How quickly does wood fragment in rivers? Methodological challenges, preliminary findings, and perspectives

Large wood plays a significant role in fluvial ecosystems, influencing river geomorphology and ecology. However, it poses both benefits to river systems and risks, making it essential to understand its dynamics for effective management. A better understand the wood breakdown process is required to evaluate the flood risk of wood in rivers. This paper aims to evaluate early-stage fragmentation of wood in rivers after being recruited through bank erosion, taking into account its mobility and residence time. Two methods for characterising and monitoring wood fragmentation are suggested and compared: 1) photo-interpretation based on ground and drone photo and 2) terrestrial LiDAR. We used Quantitative Structure Modelling (QSM) of point clouds to accurately simulate the full branching structure of trees. Close relationships exist between photo-interpreted and LiDAR-derived complexity metrics, but a scaled parameter (i.e., diameter) is needed to correlate branching complexity with volume. The debranching process occurs quickly, with a median reduction in branching complexity of over 80% within the initial two years. Further research with a larger sample size is necessary to investigate the impact of context – including transportation, submersion, accumulation, and isolation of wood pieces – on the fragmentation process. Field observations indicate that immobile wood pieces can experience a rapid reduction in their branching complexity, similar to the ones that are transported. Partial fine branching structure can be maintained on transported pieces. Both photo-interpretation and terrestrial LiDAR offer complimentary approaches to monitoring wood fragmentation. Photo-interpretation is easily implementable and may be used as a proxy for mechanical fragmentation, whilst terrestrial LiDAR may be used to monitor 3D wood fragmentation, volume and length evolution, following QSM modelling

Utilisation de la photogrammétrie pour l’étude de la végétation riveraine

International audienceThis study presents the results of methodological tests in aerial photogrammetry with the objective of a diachronic survey of riparian vegetation in three dimensions, i.e. its spatial distribution and height. The study is undertaken at two complementary scales. The first one corresponds to a 10 km reach of the Allier river floodplain. At this scale, aerial photographs were taken from a small airplane. The second level of analysis corresponds to a wooded point bar located within the 10 km reach. At this scale, the photographs were taken from an unmanned aerial vehicle (UAV). Canopy height models (CHM) are produced at two scales of analysis. The accuracy of the CHMs is then analysed by comparison with vegetation height measurements collected during a filed campaign. The results show significant agreement for the models derived from two approaches, with an error of one meter at the floodplain scale and few centimetres at the bar scale. This variation in error is mainly due to different photographic resolutions. The two approaches appeared to be complementary. The first one is adequate for mapping vegetation structure at the floodplain scale, the second for mapping vegetation in smaller areas but at a higher resolution.Cette étude présente les résultats de tests méthodologiques réalisés en photogrammétrie aérienne avec pour objectif le suivi diachronique de la végétation riveraine dans ses trois dimensions, c’est-à-dire sa distribution spatiale et sa hauteur. L’étude est menée à deux échelles d’analyse. La première échelle est celle d’un tronçon longitudinal de 10 km de la plaine alluviale de la rivière Allier. À cette échelle de travail, un avion a été utilisé pour les prises de vue aérienne. La deuxième échelle d’analyse est celle d’un banc boisé, localisé dans la zone de 10 km. À cette échelle, la couverture photographique a été faite à l’aide d’un drone. Pour les deux objets d’étude des modèles de hauteur du couvert végétal (MHC) sont produits. La précision des MHC est ensuite vérifiée à partir de la mesure de la hauteur d’arbres sur le terrain. Les résultats révèlent une très bonne précision des modèles pour les deux approches, de l’ordre du mètre à l’échelle du tronçon de 10 km et de quelques centimètres à l’échelle du banc. Cette inégalité s’explique surtout par des résolutions photographiques différentes pour les deux missions. Il ressort de ce travail que les deux approches sont complémentaires. La première permet de cartographier la structure de la végétation à l’échelle de la plaine alluviale, la deuxième permet une cartographie de la végétation à plus haute résolution mais seulement à une échelle plus réduite

Une analyse écologique orientée vers des processus d’un système riverain dynamique: la rivière Allier aval (France)

International audienceRiparian ecosystems are highly dynamic ecosystems subjected to hydrogeomorphological processes. Their geomorphological, sedimentological and hydrological heterogeneity makes them one of the richest ecosystems in terms of species diversity. In addition, riparian zones also provide numerous ecosystem functions and services to society. In the European context, the lower river Allier (France) is one of the last remaining rivers with laterally dynamic sections. Its historical evaluation has shown repeated river bed displacements during the last century. Spatio-temporal processes on a highly mobile river section of the lower river Allier have been studied analysing the mosaic of vegetation types and successional phases as well as physical habitat parameters. Understanding riparian ecosystem functioning and evolution in natural or nearly natural systems is essential for river restoration practices in highly degraded rivers. This understanding may lead to the establishment of better sustainable river rehabilitation targets that consider societal needs and natural processes.Les écosystèmes riverains sont des écosystèmes dynamiques soumis aux processus hydrogéomorphologiques avec une forte hétérogénéité géomorphologique, sédimentologique et hydrologique et de nombreux services écosystémiques rendus à la société. Ils font partie des écosystèmes les plus riches en termes de diversité des espèces. Dans le contexte européen, la rivière Allier aval est une des dernières rivières avec des sections à forte mobilité latérale. L'analyse historique a démontré cette forte mobilité latérale des chenaux durant le siècle dernier. Des processus spatio-temporels d'une section de rivière très mobile ont été étudiés à travers l'analyse de la mosaïque des types de végétation, les phases de succession, ainsi que les paramètres physiques des habitats. Comprendre le fonctionnement et l'évolution des écosystèmes riverains dans des systèmes naturels ou proche de l'état naturel est essentiel pour la restauration des cours d'eau très dégradées, afin de permettre la mise en place de stratégies de réhabilitions durables qui considèrent les besoins de la société et les processus naturels

Identification of floodwater source areas in Nepal using SCIMAP‐Flood

Practical approaches for managing flooding from fluvial sources are moving away from mitigation solely at the point of impact and towards integrated catchment management. This considers the source areas, flow pathways of floodwaters and the locations and exposure to the risk of communities. For a field site in southern Nepal, we analyse catchment response to a range of simulated rainfall events, which when evaluated collectively can help guide potential flood management solutions. This is achieved through the adoption of SCIMAP-Flood, a decision support framework that works at the catchment-scale to identify critical source areas for floodwaters. The SCIMAP-Flood Fitted inverse modelling approach has been applied to the East Rapti catchment, Nepal. For multiple flood impact locations throughout the catchment, SCIMAP-Flood effectively identifies locations where flood management measures would have the most positive effects on risk reduction. The results show that the spatial targeting of mitigation measures in areas of irrigated and rainfed agriculture and the prevention of deforestation or removal of shrubland would be the most effective approaches. If these actions were in the upper catchment above Hetauda or upstream of Manahari they would have the most effective reduction in the flood peak

Succession processes of a dynamic riparian ecosystem: the lower Allier River (France)

Riparian ecosystems are highly dynamic and are subject to hydrogeomorphological processes. In Europe, the lower Allier River (France) is one of the last remaining meandering rivers with laterally dynamic sections. For this study, we developed an original method to analyse the vegetation assemblages and succession processes. The analysis includes both spatial and temporal evolution in order to evaluate the shifting paradigm of habitat mosaic at the study site over a 60-year period. On the long-term, floods with low recurrence intervals tend to be important for small scale habitat changes, whereas major flood events lead to major habitat changes. Flood events did not change the overall habitat composition of the active tract but their spatial distribution. The lack of major hydrogeomorphological disturbance observed during the last two decades, however, led to a higher stability of floodplain ecotopes

Réponse du peuplier noir (Populus nigra L.) aux contraintes hydrogéomorphologiques : une expérimentation ex situ semi contrôlée

International audienceBased on the hypothesis of an eco-evolutionary feedback between woody riparian species and fluvial geomorphology, a semi-controlled ex situ experiment has been planned to quantify key response functional traits (morphological and biomechanical) of Populus nigra L. cuttings to simulated hydrogeomorphological constraints, as well as to dissociate the specific responses to them. The constraints tested are sediment burial and drag force exerted by floods. The characteristics of the experiment are presented as well as the experimental design. The hypothesis of a positive niche construction by P. nigra is tested to a certain degree. The results from this research will improve our understanding of riparian ecosystem functioning and specifically of the role of this key woody pioneer species within active floodplain rivers.Basé sur l’hypothèse d’existence d'une rétroaction éco-évolutive entre les espèces végétales riveraines ligneuses et la géomorphologie fluviale, une expérimentation ex situ en conditions semi-contrôlées a été planifiée afin de quantifier les traits de réponses fonctionnels (morphologique et biomécanique) de boutures de Populus nigra L. soumises à des contraintes hydrogéomorphologiques simulées : l’enfouissement sédimentaire, la force de traînée, et la combinaison des deux contraintes. Les questions scientifiques et le protocole expérimental sont présentés ici. L’hypothèse d’une construction de niche positive de P. nigra est testée à un certain degré. Les résultats de cette recherche contribueront à améliorer notre compréhension du fonctionnement des écosystèmes riverains et plus particulièrement du rôle de cette espèce ligneuse pionnière clé au sein de la bande active des cours d’eau

Towards harmonization of image velocimetry techniques for river surface velocity observations

Since the turn of the 21st Century, image based velocimetry techniques have become an increasingly popular approach for determining open-channel flow in a range of hydrological settings across Europe, and beyond. Simultaneously, a range of large-scale image velocimetry algorithms have been developed, equipped with differing image pre-processing, and analytical capabilities. Yet in operational hydrometry, these techniques are utilised by few competent authorities. Therefore, imagery collected for image velocimetry analysis, along with validation data is required both to enable inter-comparisons between these differing approaches and to test their overall efficacy. Through benchmarking exercises, it will be possible to assess which approaches are best suited for a range of fluvial settings, and to focus future software developments. Here we collate, and describe datasets acquired from six countries across Europe and Asia, consisting of videos that have been subjected to a range of pre-processing, and image velocimetry analysis (Perks et al., 2019, https://doi.org/10.4121/uuid:34764be1-31f9-4626-8b11-705b4f66b95a). Validation data is available for 12 of the 13 case studies presented enabling these data to be used for validation and accuracy assessment

Application de la photogrammétrie stéréoscopique pour l’étude de la végétation alluviale : exemple de l’Allier (France).

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Historic reconstruction of reservoir topography using contour line interpolation and structure from motion photogrammetry

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