Assessing slope-river connectivity for evaluating cascading landslide hazards: A case study of Tordera river basin, NE Spain.

Landslides and torrential flows are among the most dangerous processes that occur on hillslopes, and they are mostly triggered by intense rainfall events. These phenomena are not only hazardous in themselves, but they can also have a more significant impact downstream when they interact with channels or the river network. When multiple landslides are simultaneously triggered by a rainfall event that affects an extensive area, they can initiate chains of further hazards due to the sudden and massive influx of sediment they bring onto channels and rivers. Therefore, it is crucial to study the connectivity between slopes and the river network to evaluate areas with a potentially higher sediment contribution to the river network. Ultimately, this information will help to assess flood hazards and mitigate risks, as well as assist in the planning of protective structures, drainage works, and other relevant measures.

We conducted a study on the slopes of the Tordera River basin (NE Spain). This river flows from the Montseny (Catalan Coastal range)  into the Mediterranean Sea. The study area was affected by a regional landslide event that occurred in January 2020 during the Gloria Storm (more than 480 mm of rainfall was measured in 96 hours in the region). We employed the index of connectivity, which is based on Borselli et al. (2008), to examine the connectivity between the slopes and the river network. The outcomes of this analysis were subsequently compared to a landslide inventory (more than 1000 mass movements) to determine whether the high amount of sediment present in the lowlands could have originated from landslides in the upper part of the basin.

According to the results of this study, slopes with high connectivity experienced a high density of landslides. The sediment that flowed down the slopes and reached the rivers added to the flood that occurred downstream. This flood carried a considerable amount of sediment which caused the widening of the active channel and the growth of the Tordera delta. The impacts of the Gloria storm on the infrastructure caused significant economic losses.

 

Borselli, L.;  Cassi, P.;  Torri, D. Prolegomena to sediment and flow connectivity in the landscape: A GIS and field numerical assessment, CATENA, Volume 75, Issue 3, 2008, Pages 268-277, ISSN 0341-8162, https://doi.org/10.1016/j.catena.2008.07.006.