Search

Subsidence is a highly destructive natural hazard, which can be caused by both natural and anthropogenic causes. Its impacts include a decrease in storage capacity of aquifer systems, the creation of cracks and fissures, damages to buildings and infrastructures, and an increase of the susceptibility to flooding. In this study, Persistent Scattered Interferometry (PSI) has been used to process Synthetic Aperture Radar (SAR) images, for the detection and analysis of ground deformation and subsidence processes in the island of Mallorca. The study database is composed of 120 images captured by the Sentinel 1A and 1B satellites (between May 2016 and December 2019), from which we derived a map of accumulated displacement rates occurred during a 3 years and a half period. The results show important subsidence processes of up to 3 cm per year in large areas of the sedimentary basin of Palma, and of lesser magnitude (between 1 and 2 cm per year) in locations of the Inca basin and in small basins in the Tramuntana Range. A significant relationship has been observed between the thickness of the Quaternary sediment and the observed subsidence rates. The results highlight the high degree of geomorphological dynamism at very short time scales that characterizes Mallorca, and the vulnerability of certain urban areas, such as the city of Palma (400000 inhabitants), and agricultural areas, such as the Central Depression, facing the risk of subsidence and associated damages.

Wall collapses, sometimes accompanied by mass movements, is one of the main degradation processes in long-abandoned agricultural terraces and critically affects the hydrological and geomorphological behaviour of terraced hillslopes (e.g., development of new sediment sources, increasing soil erosion, increasing hydrological connectivity, re-establishment of natural drainage pathways). An efficient monitoring of the spatial and temporal dynamics of wall collapses is fundamental to understand all these processes. However, the identification of terrace wall collapses in the field is a time-consuming task with spatial limitations. In this study, Object Based Image Analysis (OBIA) applied on High-Resolution Topography was used (and evaluated) for detecting wall collapses in terraced hillslopes. The approach, which includes image segmentation and Support Vector Machine classification, was implemented in the Cidacos Valley (Iberian Range, Spain), extensively affected by the abandonment and degradation of agricultural terraces. Point clouds extracted from three different remote data sources (airborne LiDAR, terrestrial LiDAR and airborne photogrammetry) were used to obtain three different digital terrain models (DTM). The low spatial resolution of the DTM derived from airborne LiDAR (1 m pixel size) was not sufficient to detect any terrace wall collapse, which had a median size of 10 m2. The results showed that slope, Sky-View factor, topographic openness and curvature DTM-derivatives produced the best segmentations. Field inventories of wall collapses were used to train the classification algorithm (Support Vector Machine) and validate the results of the OBIA approach. Terrace wall collapses were identified with an Overall Accuracy of 0.80 and a Producer Accuracy of 0.50. The results of the approach and further improvements are discussed. Although promising, the detection of wall collapses in terraced hillslopes using OBIA is challenging, especially when it is compared to the detection of larger scale hillslope geomorphological processes (e.g.: landslides).

An extraordinary convective rainfall event –unforeseen by most numerical weather prediction models– led to a devastating flash flood in the town of Sant Llorenç des Cardassar, eastern Mallorca, on 9th October 2018. Four people died inside the village, while the total death toll was of 13 and economic damages amounted to 91 M€. The observed flooded extension inside the town by the Copernicus Emergency Management Service –based on Sentinel-1 imagery– far exceeded the extension for a 500-year return period flood. This extreme event has been reconstructed by implementing an integrated flood modelling approach over the semi-arid and small-sized Ses Planes basin up to Sant Llorenç (23.4 km²). This procedure is based on three components: (i) generation of high spatial and temporal resolution radar-derived precipitation estimates; (ii) modelling of the hydrologic response based on post-flood peak discharge estimates; and (iii) hydraulic simulation and mapping of the affected areas based on high water marks. Radar-derived rainfall estimates and the simulated flooding extent and water depths highly correlate with observations. The hydraulic simulation has revealed that water reached a depth of 3 m at some points inside Sant Llorenç and that water velocity greatly increased at bridges’ locations close to the town centre. Even if the catastrophic flash flood was not a debris flow, the flood bore eroded enough material to change channel geomorphology. This study also highlights how the concurrence of the very low predictability of this type of extreme convective rainfall events and the very short hydrological response times typical of small Mediterranean catchments still challenges the implementation of early warning systems, which effectively reduce people’s exposure to flash flood risk in the region.

Glaciers in the inner tropics are rapidly retreating due to atmospheric warming. In Colombia, this retreat is accelerated by volcanic activity, and most glaciers are in their last stages of existence. There is general concern about the hydrological implications of receding glaciers, as they constitute important freshwater reservoirs and, after an initial increase in melting flows due to glacier retreat, a decrease in water resources is expected in the long term as glaciers become smaller. In this paper, we perform a comprehensive study of the evolution of a small Colombian glacier, Conejeras (Parque Nacional Natural de los Nevados) that has been monitored since 2006, with a special focus on the hydrological response of the glacierized catchment. The glacier shows great sensitivity to changes in temperature and especially to the evolution of the El Niño–Southern Oscillation (ENSO) phenomenon, with great loss of mass and area during El Niño warm events. Since 2006, it has suffered a 37 % reduction, from 22.45 ha in 2006 to 12 ha in 2017, with an especially abrupt reduction since 2014. During the period of hydrological monitoring (June 2013 to December 2017), streamflow at the outlet of the catchment experienced a noticeable cycle of increasing flows up to mid-2016 and decreasing flows afterwards. The same cycle was observed for other hydrological indicators, including the slope of the rising flow limb and the monthly variability of flows. We observed an evident change in the daily hydrograph, from a predominance of days with a purely melt-driven hydrograph up to mid-2016, to an increase in the frequency of days with flows less influenced by melt after 2016. Such a hydrological cycle is not directly related to fluctuations of temperature or precipitation; therefore, it is reasonable to consider that it is the response of the glacierized catchment to retreat of the glacier. Results confirm the necessity for small-scale studies at a high temporal resolution, in order to understand the hydrological response of glacier-covered catchments to glacier retreat and imminent glacier extinction.