Evidence for Erosional Efficiency of Extreme Precipitation Events at a Multi-Decennial Time Scale

Extreme precipitation events play a pivotal role in shaping Earth’s surface through their influences on hillslope processes and sediment transport in rivers. In this study, we focus on understanding the implications of such events on sediment transport, using Réunion Island as a natural laboratory due to its intense tropical rainfall regime. Through photogrammetric techniques and subsequent sediment volume estimates spanning decades as well as cosmogenic 3He measurements, we assessed the spatio-temporal evolution of the canyon bed of the ephemeral Rivière des Remparts and the drainage of products from major landslides and/or rock avalanches between 1950 and 2011. Results show that 39.6 Mm3 of sediment was transported out of the watershed in 62 years. Furthermore, we modeled the flow dates of this ephemeral river and show that such an export of material actually happened during only 391 days over the 62 years, at an average rate of 0.1 Mm3/day. Our investigation confirms that sediment transport coincides with officially recorded extreme meteorological events such as cyclones. Moreover, our findings reveal that sediment transport predominantly occurs on days corresponding to high-percentile rank precipitation events, demonstrating that all transport is concentrated during these intense rainfall periods. Finally, this study underscores the extremely fast conveyance of material from slopes to deep-sea fans, facilitated in Réunion by the absence of a coastal platform. This rapid transfer has implications for CO2 consumption, as it should enhance the transport and burial of organic matter particles, potentially contributing significantly to the island’s overall CO2 consumption efficiency.