High rates of geomorphological processes in a tropical area: the Remparts River case study (Réunion Island, Indian Ocean)

Reunion Island is characterized by rapid landscape evolution resulting from its cyclonic tropical climate. However, local active surface processes are not well understood. The relationships between climatic events, large scale landslides and torrential transport of sediment by the rivers remain unclear. The Remparts River is an appropriate area for studying such geomorphological processes, as it deeply incises the active Piton de la Fournaise volcano. In this study, different approaches are used to analyze the morphological evolution of the river from the sediment production areas to the outlet over the last 40 years. Recurrent events of huge mass wasting occur at Mahavel Cliff, upstream of one of the river tributaries, the most recent producing around 50×10⁶ m³ of sediment in 1965. Combined analyses of the sequence of cyclonic events, major mass wasting events and aerial photography interpretation over the last 40 years led to the proposal of a functional model of river system responses to these events. The river system can be divided into three compartments, each affected by three classes of geomorphological events. The sedimentary response (erosion and/or aggradation) of each compartment to a triggering event, such as cyclonic rainfall and/or seasonal rise of water discharge, is controlled both by the magnitude of the climatic event and by the state of the compartment resulting from previous evolution. A set of five aerial photographs and a satellite image showing the evolution of the studied area during the last 40 years are examined in detail in the light of the functional model. Observations confirm a rapid and complex evolution of the river bed (erosion and aggradation), and provide information about the dynamics of the sediment transfer from the production areas to the ocean. Analysis of two distinct topographic datasets bracketing the last major cyclone Dina in 2002 allows the estimation of the river sediment budget resulting from this event. The net volume of aggraded sediments in the river bed is estimated at around 8×10⁶ m³.With no major collapse event recorded at Mahavel Cliff, sediment transfer due to the flood associated with the 3-day cyclone Dina event is responsible for this significant increase in river bed sediment volume. This quantification shows that several million cubic meters of sediment may take only a few years to spread over more than 5 km downstream. The river bed has now reached its highest elevation since the 1965 landslide, with potential consequences for natural hazards in the area of the outlet at the city of Saint-Joseph.