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

Abstract

Extreme precipitation events play a pivotal role in shaping Earth's surface through their influences on hillslope processes (triggering mass-wasting events) and on transport capacity and dynamics of rivers. This study investigates the impact of such events on sediment transport using Réunion Island as a natural laboratory due to its intense tropical rainfall regime. Through photogrammetric techniques using historical aerial images, we reconstructed nine DEMs of the entire canyon bed of the ephemeral Rivière des Remparts spanning six decades. Based on differences in DEMs and sediment volume estimates combined with cosmogenic ³He measurements, we assessed the spatio-temporal evolution of the entire canyon bed and the drainage of products from major landslides and rock avalanches between 1950 and 2011. Results indicate that 50.2 ± 8.4 Mm³ (145.5 ± 24.4 Mt) of sediment was transported out of the watershed over 62 years via bed load waves. We modeled the flow dates and showed that such an export of material occurred during only 391 days over the 62 years, at an average rate of 0.13 ± 0.02 Mm³/day (0.37 ± 0.06 Mt/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, we underscore the extremely fast conveyance of material from slopes to deep-sea fans, facilitated in Réunion by the absence of a coastal platform.