1,721,026 research outputs found
Application of structure from motion (SfM) algorithms for the historical analysis of changes in fluvial geomorphology
No full textThis work presents the workflow used to obtain spatial information to analyze geomorphological changes in the upper reach of the Cinca River (Southern Pyrenees) during the 1927-2015 period. The final products after application of the SfM-MVS methodology are orthomosaics with a root mean square error (RMSE) of 0.5-1 m, and point clouds with an RMSE of 1-2 m. The results of the study indicate that the study reach has suffered a clear reduction of the active channel width (i.e. 52%) with a high incision processes associated (e.g. >5m in some points), and a simplification in the channel pattern (i.e. reduction of the braiding index and changes from a multi-channel pattern to a single-channel one). These processes are directly influenced by the anthropogenic impacts associated with in-channel gravel mining and the associated impacts related to channel embankment (reach scale, localized impacts), and also influenced by the impacts on the production and transfer of sediment (catchment scale) due to land use changes since the 50s of the 20th century. The methodology presented in this paper is very useful for the diagnosis of the morphosedimentary state of river systems. In the particular case of the Alto Cinca River, the results are of great interest for the improvement of the understanding of the cause-effect relationships in the morphosedimentary dynamics observed for the period 1927-2015. This improvement can help modify basin management plans through a more comprehensive view of contemporary processes
Assessing landscape changes associated to anthropic disturbances by means of the application of Structure from Motion photogrammetry using historical aerial imagery
No full textIn this paper we present a methodological workflow to obtain planimetric and topographic data from historical aerial photos using photogrammetric methods through Structure from Motion (SfM) and Multi-View Stereo (MVS) algorithms. This methodology is applied in two case studies located in the Upper Cinca River catchment (Southern Pyrenees). These sites have suffered a series of direct anthropogenic disturbances that have modified landscape topography. Specifically, topographic changes associated with the construction of a road and the extraction of materials from a pit quarry have been analysed. The resultant products extracted by the application of SfM-MVS are orthomosaics with a root mean square error (RMSE) between 0.5 and 1 m, and points clouds (topography) with a RMSE between 1 and 2 m. The topography before and after each of the impacts was compared with the objective to analyse the changes attributed to these disturbances. A simple minimum Level of Detection (minLoD) was estimated based on the RMSEs in order to distinguish potential real changes from those due to the noise ascribable to the uncertainty of the topographic data sets. The significance of both impacts is relevant in terms of topographic changes (from -20 to +15 m). The dominant topographic change (in extension) in the case of the road construction (51% of the surface) is extraction (i.e. erosion). In the case of the pit quarry, the dominant process is deposition (i.e. sedimentation; 27% of the area). The extension below the minLoD is around the 23% and 48%, respectively, indicating that the magnitude of the changes is substantially higher in the case of the road construction. In both cases the net volume is negative (-913,710 and -16,197 m3 in the case of the road and the quarry, respectively), that show the differences in terms of the extension of the processes and their magnitude in each case study. Finally, both impacts had a direct effect on landscape morphometry (e.g. changes in the slope and flow direction). The developed approach provides an opportunity to analysed and quantify landscape changes that may help to improve our understanding of the long-term evolution of the transfer of water and sediment through landscapes
The effects of land use and topographic changes on sediment connectivity in mountain catchments
Full text linkUnderstanding the evolution of sediment connectivity associated with different land use and topographic changes is a prerequisite for a better understanding of sediment budgets and sediment transport processes. We used the Index of Sediment Connectivity (IC) developed by Cavalli et al. (2013) based on the original approach by Borselli et al. (2008) to study the effects of decadal-scale land use and topographic changes on sediment connectivity in mountain catchments. The input variables of the IC (i.e. land cover and topography) were derived from historical aerial photos using Structure from Motion-Multi View Stereo algorithms (SfM-MVS). The method was applied in different sub-catchments of the Upper River Cinca Catchment (Central Pyrenees), representative of three scenarios: (a) Land cover changes; (b) Topographic changes in agricultural fields (terracing); and (c) Topographic changes associated with infrastructure (road construction). In terms of land cover changes, results show that although connectivity is increased in some areas due to the establishment of new field crops, for most of the study area connectivity decreased due to afforestation caused by rural abandonment. Topographic changes due to the establishment of agricultural terraces affected connectivity to a larger degree than land cover changes. Terracing generally reduced connectivity due to the formation of flat areas in step-slopes, but in certain points, an increase in connectivity caused by the topographic convergence produced by terraces was observed. Finally, topographic changes associated with road construction greatly modified surface flow directions and the drainage network, resulting in changes in connectivity that may affect erosional processes nearby. The methodology used in this paper allows to study the effects of real decadal-scale land use and topographic changes on sediment connectivity and also evaluating and disentangling those changes. Furthermore, this approach can be a useful tool to identify potential risks associated with morphological and land use changes, involving road infrastructures
Geomorphic process signatures reshaping sub‐humid Mediterranean badlands: 2. Application to 5‐year dataset
Full text linkBadland landscapes exhibit high erosion rates and represent the main source of fine sediments in some catchments. Advances in high‐resolution topographic methods allow analysis of topographic changes at high temporal and spatial scales. We apply the Mapping Geomorphic Processes in the Environment (MaGPiE) algorithm to infer the main geomorphic process signatures operating in two sub‐humid badlands with contrasting morphometric attributes located in the Southern Pyrenees. By interrogating a 5‐year dataset of seasonal and annual topographic changes, we examine the variability of geomorphic processes at multiple temporal scales. The magnitude of geomorphic processes is linked to landform attributes and meteorological variables. Morphometric differences between both adjacent badlands allow us to analyse the role of landform attributes in the main geomorphic process reshaping landscapes subjected to the same external forcing (i.e. rainfall and temperature).
The dominant geomorphic process signatures observed in both badlands are different, despite their close proximity and the same rainfall and temperature regimes. Process signatures determining surface lowering in the gently sloping south‐facing badland, characterized by lower connectivity and more vegetation cover, are driven by surface runoff‐based processes, both diffuse (causing sheet washing) and concentrated (determining cutting and filling, rilling and gullying). The steeper, more connected north‐facing slopes of the other badland are reshaped by means of gravitational processes, with mass wasting dominating topographic changes. In terms of processes determining surface raising, both mass wasting and cutting and filling are most frequently observed in both badlands. There is a clear near‐balanced feedback between both surface‐raising and ‐lowering processes that becomes unbalanced at larger temporal scales due to the thresholds overcome, as the volume associated with surface lowering becomes higher than that associated with raising‐based processes. Rainfall variables control surface flow processes, while those variables associated with low temperature have a significant relation with mass movement‐based processes and other localized processes such as regolith cohesion loss. Finally, our results point out that morphometry (slope and connectivity) together with vegetation cover are key factors determining geomorphic processes and associated topographic changes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
From SfM photogrammetry to DoDs: a methodological workflow to monitor topographic changes in a debris-flow catchment
No full textGeomorphic effectiveness of check dams in a debris-flow catchment using multi-temporal topographic surveys
No full textTorrent control works, such as grade control dams and sediment retention dams, are structural measures for controlling debris flows and debris floods. In spite of the widespread presence of such hydraulic structures in steep mountain streams worldwide, there are still few studies monitoring the effects of check dams on sediment dynamics. The use of repeated topographic surveys allows us to characterize debris-flow activity at multiple temporal and spatial scales and its interaction with torrent control works. Structure from Motion (SfM) technique paired with Multi-View Stereo (MVS) algorithms represents a low-cost opportunity to conduct such multi-temporal surveys. This enables us to better study the effects of individual debris flows, track geomorphic changes and evaluate the effectiveness of torrent control works (e.g. check dams). The effect of check dams on sediment dynamics is investigated by means of multi-temporal topographic surveys before and after debris-flow events in the Moscardo torrent (eastern Italian Alps) where two check dams have recently been built. Multi-temporal SfM based on images taken from the ground in combination with imagery taken by an Unmanned Aerial Vehicle (UAV) is used to obtain DEMs and to study topographic changes through the comparison of repeat DEMs (DEM of Difference, i.e. DoD). The results show that the new check dams considerably modified sediment dynamics in the studied channel but their performance cannot be considered satisfactory. The sediment flowed around the upstream check dam on the right wing, while deep erosion observed downstream of the check dam threatened to undermine the foundation's stability. Moreover, debris-flow lobes deposited upstream of the check dams could act as sources of sediment further increasing downstream debris-flow volume. The analysis proposed in this work could help improve design strategies and permitting the identification of a proper site for check dam building
Recent geomorphological evolution of a natural river channel in a Mediterranean Chilean basin
No full textIn this paper we analyse the recent geomorphological evolution of the River Ñuble, an unmodified wandering river in the Mediterranean region of Chile, highlighting the relation between the observed hydroclimatic trends in the region and the morphology of the river. The work documents the recent changes (2003–2016) in channel morphology (i.e., narrowing, simplification, vegetation encroachment) and, in particular, how the river has progressively reduced its geomorphic activity during the last decade. Changes have been detected using aerial imagery and quantified by means of a series of channel form indices that allow tracking the geomorphological evolution of an 8-km river segment. The catchment has experienced a clear decrease in the frequency and magnitude of flood events, although this fact is not fully supported by a generalized reduction in the analysed rainfall series. We relate the observed river relaxation and the associated channel changes with the reported period of low hydroclimatic activity, which follows a humid period that occurred in the Pacific region during the 1970s and 1980s, connected to the PDO (Pacific Decadal Oscillations) that particularly affected the northern and the central parts of Chile. Altogether, this resulted in generalized river stability and a simplification of the drainage pattern of the channel. In absence of river training practices and other major changes in the catchment, the reduction of formative discharges appears thus to be the main control of the recent evolution of this gravel-bed river
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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