Reconstructing the Salgar 2015 Flash Flood Using Radar Retrievals and a Conceptual Modeling Framework: A Basis for a Better Flood Generating Mechanisms Discrimination

Abstract. Flash floods associated with severe precipitation events are highly destructive, often resulting in significant human and economic losses. Due to their nature, flash floods tend to occur in medium to small basins located within complex high mountainous regions. In the Colombian Andean region these basins are common, with the aggravating factor that the vulnerability is considerably high due to the presence of important human settlements frequently occupying floodplains and other flash-flood prone areas. During May 18 of 2015, severe rainfall generated a flash flood in the municipality of Salgar, in La Liboriana basin located in the northwestern Colombian Andes, resulting in more than 100 human casualties and significant economic losses. This work aims to reconstruct the 2015 La Liboriana flash flood to understand better the hydrological processes that took place during the event and to serve as a proof of concept for the development of a flash flood guidance low-cost tool when information is scarce. Radar QPE, satellite data, and post-event field visits are used to reconstruct the Salgar flash flood addressing the relationship between rainfall spatio-temporal structure, soil moisture, and runoff generation during successive rainfall events, using conceptual modeling framework including land-slide and hydraulic sub-models. The hydrological model includes the addition of virtual tracers to explore the role of runoff and subsurface flow, as well as the relative importance of convective and stratiform precipitation in flash flood generation. The assessment of the interactions between runoff, subsurface flow, and convective-stratiform rainfall allows a better understanding of the short-term hydrological mechanisms leading to the flash flood event. The overall methodology reproduces considerably well the magnitude and timing of La Liboriana flash flood discharge peak, as well as the areas of regional land-slide occurrence and flood spots location, with some limitations due to the available spatial scale of the digital elevation model. Simulation results indicate that the flash-flood and the regional land-slide features were strongly influenced by the antecedent rainfall associated with a northeasterly stratiform event that recharged the gravitational and capillary storages. The hydrologic simulation shows that the antecedent event set wet conditions in the entire basin before the occurrence of the flash flood event, governing the streamflow during the flash flood event. Evidence suggests that the spatial structure of the rainfall is at least as important as the geomorphological features of the basin regulating the generation of flash flood events. Results also suggest that the described model-based tool is useful for policy-makers not only for short-term decisions in the context of an early warning system but also as a planning resource for long-term risk management.