Showing papers by "Dimitri Lague published in 2020"

Sediment flux-driven channel geometry adjustment of bedrock and mixed gravel–bedrock rivers

Abstract: Sediment supply (Qs) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse-grained, hard, sediment on the geometry of bedrock channels from the Rangitikei river, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l/min) under different Qs (between 0 and 20 g/l) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope, and width-to-depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Qs are characterised by simple in-channel morphologies with a uniform distribution of shear stress within the channel while channels with high Qs are characterised by dynamic channels with multiple active threads and a non-uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Qs is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Qs.

HyLands 1.0: a hybrid landscape evolution model to simulate the impact of landslides and landslide-derived sediment on landscape evolution

Abstract: . Landslides are the main source of sediment in most mountain ranges. Rivers then act as conveyor belts, evacuating landslide-derived sediment. Sediment dynamics are known to influence landscape evolution through interactions among landslide sediment delivery, fluvial transport, and river incision into bedrock. Sediment delivery and its interaction with river incision therefore control the pace of landscape evolution and mediate relationships among tectonics, climate, and erosion. Numerical landscape evolution models (LEMs) are well suited to study the interaction among these earth surface processes. They enable evaluation of a range of hypotheses at varying temporal and spatial scales. While many models have been used to study the dynamic interplay between tectonics, erosion and climate, the role of interactions between landslide-derived sediment and river incision has received much less attention. Here, we present HyLands, a hybrid landscape evolution model integrated within the Topo Toolbox Landscape Evolution Model (TTLEM) framework. The hybrid nature of the model lies in its capacity to simulate both erosion and deposition at any place in the landscape due to fluvial bedrock incision, sediment transport and rapid, stochastic mass wasting through landsliding. Fluvial sediment transport and bedrock incision are calculated using the recently developed Stream Power with Alluvium Conservation and Entrainment (SPACE) model. Therefore, rivers in HyLands can dynamically transition from detachment-limited to transport-limited, and from bedrock to bedrock-alluvial to fully alluviated states. Erosion and sediment production by landsliding is calculated using a Mohr-Coulomb stability analysis while landslide-derived sediment is routed and deposited using a multiple flow direction, non-linear deposition method. We describe and evaluate the HyLands 1.0 model using analytical solutions and observations. We first illustrate the functionality of HyLands to capture river dynamics ranging from detachment-limited to transport-limited configurations. Second, we apply the model to a portion of the Namche-Barwa massif in Eastern Tibet and compare simulated and observed landslide magnitude-frequency and area-volume scaling relationships. Finally, we illustrate the relevance of explicitly simulating landsliding and sediment dynamics over longer timescales for landscape evolution in general and river dynamics in particular. With HyLands we provide a new tool to understand both the long and short-term coupling between stochastic hillslope processes, river incision, and source-to-sink sediment dynamics.

Topo-bathymetric airborne LiDAR for fluvial-geomorphology analysis

Abstract: Topographic airborne lidar using a near-infrared laser cannot penetrate water. A new generation of topo-bathymetric sensors adds a green laser to measure shallow bathymetry. We synthesize previous work and present new results using these sensors in the context of fluvial geomorphology. These sensors allow synoptic continuous topography and bathymetry with a vertical accuracy better than 10 cm and a capacity to resolve details of 20–30 cm. The maximum measurable depth can vary from 1 to 6 m depending on water turbidity, bottom reflectance, and the sensor used. Based on a 55 km survey of the Ain River (France), we illustrate the level of detail recorded in raw data, the full-waveform record, and the challenges in detecting individual bathymetric points that require a refraction correction among billions of points. We show various applications, including synoptic measurement of erosion/sedimentation, back calculation of water optical properties, and large-scale, high-resolution mapping of inundation patterns.

Use of terrestrial photosieving and airborne topographic LiDAR to assess bed grain size in large rivers: a study on the Rhine River

Abstract: 300 mots-max) Most grain size monitoring is still being conducted by manual sampling in the field, which is time consuming and has low spatial representation. Due to new remote sensing methods, some limitations have been partly overcome, but methodological progress is still needed for large rivers as well as in underwater conditions. In this paper, we tested the reliability of two methods along the Old Rhine River (France/Germany) to estimate the grain size distribution (GSD) in above-water conditions: (i) a low-cost terrestrial photosieving method based on an automatic procedure using Digital Grain Size (DGS) software and (ii) an airborne LiDAR topobathymetric survey. We also tested the ability of terrestrial photosieving to estimate the GSD in underwater conditions. Field pebble counts were performed to compare and calibrate both methods. The results showed that the automatic procedure of

Terrestrial laser scanner applied to fluvial geomorphology

Abstract: Terrestrial laser scanner (TLS) offers an unprecedented combination of sub-cm resolution, mm precision and survey extent that uniquely captures the geometry of individual pebbles and allows the spatial variability of channel evolution to be quantified precisely. Data processing can, however, be challenging, and the full scientific potential of fluvial 3D datasets remains arguably untapped. This chapter is an introduction to using TLS to solve fluvial geomorphology problems, synthesizing data acquisition, processing methods, and application examples. It covers practical aspects of field acquisition and addresses the respective benefits of TLS and structure from motion (SfM). Three-dimensional (3D) point cloud processing methods involved in data registration, vegetation classification, and spatial analysis are presented. Processing of repeat surveys and change detection methods are synthesized and the choice of raster-based or 3D point cloud differencing methods discussed in the context of fluvial processes and dynamics.

Beyond 2D inventories : synoptic 3D landslide volume calculation from repeat LiDAR data

Abstract: Efficient and robust landslide mapping and volume estimation is essential to rapidly infer landslide spatial distribution, to quantify the role of triggering events on landscape changes and to assess direct and secondary landslide-related geomorphic hazards. Many efforts have been made during the last decades to develop landslide areal mapping methods, based on 2D satellite or aerial images, and to constrain empirical volume-area (V-A) allowing in turn to offer indirect estimates of landslide volume. Despite these efforts, some major issues remain including the uncertainty of the V-A scaling, landslide amalgamation and the under-detection of reactivated landslides. To address these issues, we propose a new semi-automatic 3D point cloud differencing method to detect geomorphic changes, obtain robust landslide inventories and directly measure the volume and geometric properties of landslides. This method is based on the M3C2 algorithm and was applied to a multi-temporal airborne LiDAR dataset of the Kaikoura region, New Zealand, following the Mw 7.8 earthquake of 14 November 2016. We demonstrate that 3D point cloud differencing offers a greater sensitivity to detect small changes than a classical difference of DEMs (digital elevation models). In a small 5 km2 area, prone to landslide reactivation and amalgamation, where a previous study identified 27 landslides, our method is able to detect 1431 landslide sources and 853 deposits with a total volume of 908,055 ± 215,640 m3 and 1,008,626 ± 172,745 m3, respectively. This high number of landslides is set by the ability of our method to detect subtle changes and therefore small landslides with a carefully constrained lower limit of 20 m2 (90 % with A

Performance of automated flood inundation mapping methods in acontext of flash floods: a comparison of three methods based eitheron the Height Above Nearest Drainage (HAND) concept, or on1D/2D shallow water equations

Abstract: . Flash floods observed in headwater catchments often cause catastrophic material and human damage worldwide. Considering the large number of small watercourses possibly affected, the use of automated methods for flood inundation mapping at a regional scale can be of great help for the identification of threatened areas and the prediction of potential impacts of these floods. An application of three mapping methods of increasing level of complexity (HAND/MS, caRtino 1D, and Floodos 2D) is presented herein. These methods are used to estimate the flooded areas of three major flash floods observed during the last ten years in South-Eastern France: the 15th of June 2010 flood on the Argens river and its tributaries (585 km of river reaches), the 3rd of October 2015 flood on small coastal rivers of the French Riviera (131 km of river reaches) and the 15th of October 2018 floods on the Aude river and its tributaries (561 km of river reaches). The common features of the three mapping approaches are their high level of automation, their application based on a high-resolution (5 m) DTM, and their reasonable computation times. Hydraulic simulations are run in steady-state regime, based on peak discharges estimated using a rainfall-runoff model preliminary adjusted for each event. The simulation results are compared with the reported flood extent maps and the high water level marks. A clear grading of the tested methods is revealed, illustrating some limits of the HAND/MS approach and an overall better performance of hydraulic models solving the shallow water equations. With these methods, the inundated areas are overall well retrieved, and the errors on water levels remain mostly below 80 cm for the 2D Floodos approach. The most important remaining errors are related to limits of the digital elevation model such as the lack of bathymetric information, uncertainties on embankment elevation and to possible bridge blockages not accounted for in the models.

The impact of atmospheric pressure change and rainfall for triggering landslides during weather events

Abstract: Landslides are a complex phenomenon which triggering depends on both intrinsic properties of soils and rocks and external influences such as the action of weather conditions, or earthquakes. Around 6,000 landslides failed the 6 th of September 2018 during the Mw 6.6 Hokkaido Eastern Iburi earthquake (Japan), one day after the typhoon Jebi hit the region. If the ground acceleration induced by the seismic waves likely played a major role in the triggering of these landslides, it is unclear how it compares to the respective role of rainfall and atmospheric pressure drop induced by the typhoon. The aim of this work is therefore to investigate the influence of weather conditions on landslide triggering, and more specifically to characterize the relative contributions of rainfall and atmospheric pressure changes on slope stability.