Showing papers by "Dimitri Lague published in 2018"

River self-organisation inhibits discharge control on waterfall migration

Abstract: The action of rivers within valleys is fundamentally important in controlling landscape morphology, and how it responds to tectonic or climate change. The response of landscapes to external forcing usually results in sequential changes to river long profiles and the upstream migration of waterfalls. Currently, models of this response assume a relationship between waterfall retreat rate and drainage area at the location of the waterfall. Using an experimental study, we show that this assumption has limited application. Due to a self-regulatory response of channel geometry to higher discharge through increasing channel width, the bed shear stress at the lip of the experimental waterfall remains almost constant, so there was no observed change in the upstream retreat rate despite an order of magnitude increase in discharge. Crucially, however, the strength of the bedrock material exhibits a clear control on the magnitude of the mean retreat rate, highlighting the importance of lithology in setting the rate at which landscapes respond to external forcing. As a result existing numerical models of landscape evolution that simulate the retreat of waterfalls as a function of drainage area with a fixed erodibility constant should be re-evaluated to consider spatial heterogeneity in erodibility and channel self-organisation.

Supercritical river terraces generated by hydraulic and geomorphic interactions

Abstract: The alternating cycle of valley widening through lateral erosion ("strath planation") and valley narrowing through vertical incision into bedrock ("strath terrace abandonment") due to variations in sediment supply (Qs) relative to river transport capacity (Qsc) is a common feature in many mountainous environments, yet our understanding of the mechanics of the processes that drive this landscape change remains poorly quantified. Here, we use an experimental and numerical study to identify the geomorphic and hydraulic controls driving the response of mixed bedrock-alluvial rivers to variable sediment supply, water discharge, and tectonic tilting. The experimental channels exhibit a multistage response of channel narrowing, accompanied by stripping of the alluvial cover in a downstream-migrating incision wave, followed by destabilization of the bed and development of a single vertical step in the bed profile ("knickpoint") when the hydraulic conditions are supercritical. In our experiments, headward erosion by knickpoints is the most efficient process of strath terrace abandonment, contributing the majority of the total vertical incision in a short period of time. We show experimentally that knickpoint development under supercritical flow conditions drives the rapid response of fluvial systems to upstream perturbations in Qs/Qsc despite no base-level fall. This has implications for the understanding of distributions of strath terrace ages, the inference of base-level variations from knickpoint propagation, and how landscapes respond to climatic or tectonic perturbations.

Full-Waveform LiDAR Pixel Analysis for Low-Growing Vegetation Mapping of Coastal Foredunes in Western France

Abstract: The monitoring of coastal sand dunes requires regular high-resolution aerial photography along hundreds of kilometers of coastal strips. Light detection and ranging (LiDAR) is now the most widely used method for detailed topographic and vegetation studies. The aim of this work is to show how the full-waveform shapes returned from single or multiple targets can carry information relating to low-vegetation cover and ground roughness of dunes. This work focuses on marram grass, widely involved in the development of mobile dunes. Low-growing plants often exhibit identical pigmentary composition and can only be distinguished by the height of their foliage, which modifies the shape of the LiDAR waveform around the main returns at the top of the foliage. We show that ray tracing of full LiDAR waveforms on the regular grid of pixels of hyperspectral images, acquired synchronously, can resolve the confusion between low-vegetation gradients and bare sand by analyzing the waveform damping induced by cumulating microdiffusion on foliage height, but also with glint effects on the surface roughness of compact materials. Analysis of successive shorelines of wet to dry sand, sand to pioneer couch grass, and couch grass to consolidating marram grass can thereby be conducted routinely.

Classification de données LiDAR bi-spectral topo-bathymétriques par une approche multi-échelle : Application en milieu fluvial

Abstract: Le suivi d'un espace naturel/semi-naturel requiert souvent d'identifier automatiquement les differents objets presents tels que la vegetation, le sol, l'eau, les bâtiments, etc. Dans le contexte fluvial, la detection des classes bathymetriques (surface et fond de l'eau principalement) est essentielle pour de nombreuses applications, comme la gestion des rivieres ou cours d'eaux par exemple. Afin de les obtenir, le LiDAR topo-bathymetrique est un outil interessant car il permet de construire deux nuages de points 3D a une tres grande resolution spatiale des scenes scannees a partir de deux longueurs d'onde specifiques : 1064 nm et 532 nm. L'aspect topographique, porte par la longueur d'onde a 1064 nm, permet de recuperer la structure spatiale des differents objets. Le cote bathymetrique provient de la longueur d'onde 532 nm penetrant dans l'eau et permettant parfois d'acceder jusqu'au fond de riviere. Ainsi, classer les differents objets topo-bathymetriques a partir de ces deux nuages de points est un enjeu essentiel. Dans cet article, nous etendons l'approche multi-echelle permettant de caracteriser les structures spatiales, qui a deja demontre ses performances pour l'analyse de nuages de points, avec de nouveaux descripteurs prenant notamment en consideration l'aspect bi-spectral. La classification est realisee au moyen d'une technique de forets aleatoires qui offre la possibilite d'analyser les descripteurs les plus informatifs dans la classification, et donc de mieux comprendre l'impact de la donnee bi-spectrale.

Modeling river erosion and knickpoint propagation using a lagrangian description: numerical benefits and implications for river profiles

Abstract: Landscape evolution models generally solve for the time variation of elevation due to erosional processes on a fixed grid. This Eulerian description offers a tractable approach, despite leading to unnecessary dependency to slope, and in turn to numerical diffusion, when solving for the specific stream power incision model, e.g. with a slope exponent equal to 1. Indeed, under this condition, the stream power model reduces to a linear kinematic wave equation with a wave celerity independent of the local slope (Whitham, 1974; Whipple & Tucker, 1999). Using a Lagrangian description and solving for the horizontal advection of the topography due to erosion enables to develop a simple finite difference numerical scheme fully independent of local slope. This scheme presents numerous benefits over Eulerian descriptions, including the absence of numerical diffusion, unconditional stability and the possibility to track geomorphological features and slope instabilities such as knickpoints. We exploit the latter to track co-seismic knickpoints in simulations where uplift history is computed from a statistical model of earthquakes (Turcotte et al., 2007; Leonard 2010). This model results in spatial and temporal distributions of earthquakes that are consistent with Gutenberg-Richter frequency-magnitude scaling, a modified Bath’s law, Omori’s law in space and time (Turcotte et al., 2007) and earthquake dimension scaling laws (Leonard 2010). Motivated by recent results from sandbox experiments (Baynes, 2016), we then introduce a dependency of knickpoint celerity to knickpoint height, that is initially here equals to co-seismic displacement. This model authorizes the merging of successive and colliding knickpoints due to different celerities. We use this model to investigate the role of knickpoint height and temporal distribution and of the dependency of their celerity to height on the shape of river profiles. This model leads to river profiles with concavities deviating from predicted ones and to particular morphologies, including river profile characterized by a succession of knickzones separated by flat zones. We then discuss these implications in the light of natural river profiles.