Linking pre-event hillslope-channel connectivity to its geomorphic response during an extreme rainfall event: insights from the 2020 Gloria storm in the Tordera River basin (NE Spain)

Intense rainfall events often trigger landslides and torrential flows, which are not only hazardous processes on their own, but can also generate cascading hazards through sudden and massive sediment delivery to river networks. Slope processes are therefore key drivers of geomorphic change in mountainous catchments, enhancing hillslope-channel connectivity and promoting rapid channel reorganisation. In light of the above, it is essential to characterise structural and functional connectivity (Heckmann et al., 2018), as well as geomorphic organisation before and after intense precipitation events, to better evaluate flood hazards and associated risks. Against this background, the January 2020 Gloria storm affected the Tordera River basin (Catalonia, NE Spain), where more than 480 mm of rainfall was recorded over 96 hours, with 24-hour accumulation over 200 mm, causing widespread sediment mobilisation and channel changes, as well as significant damage due to flooding and landslides.In this study, we aim to evaluate: 1) how pre-event hillslope-channel connectivity influences the geomorphic response to extreme floods and the post-event geomorphic changes through an integrated analysis of the index of connectivity (IC); 2) the spatial distribution patterns of erosion and sedimentation through the geomorphic mapping of the active riverbed and sediment bars (both active and stable), before and after the Gloria storm, and the existing inventory of landslides caused by the event. High-resolution DTMs (Digital Terrain Models) were generated from airborne LiDAR surveys conducted in 2011, 2016, and 2023. The IC was derived from a pre-event DTM to characterise structural sediment connectivity following Cavalli et al. (2013), while erosion and sedimentation processes were quantified using the difference between DTMs (DTMs of Difference, DoDs) for pre-event (2016-2011) and post-event (2023-2016) periods. GIS-based geomorphic mapping of active channels and sediment bars before and after Gloria was used to assess event-scale channel reorganisation.Preliminary results indicate a clear spatial correspondence between pre-event connectivity patterns and the magnitude of the geomorphic change observed during that extreme flood. Areas characterised by high pre-event erosion rates, identified from 2016-2011 DoDs, largely coincide with sectors where numerous landslides were triggered during the Gloria storm. High connectivity values also correspond to areas dominated by erosion and deposition in the 2016-2011 DoDs, highlighting the role of pre-event structural connectivity in conditioning sediment transfer pathways. Furthermore, active bars mapped after the event predominantly overlap with areas affected by pre-event erosion, whereas bars that remained stable during the storm are mainly associated with zones characterised by pre-event deposition. The active channel also experienced noticeable widening during the event, while the majority of vegetated bars that were stable before Gloria remained stable throughout the storm, reinforcing the link between pre-event geomorphic organisation and flood response. These findings highlight the importance of pre-event structural connectivity in controlling geomorphic response during extreme rainfall events, providing valuable insight for hazard assessment and river management. Cavalli, M. et al. (2013). Geomorphometric assessment of spatial sediment connectivity in small Alpine catchments. Geomorphology, 188, 31-41.Heckmann, T. et al. (2018). Indices of sediment connectivity: opportunities, challenges and limitations. Earth-Science Reviews, 187, 77-108.