Flood Modelling Evaluation in a Semiarid Catchment: The Role of a Soil Mineralogical Index From Satellite Imagery

Abstract

The Mediterranean region is characterized by the occasional occurrence of intense rainfall events. Along with lithological, geomorphological, and land use factors, these events influence the hydrosedimentary dynamics of drainage basins, potentially causing torrential floods. In the present study, a hydrological and hydraulic analysis of a cut-off low is performed. This event occurred between September 11 and 13, 2019 and affected the Rambla del Beal, an ephemeral stream in a context of semiarid climate in southeast Spain, where no water discharge data are available. The headwaters of this drainage basin are distinguished by mining waste deposits accumulated over decades without any treatment. The HEC-HMS model was used for hydrological analysis, and the HEC-RAS hydraulic model was applied to simulate the flood extension around the rambla. A total rainfall accumulation of 226 mm was recorded, concentrated mainly within a 6 h period, with a maximum hourly value of 76.71 mm. The hydrograph indicated a peak flow of 171.73 m³/s. The results were evaluated using a map of sediments deposited during the flood. This cartography was generated using an index band to detect iron oxides and hydroxides with Sentinel-2 satellite images. This index was applied to images before and after the flood. The difference in the index between both dates was then calculated. The result displayed positive values in areas where deposits occurred during the flash flood. Therefore, these accumulations indicate the minimum flooded area. Furthermore, the spatial distribution of the deposits reflected the hydrological behavior of alluvial fan dynamics as marked on the geological map, albeit without detail. The exact places where alluvial fans deposited their sediments were confirmed with the analysis of the historical aerial photographs. These features are not currently visible as agricultural and urban development has altered the topography. In conclusion, iron oxide and hydroxide mineral mapping proved valuable in validating the results of hydraulic modeling in basins influenced by acid mine drainage.