Applying Independent Component Analysis on Sentinel-2 Imagery to Characterize Geomorphological Responses to an Extreme Flood Event near the Non-Vegetated Río Colorado Terminus, Salar de Uyuni, Bolivia

Abstract

In some internally-draining dryland basins, ephemeral river systems terminate at the margins of playas. Extreme floods can exert significant geomorphological impacts on the lower reaches of these river systems and the playas, including causing changes to flood extent, channel-floodplain morphology, and sediment dispersal. However, the characterization of these impacts using remote sensing approaches has been challenging owing to variable vegetation and cloud cover, as well as the commonly limited spatial and temporal resolution of data. Here, we use Sentinel-2 Multispectral Instrument (MSI) data to investigate the flood extent, flood patterns and channel-floodplain morphodynamics resulting from an extreme flood near the non-vegetated terminus of the Río Colorado, located at the margins of the world's largest playa (Salar de Uyuni, Bolivia). Daily maximum precipitation frequency analysis based on a 42-year record of daily precipitation data (1976 through 2017) indicates that an approximately 40-year precipitation event (40.7 mm) occurred on 6 January 2017, and this was associated with an extreme flood. Sentinel-2 data acquired after this extreme flood were used to separate water bodies and land, first by using modified normalized difference water index (MNDWI), and then by subsequently applying independent component analysis (ICA) on the land section of the combined pre-and post-flood images to extract flooding areas. The area around the Río Colorado terminus system was classified into three categories: water bodies, wet land, and dry land. The results are in agreement with visual assessment, with an overall accuracy of 96% and Kappa of 0.9 for water-land classification and an overall accuracy of 83% and Kappa of 0.65 for dry land-wet land classification. The flood extent mapping revealed preferential overbank flow paths on the floodplain, which were closely related to geomorphological changes. Changes included the formation and enlargement of crevasse splays, channel avulsion, and the development of erosion cells (floodplain scour-transport-fill features). These changes were visualized by Sentinel-2 images along with WorldView satellite images. In particular, flooding enlarged existing crevasse splays and formed new ones, while channel avulsion occurred near the river's terminus. Greater overbank flow on the floodplain led to rapid erosion cell development, with changes to

Cascades of sub‐decadal, channel‐floodplain changes in low‐gradient, non‐vegetated reaches near a dryland river terminus: Salar de Uyuni, Bolivia

“…() argued for clear links between precipitation events, discharge, and channel and floodplain changes. That precipitation, flooding and morphodynamic changes are linked is not surprising, as has been shown by many other studies of dryland river response to individual flood events (see Tooth, ), including in the study area (Li et al, ). Rarely, however, has there been detailed study of the impacts of two or more closely‐spaced floods on dryland river morphology (Milan et al, in press) but the rainfall record, availability of high‐resolution satellite imagery, and rapidity of change on the lower Río Colorado provides an unusual opportunity to do so.…”

“…Salar de Uyuni is the world's largest salt lake, and is periodically supplied with water and sediment by low‐gradient, ephemeral rivers such as the Río Colorado. This river has been the focus of extensive previous studies that have shown, inter alia , how high‐resolution satellite imagery can be useful for quantifying channel‐floodplain morphology and dynamics in this poorly or non‐vegetated dryland setting, including meander bend dynamics, the development of crevasse splays, and avulsions (Donselaar et al, ; Torres Carranza, ; Li, , in press; Li et al, , b, , b, ; Li and Bristow, ; Sandén, ). Erosion cells have been noted in the upstream parts of the Río Colorado catchment (Li et al, ) and are also prominent features farther downvalley but have not been investigated in any detail.…”

Sentinel-1 Based Flood Mapping Using Interferometric Coherence and Intensity Change Detection Approach

“…Satellite images enable fast, reliable and costeffective monitoring of large coverages, thus provide geospatial information to support disaster monitoring and management in an effectively (Amadio, 2016). Identification of flooded areas become much effective and profitable by use of multispectral satellite data, Synthetic Aperture Radar (SAR) data or a combination of them (Notti et al, 2018), (Rahman and Li, 2017), (J Li et al, 2018,) and (Muro et al, 2016). Many studies have proven that SAR systems are very effective and reliable tools for flood mapping especially on vegetated areas and the bare soil as well, by benefiting from their sensitivity to surface roughness and soil moisture changes (Schlaffer et al, 2015) and .…”

“…In sense of feature selection to define and model the change between two scenes; there are several methods used in literature both for pixel-based or texture-based models (Ouled Sghaier et al, 2018) and (J Li et al, 2018). Commonly used change indicators for pixel-based models are the ratio, log-ratio (Y or difference operator, which suppresses the background information to extract the changed region.…”

Preface: Remote Sensing for Flood Mapping and Monitoring of Flood Dynamics

“…In the context of extreme flood events or flood disasters, Li et al [7] applied an independent component analysis of Sentinel-2 imagery in order to characterize geomorphological responses to an extreme event in Bolivia. For flood disaster monitoring, two case studies of Hurricane Harvey were contributed.…”