3D terrestrial lidar data classification of complex natural scenes using a multi-scale dimensionality criterion: applications in geomorphology

International audience3D point clouds of natural environments relevant to problems in geomorphology (rivers, coastal environments, cliffs, ...) often require classification of the data into elementary relevant classes. A typical example is the separation of riparian vegetation from ground in fluvial environments, the distinction between fresh surfaces and rockfall in cliff environments, or more generally the classification of surfaces according to their morphology (e.g. the presence of bedforms or by grain size). Natural surfaces are heterogeneous and their distinctive properties are seldom defined at a unique scale, prompting the use of multi-scale criteria to achieve a high degree of classification success. We have thus defined a multi-scale measure of the point cloud dimensionality around each point. The dimensionality characterizes the local 3D organization of the point cloud within spheres centered on the measured points and varies from being 1D (points set along a line), 2D (points forming a plane) to the full 3D volume. By varying the diameter of the sphere, we can thus monitor how the local cloud geometry behaves across scales. We present the technique and illustrate its efficiency in separating riparian vegetation from ground and classifying a mountain stream as vegetation, rock, gravel or water surface. In these two cases, separating the vegetation from ground or other classes achieve accuracy larger than 98%. Comparison with a single scale approach shows the superiority of the multi-scale analysis in enhancing class separability and spatial resolution of the classification. Scenes between 10 and one hundred million points can be classified on a common laptop in a reasonable time. The technique is robust to missing data, shadow zones and changes in point density within the scene. The classification is fast and accurate and can account for some degree of intra-class morphological variability such as different vegetation types. A probabilistic confidence in the classification result is given at each point, allowing the user to remove the points for which the classification is uncertain. The process can be both fully automated (minimal user input once, all scenes treated in large computation batches), but also fully customized by the user including a graphical definition of the classifiers if so desired. Working classifiers can be exchanged between users independently of the instrument used to acquire the data avoiding the need to go through full training of the classifier. Although developed for fully 3D data, the method can be readily applied to 2.5D airborne lidar data

Modèle minimal pour les dunes transverses.

National audienceNous présentons ici un modèle minimal pour décrire la stabilité des dunes transverses. Ce modèle qui s'inspire de celui proposé par NIIYA et al. (2010) est basé sur une représentation simplifiée d'une dune transverse. Celle-ci est décrite par une succession de sections longitudinales qui interagissent entre elles par l'intermédiaire de flux sédimentaires latéraux. Les coupes longitudinales sont définies à partir de caractéristiques géométriques simples. La position et la hauteur de leur sommet suffisent ainsi à les caractériser entièrement. Un bilan des flux sédimentaires dans la direction longitudinale et latérale permet d'obtenir un système de deux équations couplées décrivant l'évolution spatio-temporelle de la position et de la hauteur de la ligne de crête. Ce modèle nous permet de conduire une analyse de stabilité linéaire d'une dune transverse rectiligne et d'identifier les processus physiques susceptibles de la déstabiliser au profit d'une dune sinueuse ou de la fragmenter en une multitude de dunes plus petites. Nous avons identifié deux paramètres importants dans le processus de stabilité : le taux de capture de sédiment par la face d'avalanche et les coefficients de diffusion transverses. Ce modèle simple constitue un outil intéressant pour étudier la dynamique complexe de dunes 3D

Fluvial incision into bedrock: Insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys (Western Alps, France)

International audienceBedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief. Using digital elevation models, topographic maps, and field reconnaissance, we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the Romanche watershed, French Western Alps. Longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches. We reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat. From morphometric analyses, we find that mean channel gradients and widths, as well as knickpoint retreat rates, display a drainage area dependence modulated by bedrock lithology. However, there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints. Assuming a postglacial origin of these gorges, our results imply high postglacial fluvial incision (0.5-15 mm yr−1) and knickpoint retreat (1-200 mm yr−1) rates that are, however, consistent with previous estimates. Numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges. Results from simple end‐member models suggest transport‐limited behavior of the bedrock gorges. A more sophisticated model including dynamic width adjustment and sediment‐dependent incision rates predicts present‐day channel geometry only if a significant supply of sediment from the gorge sidewalls (∼10 mm yr−1) is triggered by gorge deepening, combined with pronounced inhibition of bedrock incision by sediment transport and deposition

Extreme flood-driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro-acoustic techniques: Direct measurement of flood-driven erosion using MLS and MBES

Copyright © 2016 John Wiley & Sons, Ltd. This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone-driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone-affected wet season. Copyright © 2016 John Wiley & Sons, Ltd

Seaward expansion of salt marshes maintains morphological self-similarity of tidal channel networks

International audienceTidal channel networks (TCNs) dissect ecologically and economically valuable salt marsh ecosystems. These networks evolve in response to complex interactions between hydrological, sedimentological, and ecological processes that act in tidal landscapes. Thus, improving current knowledge of the evolution of salt-marsh TCNs is critical to providing a better understanding of bio-morphodynamic processes in coastal environments. Existing studies of coastal TCNs have typically focussed on marshes with either laterally stable or eroding edges, and suggested that TCN morphology evolves primarily through the progressive landward erosion of channel tips, that is, via channel headward growth. In this study, we analyze for the first time the morphological evolution of TCNs found within salt marshes that are characterized by active lateral expansion along their seaward edges and anthropogenically-fixed landward boundaries. We use remote-sensing and numerical-modeling analyses to show that marsh seaward expansion effectively limits headward channel growth and prompts the evolution of TCNs that maintain self-similar morphological structures. In particular, we demonstrate that the overall TCN length increases proportionally to the rate at which marshes expand laterally and that these morphological changes do not significantly alter the drainage properties of the coupled marsh-TCN system. Such behavior is not observed in marshes that are not expanding laterally. Our results allow for elucidating the mechanisms of TCN formation and evolution in tidal wetlands, and are therefore critical to improving our current understanding of coastal-landscape ecomorphodynamics, as well as to developing sustainable strategies for the conservation and restoration of these environments

Analyzing high resolution topography for advancing the understanding of mass and energy transfer through landscapes: A review

International audienceThe study of mass and energy transfer across landscapes has recently evolved to comprehensive considerations acknowledging the role of biota and humans as geomorphic agents, as well as the importance of small-scale landscape features. A contributing and supporting factor to this evolution is the emergence over the last two decades of technologies able to acquire high resolution topography (HRT) (meter and sub-meter resolution) data. Landscape features can now be captured at an appropriately fine spatial resolution at which surface processes operate; this has revolutionized the way we study Earth-surface processes. The wealth of information contained in HRT also presents considerable challenges. For example, selection of the most appropriate type of HRT data for a given application is not trivial. No definitive approach exists for identifying and filtering erroneous or unwanted data, yet inappropriate filtering can create artifacts or eliminate/distort critical features. Estimates of errors and uncertainty are often poorly defined and typically fail to represent the spatial heterogeneity of the dataset, which may introduce bias or error for many analyses. For ease of use, gridded products are typically preferred rather than the more information-rich point cloud representations. Thus many users take advantage of only a fraction of the available data, which has furthermore been subjected to a series of operations often not known or investigated by the user. Lastly, standard HRT analysis work-flows are yet to be established for many popular HRT operations, which has contributed to the limited use of point cloud data.In this review, we identify key research questions relevant to the Earth-surface processes community within the theme of mass and energy transfer across landscapes and offer guidance on how to identify the most appropriate topographic data type for the analysis of interest. We describe the operations commonly performed from raw data to raster products and we identify key considerations and suggest appropriate work-flows for each, pointing to useful resources and available tools. Future research directions should stimulate further development of tools that take advantage of the wealth of information contained in the HRT data and address the present and upcoming research needs such as the ability to filter out unwanted data, compute spatially variable estimates of uncertainty and perform multi-scale analyses. While we focus primarily on HRT applications for mass and energy transfer, we envision this review to be relevant beyond the Earth-surface processes community for a much broader range of applications involving the analysis of HRT

Modelling long-term mountain river dynamics: is the devil in the details ?

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Dynamique de l'érosion continentale aux grandes échelles de temps et d'espace : modélisation expérimentale, numérique et théorique

Thèse publiée dans la collection des Mémoires du CAREN (ISSN 1761-2810) : Mémoire n° 1 (ISBN 2-914375-04-2) La thèse a obtenu le prix 2003 du Comité National Français de Géologie (CNFG)ForthcomingL'évolution à long terme des reliefs continentaux résulte de couplages complexes entre processus d'érosion et mouvements tectoniques qui tendent à s'équilibrer pour atteindre éventuellement un état stationnaire. La partie érosion de ces couplages est gouvernée par l'interaction entre des processus élémentaires de versant et de rivière, étroitement couplés, qui contrôlent l'organisation spatiale du réseau de drainage. Ces processus sont caractérisés par des vitesses d'érosion et de transport lentes sur les versants et très rapides dans les rivières, et dépendent souvent non-linéairement de la pente topographique et du flux d'eau. Il en résulte une dynamique complexe à l'échelle locale dont les conséquences à l'échelle continentale sur la vitesse et les volumes de matière transportés ne sont pas encore comprises. L'objectif de cette thèse a été de comprendre et modéliser cette dynamique à l'échelle des temps géologiques, dans le cadre de perturbations tectoniques simples, et pour des conditions climatiques constantes. Nous utilisons un modèle numérique permettant de simuler l'action de différents processus d'érosion de versant et de transport fluvial. Les solutions hors équilibre prédites par ce modèle ne pouvant être validées analytiquement ou à partir de données naturelles, une approche expérimentale inédite a été développée. Elle permet d'étudier précisément l'évolution d'une topographie soumise à l'action simultanée d'une surrection simple et de processus d'érosion et de transport par ruissellement. Au cours d'une expérience, la surface s'auto-organise d'abord en une série de bassins versants de géométrie similaire aux systèmes naturels, puis l'altitude moyenne du système s'approche quasi-exponentiellement d'une valeur constante, traduisant un état d'équilibre macroscopique. L'analyse couplée des expériences et des simulations numériques montre que cette dynamique dépend fortement de la présence de zones de drainages internes non connectées aux conditions limites du système, du degré de non-linéarité entre érosion et pente topographique et de l'existence d'un seuil d'érosion. A partir de l'analyse de systèmes naturels, via le formalisme de la relation pente-aire drainée, nous mettons en évidence l'existence d'un tel seuil dans les systèmes naturels. Pour ce faire nous avons développé un nouveau critère morphologique permettant de caractériser l'état dynamique des reliefs naturels. L'hypothèse d'équilibre dynamique entre érosion et tectonique, nécessaire à l'interprétation correcte des relations entre formes topographiques et processus d'érosion, peut être ainsi (in)validée. Ces résultats suggèrent que le seuil d'érosion habituellement négligé dans le calibrage des lois d'érosion élémentaires doit être explicitement pris en compte pour prédire correctement les formes topographiques naturelles, la dynamique de la croissance des réseaux de drainage et les relations à l'échelle continentale entre taux de dénudation et paramètres topographiques

What's the Point of a Raster ? Advantages of 3D Point Cloud Processing over Raster Based Methods for Accurate Geomorphic Analysis of High Resolution Topography

International audienceHigh Resolution Topographic (HRT) datasets are predominantly stored and analyzed as 2D raster grids of elevations (i.e., Digital Elevation Models). Raster grid processing is common in GIS software and benefits from a large library of fast algorithms dedicated to geometrical analysis, drainage network computation and topographic change measurement. Yet, all instruments or methods currently generating HRT datasets (e.g., ALS, TLS, SFM, stereo satellite imagery) output natively 3D unstructured point clouds that are (i) non-regularly sampled, (ii) incomplete (e.g., submerged parts of river channels are rarely measured), and (iii) include 3D elements (e.g., vegetation, vertical features such as river banks or cliffs) that cannot be accurately described in a DEM. Interpolating the raw point cloud onto a 2D grid generally results in a loss of position accuracy, spatial resolution and in more or less controlled interpolation. Here I demonstrate how studying earth surface topography and processes directly on native 3D point cloud datasets offers several advantages over raster based methods: point cloud methods preserve the accuracy of the original data, can better handle the evaluation of uncertainty associated to topographic change measurements and are more suitable to study vegetation characteristics and steep features of the landscape. In this presentation, I will illustrate and compare Point Cloud based and Raster based workflows with various examples involving ALS, TLS and SFM for the analysis of bank erosion processes in bedrock and alluvial rivers, rockfall statistics (including rockfall volume estimate directly from point clouds) and the interaction of vegetation/hydraulics and sedimentation in salt marshes. These workflows use 2 recently published algorithms for point cloud classification (CANUPO) and point cloud comparison (M3C2) now implemented in the open source software CloudCompare

Un peu de physique sale pour des lois d'incision fluviatiles plus propres

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