Budgeting sediment volumes mobilised during Storm Alex in the upper part of the Roya valley

Abstract

2nd October 2020, the Atlantic storm « Alex » triggered an exceptional Mediterranean flood event in the south-eastern region of France, affecting the coastal Alpine valleys of the Tinée, Vésubie and Roya rivers. The event is considered exceptional due to the unprecedented rainfall recorded within a 24-hour period, surpassing 650 mm at Mesche dam and the elevated liquid peak flows around 1 350 m3/s at Breil-sur-Roya. Geomorphological changes consequence of the flood event includes river-related effects such as debris flows, landslides and partial destruction of the forest cover on the slopes, together with the destruction of bridges, roads, and houses. Our study focuses on the Viévola sub-watershed upstream of the Roya river, made up of 4 confined torrential sub-valleys: Dente, Morte, Para, Scabrie and Rabay valleys. Theses generated numerous of debris flows feeding the Viévola alluvial fan. Our observations show that these valleys were responsible for massive input of sediment which led to widening of the active channel of Roya river,bank erosion and embankments during the Alex flood event. The major areas of sediment accumulation were quantified in order to determine the balance between the 'initial' volumes eroded such as upstream gullies sediments and landslide deposits and the 'deposited' volumes (Viévola alluvial fan). The sediment volumes were quantified by Digital Elevation Model of Differences (DoD) from aerial imagery available before and after the flood. Pre-flood topographical data is of low quality while post-flood topography are better thanks to LiDAR datasets acquired in october 2020 and june 2021. A statistical study of tree heights on tributary slopes, using aerial and infrared images, allowed us to subtract them from the pre-flood digital surface model, creating a tree-free pseudo topographic surface. Our results show that the Viévola sub-watershed produced approximately 304,000 m3 of sediment within a 24-hour period, with a margin of error of +/- 40,000m3 (an average error of 13%). The gullies at the head of the Dente and Rabay valleys produced 125,000 m3 of sediment, with an error margin of +/- 21,000 m3. These initial inputs from these gullies caused significant bank erosion in the Dente valley, resulting in the release of over 140,000 m3 of sediments, with an error margin of +/- 5,000 m3. This erosion led to a considerable widening of the Dente torrent channel, which expanded from an average width of 6.1m before the flood to 18.6m after the event. Based on field studies, we found that the cause of the bank erosion is linked to the debris flows that occurred in the Dente valley. Comparatively, the Morte, Para and Scabrie valleys primarily contributed sediment from landslides, totalling around 21,000 m3, with an error margin of +/- 6.300m3. On average, the width of the torrents in these valleys doubled, with a factor of 2.1. Moreover, an alluvial cone formed at the Vievola holiday resort, resulting in a deposited volume of approximately 90,000 m3, with an error margin of +/- 20,000 m3. Consequently, an estimated 214,000m3 of sediment was exported to the Roya River.