répondant aux processus. Chaque unité inclut de façon simple les informations géomorpologiques et celles du flux sédimentaire dérivées à partir de SIG et de cartographie de terrain. Cet outil permet la simulation des transferts de masse dans les chenaux, considérants la variabilité de l'apport et son transport, et aide à réduire l'incertitude liée à la prédiction de bilans sédimentaires torrentiels. Ce travail vise très humblement d'éclairer quelques aspects de la dynamique sédimentaire en milieu torrentiel