Feedback (2024-2025) on flood risk in Réunion Island (France, Indian Ocean): From the analysis of temporal and spatial dimensions to the implementation of mitigation strategies

This paper aims to assess flood risk in Réunion Island, taking into account historical data as well as the recent impacts of cyclones Belal (2024) and Garance (2025). It seeks to demonstrate that solutions grounded in a hydro-geomorphological approach can effectively mitigate flood risk, notwithstanding the challenges posed by increasing urbanisation in valley floors. Volcanic in origin, Réunion Island is situated in the Indian Ocean, approximately 678 km east of Madagascar. Its mountainous landscape and subtropical climate make its hydrographic network particularly dynamic, leaving its inhabitants vulnerable to extreme hydrometeorological events, thereby contributing to a heightened flood risk. The study employs a multidisciplinary approach, integrating historical data (inclusive of texts, maps, and photographs) alongside field data (encompassing geomorphology, stratigraphy, hydraulics, and land use). The findings reveal historical settlement patterns across four types of geomorphological units: ‘planèzes’, slopes, valley floors, and coastal plains. Presently, the rivers are not adequately managed to address the hydro-geomorphological dynamics, which include the need for discharging substantial volumes of water and sediment through channels of appropriate capacity, lateral migration of sinuous channels, channel avulsions, and the incorporation of stormwater runoff into river flows. Alarmingly, 61 % of the population residing in valley floors are at risk of flooding. The discussion highlights the ineffectiveness of current flood-control strategies, which have inadvertently exacerbated vulnerability. It underscores the urgent necessity for an integrated approach to flood-risk management, incorporating dynamic hydro-geomorphological mapping and an enhanced sensitivity to the ramifications of climate change. Raising community awareness and reducing vulnerability through improved regulation and planning are crucial steps to address the challenges that lie ahead.