Post-event forensic survey after a recent catastrophic flash flood in Central Spain: morphosedimentary and hydrodynamic reconstruction

Catastrophic flash floods are among the deadliest and most damaging natural processes worldwide. Despite this, they are rarely well recorded in instrumental (e.g. rain gauges, gauging stations) and documentary records (archives and newspaper archives). For their analysis and future prevention, it is therefore essential to carry out post-event forensic studies to collect as much information as possible in the field, from which the morphodynamic, hydrological and hydraulic characteristics of these events can be reconstructed.

In early September 2023, an exceptional ‘cut-off low’ weather situation (DANA) crossed the centre of the Iberian Peninsula, causing heavy rainfall and flash floods in several river basins (Alberche, Perales, Grande, Guadarrama). There were seven deaths and hundreds of millions of euros of damage to property and infrastructure.

This work summarises all the post-event forensic analyses and field observations collected after this episode along the Grande-Perales-Alberche river system, consisting of: (i) documentation of the historical morphological changes of these rivers, obtained from old cartographies, geomorphological maps, aerial photographs and recent orthoimages; (ii) compilation of all meteorological (rainfall) and hydrological (flow) information available for the event; (iii) acquisition of aerial images and video recordings using drones; iv) field georeferencing with differential GPS of high water marks (HWM) and paleo-stage indicators (PSI); v) field topographic measurements; vi) detailed measurement of bedform parameters such as wavelength and amplitude of current ripples; vii) grain size and composition sampling of flood deposits.

With all this information and other still being collected (such as orthophotographs and post-event DEMs generated by digital photogrammetry techniques based on images taken by drones), detailed digital elevation models are obtained. All this information will be used as calibration and validation information for 2D hydrodynamic and landscape evolution numerical models that attempt to reproduce and predict this type of event in the study rivers.