This paper presents a holistic analysis of the September 12–15, 2019, flash flood in the Mar Menor basin (Spain). The research aims to analyse the causal factors, define the flooding processes and establish criteria to detect the active zone in highly anthropized alluvial apron systems. For this purpose, several data sources were combined: digital elevation models, ortophotos, precipitation and gauging data, geomorphological and flood hazard mapping and second-generation Sentinel-2 images, which were interpreted using a turbid water index. Results allow a complete characterization of flash flood processes in alluvial fan apron systems and highlight different behaviors in the different sectors of these complex fan systems. They show an important discrepancy between the actual flood processes and the official hazard mapping, which highlights the need to develop quali-quantitative approaches to better understand flood processes in alluvial apron systems. The research also shows that floods in Mediterranean alluvial apron systems must be considered as mixt processes, resulting of nature and culture interactions, because of the longstanding tradition of runoff use and management through different traditional infrastructures. Future flood mitigation strategies should recover or imitate these ancestral solutions and should lead to a radical change in urban planning design on these areas.