Monitoring the Transport of Sediment During Tropical Cyclones From High-frequency Seismic Noise in Two Rivers of La Réunion Island

Abstract : La Réunion Island is a basaltic shield volcano located in the western Indian Ocean. The island undergoes heavy annual precipitations during tropical depressions and cyclones. These rainfalls modify the stream dynamics and sediment transport of rivers. The transport of sediment participates to the erosion of the volcanic island, however, in situ characterization is difficult during high water stage. In the frame of the Rivière des Pluies project, we are deploying a temporary seismic network of 10 three-component broadband seismometers around two rivers: Rivière des Pluies and Rivière du Mât. The goal of the project is to monitor spatial and temporal variations of the river's bed-load during tropical cyclones with high-frequency noise. Meteorological and hydrological stations are installed at both rivers providing valuable data such as precipitations, water discharge and water level. We will also sample the bed surface grain size distribution by visual count to determine its influence on the seismic noise. We present preliminary results from two broadband seismic stations located near instrumented streams. SALA station from the temporary RHUM-RUM seismic network (http://www.rhum-rum.net/en/) was installed close to the Rivière du Mât and the permanent GEOSCOPE RER station is located close to the Rivière de l'Est. We analyzed the footprint of the cyclone Bejisa in January 2014. We observe a significant increase of the precipitation when the cyclone eye is 300 km close to the island followed by the increase of the water discharge. Simultaneously the seismic signal shows a sudden increase of the power spectral density visible above 1 Hz. Further investigations on the relationship between the seismic noise and the hydrological and meteorological parameters will help us quantifying the river bed-load.