The geomorphological and sedimentological signature of extreme storm events in the southernmost Atacama Desert :
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Universidad Católica del Norte,
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The impact of extreme storm events in arid zones has enormous potential to study and understand natural processes of erosion and sedimentation. The high preservation and quality of geomorphic outcrops, together with the lack or extremely scarce modification of storm-related geomorphological features facilitates the descriptions. In that sense, several authors have studied arid landscape modification fromstor my events since the early twentieth century. Nevertheless, the lack of data allowing one to establish, test and validate spatial erosion models associated with individual rainfall events requires data of spatial variations of erosion and sedimentation over wide areas affected by one event that are still lacking. Consequently, predicting where erosion or sedimentation will occur during a storm remains highly uncertain. On other hand, to minimize flash-flood and debris flows hazards triggered by individual storm events, the impacted areas of geomorphic change and the sedimentology need to be understood at catchment scale under increasing human population in the fluvial valleys of the Atacama Desert.In this thesis, (1) I understand how extreme storm events impact the landscapes at the southernmost Atacama Desert at a catchment scale (<~100km2), to (2) uncover the regional influence of extreme storm events on facilitating the (dis)connectivity of these arid fluvial systems and to (3) uncover the role of extreme storms on the long-term geomorphological evolution of arid fluvial systems. To do this, the erosion zones after a storm event (March 2015) in the Atacama Desert are mapped and the processes that form alluvial fans have been studied for the debris flows deposits of the March 2015 event and compared to the Holocene Stratigraphy of the fluvial valley. These scopes are structured in this thesis following the source to sink sediment transference approach of fluvial systems. To do this, I first study the erosion processes and its conditioning factors within the tributary catchments to lately present a comprehensive interpretation of tenacees of the March 2015 storm and of the role of episodic storms in the Holocene evolution of fluvial systems situated at the southernmost Atacama Desert. A remote sensing technique with radar satellite imagery has been developed to identify temporal changes related with erosion or deposition processes after storm at catchment scale. The identification has been based on the use of the coherence change detection (CCD) derived from calculated interferograms onInSAR (chapter 3). With this approach a quantitative analysis of this derived values is not straightforward for the following reasons. First, it is not possible to interpret the origin of the process which lead to a decrease of the coherence value. Second, even if the geomorphic process is known on a given area, it is not possible to use this value as a continuous scale giving a reliable representation of the impact of erosion orde position. As we clearly observed without ambiguity in the field that a coherence decrease value lower than-0.3 is always associated with a strong ground change related to erosion or deposition processes. Here I use this threshold for the interpretation of the coherence loss map as a signature of erosion and deposition. A great modification by erosion and/or deposition of alíviate channels suggests that sediment from the relatively low slopes of catchments have a significative contribution of sediment entrainment after storm whilst this behaviour is generally not observed for steep hillslopes (>30º). The change in surface characteristics directly relates to the thickness of the regolith and/or sediment cover results in greater erosion zones after the March 2015 event. The results show that the soil disturbance pattern during an individual rainfall event can contradict the landscape models predicting higher erosion on steeper hillslopes. Complementary, when analysing the factors that contribute in debris flow generation, the sediment storage, which depends in topographical attributes, dictates whether the catchments are eroded or not during extreme storm events (chapter 4 and 5). The sediment is stored during the inter-storm periods preferably in larger catchments with low-relief surfaces controlled by both the channel development and the preservation of perched low-relief surfaces on the headwaters of these arid catchments. Therefore, sediments are susceptible to be transferred by debris-flows by any extreme storm that impacts the area because there is abundant sediment supply. The alluviated channels are the main entrainment sediment zones during extreme storms whilst debris-supply from landslides on steep hillslope are not required for the debris flow generation during extreme storm events. The high altitude of the zero-isotherm and the heterogeneous cellstorms distribution instantaneously increase run-off during storms and thus the entrainment of sediment impacts larger areas where stored sediments and regolith are present. Future studies of debris flow susceptibility assessment in arid catchments of the Atacama Desert can benefit by incorporating such topographic predictors in the classification of catchment-clustering proposed in chapter 4.The alluvial fan formation in these arid fluvial systems strongly depends on sediment supply transferred from tributary catchments. Here, I present the first empirical example of a typical sequence of aggradation for an El Niño rainstorm event described in the Andes (chapter 5). The dynamics of the evolving sedimentary flow rheologies (five facies characterised) through the rainfall event, provides important insights into the resulting sedimentary signature of such rainstorm events that can be used to aid interpretation of the paleo-record and landslide hazard assessment. These findings suggest that the responséis broadly related to flow rheologies that are mostly controlled by connectivity of both lateral and longitudinal sediment supply. El Niño-signature on the alluvial fill comprises significant episodic storm and flood events and can be identified in the older record where is likely to be similarly spatially localised innature and may account for the localised distribution of some facies noted in the study of Holocene deposits that finally control the geomorphological evolution of fluvial systems in the southernmost portion of the Atacama Desert (Chapter 6a and 6b). In fact, the study of the Holocene fluvial and alluvial stratigraphy arrangements has shown that the evolution of El Huasco river valley is controlled by lateral sedimentary inputs of tributary-junction alluvial fans like El Niño-signature rather than by the para glacial response as previously stated in literature. The alluvial fans in fluvial systems situated at the southernmost Atacama Desert present phases of increased alluvial fan formation between (~11 ka. to 8 ka.), and (2 ka to the present). For other hand the erosion measured after March 2015 storm agrees with millennial erosion rates and with return time (~100 years) of high sediment discharge events previously reported in the study area. So, in the Holocene denudation of these fluvial systems the El Niño rainstorms contribute significantly. In short, this thesis shows how the impacted zones of a storm event, that triggered debris flows and flash floods, can be mapped with the coherence change detection and how the erosion and alluvial-aggradation from an episodic sediment discharge event, dictates the evolution of the fluvial systems of the southernmost Atacama Desert. Finally, mapping erosion areas and sediment pathways in ephemeral dry-land channels is of great importance to calibrate landscape evolution models in arid zones and to evaluate hydro meteorological hazards triggered by extreme storms.
Doctorado.
Incluye bibliografía h.: 127-139.
Doctorado.
Incluye bibliografía h.: 127-139.