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Constraining depositional phases in braided river deposits can be challenging if only based on sedimentology or geochronology. In this paper, we explored how different data sources can provide complementary information on the depositional history of Pleistocene fluvial deposits. The study was realized on the Zutendaal gravels that outcrop in Northeastern Belgium. In an 8 m high exposure of gravel sheets, we collected bulk samples that were processed for grain size, elemental geochemistry and cosmogenic radionuclides. After dry sieving, we derived the D10, D50, D90, gravel and sand content. From the elemental geochemistry of the bulk samples, we calculated the Ti/Zr and Cr/Zr ratios as proxies for provenance, the Al2O3/SiO2 and Ba/Sr ratios for hydrolysis, and the MnO, Fe2O3 concentrations and the ratio for oxidation. The 10Be concentration-depth profiles are informative to identify phases of landscape stability characterized by depositional hiatuses. To verify the existence of any aberrant variation in 10Be concentration associated to provenance, the in-situ 10Be concentrations were contrasted with provenance and weathering indices. Differences in grain size, elemental geochemistry and in-situ 10Be between sedimentary units were evaluated based on Mann-Whitney tests, while Spearman correlation tests informed us on the co-variation between data sources. The outcrop in the Zutendaal gravels contains three depositional units with significant differences in grain size and chemical weathering extent, and representing distinct 10Be accumulation phases. The absence of correlation between provenance and oxidation proxies indicates that weathering is essentially intraformational or post-depositional. Sedimentary hiatuses were identified based on fining upward sequences and 10Be concentration-depth profiles, whereby local deviation in 10Be concentration at the top of the fining upward sequence provided information on the duration of the hiatus. The case study illustrates how

Pleistocene braided river deposits commonly represent long periods of non-deposition or erosion that are interrupted by rapid and short aggradation phases. When dating these sedimentary sequences with in-situ produced cosmic radionuclides (CRN), simple concentration-depth profiling approaches fall often short as they assume that the alluvial sedimentary sequence has been deposited with a constant and rapid aggradation rate and been exposed to cosmic radiations afterwards. Numerical modelling of the evolution of CRNs in alluvial sequences permits to account for aggradation, non-deposition and erosion phases, and can simulate which scenarios of aggradation and preservation are most likely representing the river dynamics. In this study, such a model was developed and applied to a Middle Pleistocene gravel sheet (Zutendaal gravels) exposed in NE Belgium. The model parameters were optimized to the observed 10Be and 26Al concentrations of 17 sediment samples taken over a depth interval of 7 m. In the sedimentary sequence, (at least) three individual aggradation phases can be distinguished that were interrupted by non-deposition or erosion lasting each ~40 kyr. The age for the onset of aggradation was further constrained to kyr, and further narrows down the anticipated age window [500;1000] Ma of the terrace gravels. This age, within error limits, does not invalidate previous correlations of this gravel sheet with the Cromerian Glacial B, and marine isotope stage (MIS) 16. The deposition of the entire sedimentary sequence likely represents more than one climatic cycle, and demonstrates the importance of accounting for the depositional modes of braided rivers when applying in-situ cosmogenic radionuclide techniques.

The subtropical dry forests of South-America have been impacted by deforestation and degradation over the last two centuries. The forests still provide important ecosystem services but overexploitation of the forested ecosystems may lead to irreversible loss of provision and regulation services. Remote sensing techniques enable us to monitor changes in forest cover at regional scale. Field-based forest inventories remain essential to measure more specific forest attributes and functions. However, the scale at which such field-based forest inventories can be organized is limited for the detection of slow and subtle changes in the forest at regional scale. Therefore, this thesis analyzed the potential of 3D vegetation models based on low-cost stereo-photos taken from unpiloted aerial vehicles to map forest degradation in dry forests. The ecosystem of the Dry Chaco in Argentina, which is considered as one of the major hotspots of deforestation worldwide, was taken as a case study. Therefore, new data on surface and vegetation height were collected during UAV-flights. Flights were realized at 44 locations in contrasting ecological settings of the Dry Chaco. The results of this research show that structure-from-motion algorithms are capable of reconstructing high resolution 3D surface models of the forest cover and this at a much lower cost than LIDAR derived products. Furthermore, this thesis proposes a set of relevant forest attributes that can be derived from these 3D surface models and compares these with indicators from traditional field-based forest inventories. In a next step, the correspondence between the forest structural indicators and degradation states was examined. Finally, the indicators were compared with high resolution laser data from a recently launched spaceborne sensor, which facilitates the assessment of forest degradation patterns at sub-continental scale., (SC - Sciences) -- UCL, 2022

Arising on the island of Crete around 2700 BC, the Minoans are traditionally regarded as the first advanced civilization on the European continent in its modern meaning. The safeguarding of this primordial heritage faces multiple challenges. Besides extrinsic natural and anthropic threats, Minoan remains are also jeopardized by some of their own intrinsic properties. This paper aims to address two of these hazards: first, the preservation state of Minoan sites, leading to their restricted comprehension; secondly, their complex, “labyrinthine”, architecture, further limiting this intelligibility but also challenging the physical access to the remains. The on-going research presented here intends to instrumentalize pathways as a solution to these drawbacks: it seeks to demonstrate that paths can not only be used as mobility vectors to guide and control visitors’ movement but also, when context-aware, as interpretation media to improve on-site experience. Drawing upon phenomenological theories, this paper focusses in particular on the integration of the visitors’ interaction with their surrounding as an innovative approach in the design of such well-informed paths. Based on the outcome of an original experiment conducted among 73 participants on the archaeological site of Malia, this study explores the possibilities of a combined qualitative and quantitative analysis of the visitors’ movement in informing recommendations to increase the visitors’ understanding and orientation abilities on site. The visitors-based approach discussed in this paper is only but one of the three axes to be combined in the general workflow advocated for the formalization of curated visitors’ paths on Minoan archaeological sites.

Knowledge of sediment production mechanisms and their potential controlling factors contributes to our understanding of geomorphological processes. Previous studies highlighted the link between sediment yield and anthropogenic (e.g. agricultural practices) and natural factors (e.g. climate, topography, river runoff, lithology and vegetation cover). The rugged western slopes of the Peruvian Andes shows different climate regimes with arid climates and desertic vegetation in the south and tropical climates and abundant vegetation in the north. In this region, agricultural and water regulation projects are concentrated. However, the number of studies on sediment yield and its controlling factors are limited along the Peruvian Andes. Most studies on the western slopes have focused on the Northern part of Peru and showed a 3 to 60 times increase of sediment yield during El Niño (ENSO) events compared to normal years. The storm events rapidly mobilized sediment that was accumulated in the mountain and piedmont areas during dry normal years. In the central part of Peru where the main reservoirs, irrigation systems, and water supply plants are located, few studies were realized. They concluded that the relation between sediment production and both environmental and anthropogenic controlling factors needs to be further explored. In this study, we identify the spatial patterns of sediment yield along the western slopes of the Peruvian Andes and analyze the main environmental controlling factors. Our study presents data on sediment yield of 20 catchments. The data contains information on suspended sediment load from gauging stations, reservoir sedimentation and water turbidity. We used satellite-based data to derive topographic information (SRTM v.3 DEM by NASA), daily precipitation covering the period 1981 to 2016 (PISCO product by Senamhi), daily discharge covering 1970 to 2020 (GloFAS-ERA5 global river dataset), lithological strength (global lithological map GLiM), and veget

To constrain the timing of the sediment cascade, the inherent stochastic nature of sediment and transport through landsliding requires an integrated approach accounting for different space scales and timescales. In this contribution, we examine the sediment production on hillslopes and evacuation to the river network of one landslide affecting a regional-scale river catchment located in the foothills of the Central Swiss Alps. We quantified sediment fluxes over annual, decadal, and millennial timescales using respectively unmanned aerial vehicle (UAV)–structure-from-motion (SfM) techniques, classic photogrammetry, and in situ produced cosmogenic radionuclides. At the decadal scale, sediment fluxes quantified for the period 1962–1998 are highly variable and are not directly linked to the intensity of sediment redistribution on the hillslope. At the millennial scale, landslide occurrence perturbs the regional positive linear relationship between sediment fluxes and downstream distance as the landslide-affected catchment is characterized by a decrease in sediment fluxes and a strong variability. Importantly, the average decadal sediment flux of the landslide-affected catchment is 2 orders of magnitude higher than millennial sediment fluxes computed over the same spatial extent. The discrepancy between decadal and millennial sediment fluxes, combined to the highly variable annual sediment evacuation from the hillslopes to the channel network suggest that phases of hillslope–channel geomorphic coupling are short and intermittent. During most of the time, the first-order catchments are transport-limited and sediment dynamics in the headwaters are uncoupled from the fluvial systems. In addition, our unique spatio-temporal database of sediment fluxes highlights the transient character of the intense geomorphic activity of the landslide-affected catchment in a regional context. Our data support the fact that episodic supply of sediment from landslides during intermittent pha

Unmanned aerial vehicles (UAVs) are being increasingly used to provide high resolution imagery which, through photogrammetric method Structure-from-Motion (SfM), can be used for high resolution topographic reconstruction. PPK (Post-Processing Kinematic) positioning solution offers the potential of accurate image georeferencing without ground control points (GCPs). By integrating consumer-grade cameras and an onboard RTK/PPK GNSS receiver, we managed to have a low-cost and effective approach involving topographic change-detection. Here, we evaluated the positional accuracy and reproducibility of DSMs generated from PPK-SfM workflow with two camera setups by conducting multiple UAV surveys over a test field. Our results showed that the PPK solution has the same accuracy (mean: ca. 0.01 m, RMSE: ca. 0.03 m) as the more traditional georeferencing method based on ground control points. Furthermore, our results indicated that camera properties (i.e. focal length, resolution, sensor quality) have a large impact on the accuracy: a DSLR camera (Canon EOS), with 0.63 cm px-1 GSD, had 0.011 m planimetric error and 0.012 m altimetric error; while a cheap and light Action camera (GoPro) with 3.11 cm px-1 GSD had 0.019 m planimetric error and 0.024 m altimetric error at the same flight height. The repeatability of DSM construction was assessed by a DoD (DEMs of Difference) analysis. The LoD95% and LoDmin were ca. ±0.08 m and ca. ±0.04 m for the DSLR camera and ca. ±0.16 m and ca. ±0.08 m for the action camera (for a flight altitude of 45 m). The level of detection substantially improved when reducing the UAV flight altitude. We found that tie point density greatly controlled the error of the topography reconstruction for the DSLR camera. By exploiting the relation between error and tie point density, we demonstrated that a spatially explicit DoD threshold, according to the local surface condition, can greatly improve surface change detection. Volume estimation was conducted over

Images captured by unmanned aerial vehicles (UAVs) and processed by structure-from-motion (SfM) photogrammetry are increasingly used in geomorphology to obtain high-resolution topography data. Conventional georeferencing using ground control points (GCPs) provides reliable positioning, but the geometrical accuracy critically depends on the number and spatial layout of the GCPs. This limits the time and cost effectiveness. Direct georeferencing of the UAV images with differential GNSS, such as PPK (post-processing kinematic), may overcome these limitations by providing accurate and directly georeferenced surveys. To investigate the positional accuracy, repeatability and reproducibility of digital surface models (DSMs) generated by a UAV–PPK–SfM workflow, we carried out multiple flight missions with two different camera–UAV systems: a small-form low-cost micro-UAV equipped with a high field of view (FOV) action camera and a professional UAV equipped with a digital single lens reflex (DSLR) camera. Our analysis showed that the PPK solution provides the same accuracy (MAE: ca. 0.02 m, RMSE: ca. 0.03 m) as the GCP method for both UAV systems. Our study demonstrated that a UAV–PPK–SfM workflow can provide consistent, repeatable 4-D data with an accuracy of a few centimeters. However, a few flights showed vertical bias and this could be corrected using one single GCP. We further evaluated different methods to estimate DSM uncertainty and show that this has a large impact on centimeter-level topographical change detection. The DSM reconstruction and surface change detection based on a DSLR and action camera were reproducible: the main difference lies in the level of detail of the surface representations. The PPK–SfM workflow in the context of 4-D Earth surface monitoring should be considered an efficient tool to monitor geomorphic processes accurately and quickly at a very high spatial and temporal resolution.

The movement of sediment from mountainous uplands to continental margins, i.e. the sediment cascade, modifies the terrestrial surface and eventually drives the rate of topographic evolution. Understanding the pattern and rates of geomorphic mechanisms involved in the sediment cascade is of crucial importance to constrain biogeochemical cycles, long-term landscape evolution and potential feedbacks between tectonics, erosion and climate. Sediment cascading systems are still poorly constrained, as sediment budgets and connectivity can be highly variable in space and time due to the stochastic character of geomorphic processes. In an effort to improve our quantitative understanding of the sediment cascade in landslide-prone environments, this doctoral dissertation aims at constraining spatio-temporal sediment dynamics over annual to millennial time scales. We applied state-of-the-art geomorphic techniques to constrain sediment budgets and connectivity for a mountainous catchment located in the foothills of the Central Swiss Alps. At the annual time scale, we monitored topographic changes using 3D topographic reconstructions derived from aerial pictures acquired by a drone, and at the millennial time scale, we quantified catchment-scale denudation rates using in-situ produced cosmogenic radionuclides. By integrating sediment fluxes over different temporal scales, it becomes clear that geomorphic process rates in landslide-prone terrain are highly variable in space and time. Periods of sediment remobilisation by landsliding on the hillslopes do not necessarily coincide with sediment pulses in the river network. Only when hillslopes are geomorphically connected to the river channels, the sediment mobilised by landsliding on the slopes is effectively transferred to the channel network. Therefore, phases of low or high geomorphic activity in upland catchments are not necessarily indicative for the long-term sediment fluxes of mountainous catchments. Their spatio-temporal sed, (SC - Sciences) -- UCL, 2018

Accurately assessing geo-hazards and quantifying landslide risks in mountainous environments are gaining importance in the context of the ongoing global warming. For an in-depth understanding of slope failure mechanisms, accurate monitoring of the mass movement topography at high spatial and temporal resolutions remains essential. The choice of the acquisition framework for high-resolution topographic reconstructions will mainly result from the trade-off between the spatial resolution needed and the extent of the study area. Recent advances in the development of unmanned aerial vehicle (UAV)-based image acquisition combined with the structure-from-motion (SfM) algorithm for three-dimensional (3-D) reconstruction make the UAV-SfM framework a competitive alternative to other high-resolution topographic techniques. In this study, we aim at gaining in-depth knowledge of the Schimbrig earthflow located in the foothills of the Central Swiss Alps by monitoring ground surface displacements at very high spatial and temporal resolution using the efficiency of the UAV-SfM framework. We produced distinct topographic datasets for three acquisition dates between 2013 and 2015 in order to conduct a comprehensive 3-D analysis of the landslide. Therefore, we computed (1) the sediment budget of the hillslope, and (2) the horizontal and (3) the three-dimensional surface displacements. The multitemporal UAV-SfM based topographic reconstructions allowed us to quantify rates of sediment redistribution and surface movements. Our data show that the Schimbrig earthflow is very active, with mean annual horizontal displacement ranging between 6 and 9 m. Combination and careful interpretation of high-resolution topographic analyses reveal the internal mechanisms of the earthflow and its complex rotational structure. In addition to variation in horizontal surface movements through time, we interestingly showed that the configuration of nested rotational units changes through time. Although ther

Tectonic and geomorphic processes drive landscape evolution. In mountainous environments, river incision resets hillslope gradients and drives hillslope adjustment through mass wasting and surface erosion. The mobilisation and transport of sediment within mountain basins can be synthetized as a sediment cascade, where erosion of material, sediment transport, deposition and remobilisation of sediment control sediment discharge. Mass movement processes, i.e. stochastic events, are mobilizing and reorganizing loose sediments on slopes while gravity and water are transporting material into rivers. However, the space and time scale of sediment cascades is still subject to debate, and will be explored in this presentation. In the foothills of the Central Swiss Alps, we selected a watershed affected by an active earthflow in its intermediate part and rockfalls in its upper part. We sampled river sediments in order to quantify CRN-derived long-term erosion rates. Our data show that CRN-derived denudation rates are high in the rivers that are directly connected to mass-wasting phenomena, i.e. first-order rivers, which suggests that the landslide-derived material is rapidly entering the river network. However, in higher-order rivers, denudation rates are systematically lower and similar to denudation rates measured in adjacent watersheds. Our data suggests that there is a clear time lag between sediment mobilization by mass-movement processes and transportation of mobilized sediment into lower reaches of watersheds.

Combination of UAV-based aerial pictures and Structure-from-Motion (SfM) algorithm provides an efficient, low-cost and rapid framework for remote sensing and monitoring of dynamic natural environments. This methodology is particularly suitable for repeated topographic surveys in remote or poorly accessible areas. However, temporal analysis of landform topography requires high accuracy of measurements and reproducibility of the methodology as differencing of digital surface models leads to error propagation. In order to assess the repeatability of the SfM technique, we surveyed a study area characterized by gentle topography with an UAV platform equipped with a standard reflex camera, and varied the focal length of the camera and location of georeferencing targets between flights. Comparison of different SfM-derived topography datasets shows that precision of measurements is in the order of centimetres for identical replications which highlights the excellent performance of the SfM workflow, all parameters being equal. The precision is one order of magnitude higher for 3D topographic reconstructions involving independent sets of ground control points, which results from the fact that the accuracy of the localisation of ground control points strongly propagates into final results. © 2015 Elsevier B.V.

During the last years, exploratory research has shown that UAV-based image acquisition is suitable for environmental remote sensing and monitoring. Image acquisition with cameras mounted on an UAV can be performed at very-high spatial resolution and high temporal frequency in the most dynamic environments. Combined with Structure-from-Motion algorithm, the UAV-SfM framework is capable of providing digital surface models (DSM) which are highly accurate when compared to other very-high resolution topographic datasets and highly reproducible for repeated measurements over the same study area. In this study, we aim at assessing (1) differential movement of the Earth's surface and (2) the sediment budget of a complex earthflow located in the Central Swiss Alps based on three topographic datasets acquired over a period of 2 years. For three time steps, we acquired aerial photographs with a standard reflex camera mounted on a low-cost and lightweight UAV. Image datasets were then processed with the Structure-from-Motion algorithm in order to reconstruct a 3D dense point cloud representing the topography. Georeferencing of outputs has been achieved based on the ground control point (GCP) extraction method, previously surveyed on the field with a RTK GPS. Finally, digital elevation model of differences (DOD) has been computed to assess the topographic changes between the three acquisition dates while surface displacements have been quantified by using image correlation techniques. Our results show that the digital elevation model of topographic differences is able to capture surface deformation at cm-scale resolution. The mean annual displacement of the earthflow is about 3.6 m while the forefront of the landslide has advanced by ca. 30 meters over a period of 18 months. The 4D analysis permits to identify the direction and velocity of Earth movement. Stable topographic ridges condition the direction of the flow with highest downslope movement on steep slopes, and diffuse mo

During the last years, exploratory research has shown that UAV-based image acquisition is suitable for environmental remote sensing and monitoring. Image acquisition with cameras mounted on an UAV can be performed at very-high spatial resolution and high temporal frequency in the most dynamic environments. Combined with Structure-from-Motion algorithm, the UAV-SfM framework is capable of providing digital surface models (DSM) which are highly accurate when compared to other very-high resolution topographic datasets and highly reproducible for repeated measurements over the same study area. In this study, we aim at assessing (1) differential movement of the Earth’s surface and (2) the sediment budget of a complex earthflow located in the Central Swiss Alps based on three topographic datasets acquired over a period of 2 years. For three time steps, we acquired aerial photographs with a standard reflex camera mounted on a low-cost and lightweight UAV. Image datasets were then processed with the Structure-from-Motion algorithm in order to reconstruct a 3D dense point cloud representing the topography. Georeferencing of outputs has been achieved based on the ground control point (GCP) extraction method, previously surveyed on the field with a RTK GPS. Finally, digital elevation model of differences (DOD) has been computed to assess the topographic changes between the three acquisition dates while surface displacements have been quantified by using image correlation techniques and cloud comparison algorithm. Our results show that the digital elevation model of topographic differences is able to capture surface deformation at cm-scale resolution. The mean annual displacement of the earthflow is about 3.6 m while the forefront of the landslide has advanced by ca. 30 meters over a period of 18 months. The 4D analysis permits to identify the direction and velocity of Earth movement. Stable topographic ridges condition the direction of the flow with highest downslope movement

UAV-based aerial pictures processed with Structurefrom- Motion algorithms provide an efficient, low-cost and rapid framework for remote monitoring of dynamic environments. This methodology is particularly suitable for repeated topographic surveys in remote or poorly accessible areas. The aim of the study is to assess differential movement of surface material within a landslide located in the northern foothills of the Swiss Alps based on a time series of digital surface models derived from the UAVSfM framework. Based on two digital surface models reconstructed with aerial pictures taken within a 6-months time interval, first results show that the central part of the landslide is very dynamic with horizontal movements up to several meters.

A representation of the earth surface at very high spatial resolution is crucial to accurately map small geomorphic landforms with high precision. Very high resolution digital surface models (DSM) can then be used to quantify changes in earth surface topography over time, based on differencing of DSMs taken at various moments in time. However, it is compulsory to have both high accuracy for each topographic representation and consistency between measurements over time, as DSM differencing automatically leads to error propagation. This study investigates the reproducibility of reconstructions of earth surface topography based on structure-from-motion (SFM) algorithms. To this end, we equipped an eight-propeller drone with a standard reflex camera. This equipment can easily be deployed in the field, as it is a lightweight, low-cost system in comparison with classic aerial photo surveys and terrestrial or airborne LiDAR scanning. Four sets of aerial photographs were created for one test field. The sets of airphotos differ in focal length, and viewing angles, i.e. nadir view and ground-level view. In addition, the importance of the accuracy of ground control points for the construction of a georeferenced point cloud was assessed using two different GPS devices with horizontal accuracy at resp. the sub-meter and sub-decimeter level. Airphoto datasets were processed with SFM algorithm and the resulting point clouds were georeferenced. Then, the surface representations were compared with each other to assess the reproducibility of the earth surface topography. Finally, consistency between independent datasets is discussed.

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This study investigates the construction of a digital elevation model (DEM) using aerial photographs taken from a drone in the perspective of geomorphic applications. In this context, a representation of the earth surface at very-high resolution is crucial to accurately quantify small geomorphic landforms with high precision. These very high resolution digital elevation models can then theoretically be used to quantify changes in earth surface topography over time, based on differencing of DEMs taken at various time periods. However, high accuracy topographic representation is compulsory in this case as DEM differencing automatically leads to error propagation. As a low-cost alternative, we equipped an eight-propeller drone with a standard reflex camera. This equipment can easily be deployed in the field, as it is a lightweight, low-cost system in comparison with classic aerial photo surveys and terrestrial or airborne LiDAR scanning. Here, we tested to one method to extract a high-resolution DEM on the basis of the aerial photos: aerial photos are first processed into free software which generates 3D point clouds from arbitrary image configuration. The entire generated point cloud is then georeferenced. The accuracy and precision of the earth surface representation is then assessed by comparing it with very high resolution topographic data from airborne LiDAR and very high-resolution GPS measurements.