Geomorphic impact and assessment of flexible barriers using multi-temporal LiDAR data: The Portainé mountain catchment (Pyrenees)

Multi-temporal digital elevation models (DEMs) obtained from airborne LiDAR surveys are widely used to detect geomorphic changes in time and quantify sediment budgets. However, they have been rarely applied to study the geomorphic impact of engineering structures in mountain settings. In this study, we assessed the influence and behavior of flexible sediment retention barriers in the Portainé catchment (Spanish Pyrenees), using three LiDAR data sets (2009, 2011 and 2016) that covered a 7-year period. Densely forested mountainous areas present some limitations for reliable DEM analysis due to spatial variabilities in data precision, accuracy and point density. A new methodological approach for robust uncertainty analysis along channels, based on changes in cross-sectional elevations, was used to discriminate noise from real geomorphic changes. The obtained results indicated that erosion occurs along most reaches covering a large area, whereas deposition is localized in specific areas such as those upstream of sediment retention barriers and in the debris cone. Despite the presence of 15 flexible sediment retention barriers, the channels presented net degradation during both 2009-2011 and 2011-2016, with 2838 and 147m3 of material exported from the basin, respectively. For the same periods, the barriers retained 33% and 25% of the total deposition (up to 1300m3 per barrier), respectively, but also induced lateral and downstream incision, the latter reaching 703m3 for a single barrier. We detected a horizontal displacement of the net of up to 1.2m in filled barriers, resulting from net flexion. The interference of the natural river evolution by defense measures has resulted in a complex erosion-deposition pattern. The presented methods show high potential for the hydrogeomorphic study of mountain catchments, especially for a high-resolution assessment of flexible barriers or other engineering structures in remote areas

Dendrogeomorphological evidence of flood frequency changes and human activities (Portainé basin, spanish Pyrenees)

The Portainé mountain catchment, containing the Port Ainé ski resort (Lleida, Spanish Pyrenees), displays active erosional and depositional phenomena caused by periodic torrential floods. These events present a potential risk and incur significant economic losses. In ungauged remote catchments (like Portainé), trees might be the only paleohydrological source of information regarding past floods. Thus, we estimated the temporal and spatial distribution of torrential floods by dendrogeomorphological techniques to assess whether human impact (land-use changes and infrastructure works) affected their frequency and magnitude. One-hundred and sixty-six samples from 67 trees belonging to 10 different species were analyzed; past flood events of the last 50 years were identified by dating and relating evidence between them. Moreover, a detailed geomorphological study was performed and the available historical data compiled. Our multi-evidence analysis provides new insight into the occurrence of paleofloods. Changes in flood frequency since 2006, especially from 2008, suggest that the geomorphological equilibrium has been disturbed, coinciding with both major earthworks within the ski resort and intense but not extraordinary rainfall. This conclusion has important implications for land planning and the design of future projects in the mountain watersheds

Debris flow cartography using differential GNSS and Theodolite measurements

The presented results form part of a CHARMA project, which pursues a broad objective of reducing damage caused by uncontrolled mass movements, such as rockfalls, snow avalanches and debris flows

Datación de avenidas torrenciales y flujos de derrubios mediante metodologías dendrogeomorfológicas (barranco de Portainé, Lleida, España).

La dendrogeomorfología resulta una herramienta muy útil en el análisis científico-técnico de las avenidas y flujos de derrubios en torrentes de alta montaña, en los que las fuentes de datos convencionales (caudales o precipitaciones) son escasas o inexistentes y presentan una cierta problemática social. El barranco de Portainé, en cuya cabecera se sitúa la estación de esquí homónima de Port Ainé (Lleida, España), cumple estas características. Fenómenos erosivos y deposicionales asociados, sobre todo, a períodos de lluvias intensas de verano, afectan periódicamente a la carretera de acceso a la estación de esquí, que atraviesa el barranco en varios puntos, con las consiguientes pérdidas económicas y potenciales daños personales y a los servicios. Con este trabajo se pretende estimar, mediante técnicas dendrogeomorfológicas, el número y la distribución temporal de avenidas y flujos torrenciales en el barranco, para conocer si las interacciones antrópicas en la red hidrográfica, asociadas a la apertura y operación de la estación de esquí y los accesos a la misma, pueden haber modificado su frecuencia y magnitud. Se han muestreado principalmente aquellos ejemplares arbóreos que presentaban daños externos manifiestos, originados por el impacto de los materiales arrastrados por las avenidas. Además se ha realizado un detallado estudio geomorfológico y se han recopilado los datos históricos disponibles. Con toda esta información se ha generado una base de datos de eventos pretéritos de avenidas para un periodo de 50 años, en la que se ha determinado un incremento significativo de estos eventos en las últimas décadas

Suitability of the SL index for the semi-automatic identification of mass movements affecting the drainage network

Los índices geomorfológicos, como el índice del gradiente del perfil longitudinal de los ríos (Stream Length- Gradient index, SL) han demostrado ser buenos identificadores de anomalías en los perfiles longitudinales de los ríos. Generalmente, los factores más comunes que producen estas anomalías (knickpoints) son principalmente cambios litológicos, estructuras tectónicas activas y grandes movimientos de ladera. Este estudio se propone detectar zonas de la red de drenaje afectadas por movimientos en masa utilizando el índice SL en las cuencas de La Vall d’Àssua, Romadriu y La Vall de Tor (Pirineos Centrales). La metodología utilizada consiste en calcular el índice SL utilizando la herramienta Chi-Map y, después, se han aplicado las herramientas Análisis de Puntos Calientes (Hotspot and Cluster Analysis, HCA) y Densidad de Kernel (Kernel Density, KD) de ArcGIS® para proporcionar una mejor visualización y localización de los valores anómalos del índice SL. Las anomalías de SL han sido comparadas con el inventario de inestabilidades de ladera existente en la cuenca de La Vall d’Àssua. Los resultados obtenidos de este análisis indican que hay una buena correlación entre anomalías de SL y los movimientos en masa. Además, este índice geomorfológico ha ayudado en la identificación de 13 nuevas inestabilidades de ladera en el conjunto de las tres cuencas analizadas. En conclusión, el estudio realizado ha permitido comprobar la utilidad del índice SL para detectar aquellos puntos de la red de drenaje que están afectados por movimientos en masa.Geomorphological indices, such as the Stream Length-Gradient (SL) index, have been demonstrated to be suitable for detecting anomalies in stream-profiles. These anomalies (knickpoints) are generally related to active tectonics, lithological changes and large mass movements. This study aims to detect drainage areas affected by mass movements using SL index in La Vall d’Àssua, Romadriu and La Vall de Tor basins (Central Pyrenees). The methodology used consists of computing the SL index using the Chi-Map script and then, applying two ArcGIS® tools, which are the Hotspot and Cluster Analysis (HCA) and Kernel Density (KD), to provide a better visualisation and location of the knickpoints. SL anomalies have been compared with the slope failure inventory of La Vall d’Àssua basin, indicating a good correlation between SL anomalies and slope failures. Additionally, this geomorphological index provided the identification of 13 mass movements which had not previously been described. To sum up, this geomorphological parameter is a useful tool for the identification of drainage network areas affected by mass movements.Este trabajo ha sido financiado por los Proyectos CHARMA (CGL2013-40828-R) y PROMONTEC (MINEICO- FEDER Ref.: CGL2017-84720-R)

Four-topic correlation between flood dendrogeomorphological evidence and hydraulic parameters (the Portainé stream, Iberian Peninsula)

Torrential floods are hazardous hydrological phenomena that produce significant economic damage worldwide. Flood reconstruction is still problematic in ungauged mountainous areas due to the lack of systematic data, so indirect techniques are required. This paper presents an integrated palaeoflood study of a Pyrenean stream that combines fluvio-torrential geomorphology, dendrogeomorphology, palaeoflood discharge estimation and flow hydraulics. The use of a total station and airborne LiDAR data allows detailed topography for geomorphological mapping and running a one-dimensional hydraulic model. Based on the height of scars on several damaged trees, we obtained palaeodischarges of 316 m3 s−1 and 314 m3 s−1 for the 2008 and 2010 floods. The hydraulic parameters were related to the geomorphic position of trees, showing a positive relation between most energetic geomorphic elements and both flow depth and velocity values. The most affected trees are located in intermediate energy geomorphic positions. Analysing variation in scar height and flow stage differences, we suggest that most reliable trees for peak discharge estimation correspond to those in areas related with fluvio-torrential processes of intermediate energy. This multidisciplinary palaeohydrological study relates flood hydrodynamics with the damage to trees and their geomorphological characteristics, focusing on the hydraulic parameters of the peak flow (depth, velocity and unit stream power), which has never been performed before. The proposed approach shows strong potential for palaeoflood analysis in ungauged mountain catchments with scarce nonsystematic data

Suitability of geomorphological indices for the automatic identification of landslides affecting the drainage network: Examples from the Pyrenees

Quantitative geomorphic parameters of drainage networks, like the normalized channel steepness index (Ksn) and the Stream Length-Gradient index (SL), have been demonstrated to be suitable for detecting anomalies on stream-profiles. Ksn and SL indices allow to detect knickpoints, which are generally related to active tectonics, lithological changes and large landslides. In some cases, landslides reaching the river bed generate anomalies with long-stretch influence, which have been defined as knickzones. Often, this type of studies have to deal with certain limitations such as the difficulty to detect landslides located in the headwaters, the lack of unequivocal signs of the nature of the anomalies and the confusing use of term “anomaly”. In this work, the comparison of different indexes including Ksn, SL and slope derivative, allowed us to better define the term “anomaly”, by: 1) using negative and positive values; 2) comparing these values with those obtained from “ideal” river equilibrium profiles; 3) differentiating between knickzones and knickpoints, and; 4) analysing the intensity and extent of the anomalies. The proposed methodology consists of computing the Chi, ksn and SL geomorphological indices using a python script and ArcGIS Toolboxes to provide a better localisation of knickpoints and knickzones. Three pilot areas were analysed: La Vall d’Àssua, Romadriu and La Vall de Tor basins (Catalan Axial Pyrenees). The geomorphologic anomalies within these valleys have been compared with the landslide inventory, indicating a well-correlation between anomalies and landslides. Additionally, these geomorphic indices provided the identification of 13 new mass movements, which had not been described before. In this work, the stream-slope derivative and its comparison with that of an ideal profile add new insights for the identification of the landslides near the headwaters of the basins, which were hidden in the Ksn and SL indices analysis. The proposed methodology shows improvements in the identification of drainage network areas affected by landslides. This should benefit detection and characterization of landslides improving the completeness of landslide inventory maps. Therefore, it should has an impact on the improvement of hazard and risk analysis associated with landslides.Peer Reviewe

Datación de avenidas torrenciales y flujos de derrubios mediante metodologías dendrogeomorfológicas (barranco de Portainé, Lleida, España).

La dendrogeomorfología resulta una herramienta muy útil en el análisis científico-técnico de las avenidas y flujos de derrubios en torrentes de alta montaña, en los que las fuentes de datos convencionales (caudales o precipitaciones) son escasas o inexistentes y presentan una cierta problemática social. El barranco de Portainé, en cuya cabecera se sitúa la estación de esquí homónima de Port Ainé (Lleida, España), cumple estas características. Fenómenos erosivos y deposicionales asociados, sobre todo, a períodos de lluvias intensas de verano, afectan periódicamente a la carretera de acceso a la estación de esquí, que atraviesa el barranco en varios puntos, con las consiguientes pérdidas económicas y potenciales daños personales y a los servicios. Con este trabajo se pretende estimar, mediante técnicas dendrogeomorfológicas, el número y la distribución temporal de avenidas y flujos torrenciales en el barranco, para conocer si las interacciones antrópicas en la red hidrográfica, asociadas a la apertura y operación de la estación de esquí y los accesos a la misma, pueden haber modificado su frecuencia y magnitud. Se han muestreado principalmente aquellos ejemplares arbóreos que presentaban daños externos manifiestos, originados por el impacto de los materiales arrastrados por las avenidas. Además se ha realizado un detallado estudio geomorfológico y se han recopilado los datos históricos disponibles. Con toda esta información se ha generado una base de datos de eventos pretéritos de avenidas para un periodo de 50 años, en la que se ha determinado un incremento significativo de estos eventos en las últimas décadas