Sediment contribution of shallow landslides and flux connectivity of transfer paths in mountainous areas under climate change projections. A case study for the Saldes River basin (Pyrenees, Spain)

In the context of soil erosion modeling, coarse-grained sediments present considerable challenges, particularly concerning sediment production and quantification. This study proposes a module-based quantification approach that integrates different coarse-grained production processes, where one of the main outputs is the source area delimitation and the quantification of mobilizable sediment.

The present study focuses on analyzing shallow landslides and various scenarios of sediment transport to the nearest fluvial system, by implementing the newly developed “Random Connect” code. This code calculates the accumulated volume that travels from the source areas into the fluvial system based on the connectivity index. The chosen case study is the Saldes River basin in the Pyrenees (Spain) The outlet point of this basin is La Baells water reservoir, presently facing siltation challenges arising from sediment transport across the entire drainage area. Reported by CEDEX (2002), the sediment yield delivered to a La Baells Reservoir from the entire drainage area was 4.54 Mg ha⁻¹yr⁻¹ in 2002. In this sense, this water reservoir is utilized for calibrating and validating our model. The quantification of sediment in water reservoirs does not allow to separate the contributions of the different erosive processes at the basin, thus highlighting the importance of the study of the river section to better understand the sediment production.

For model calibration, field surveys were conducted to ascertain the connectivity index to the main river, identify (dis)connectivity factors, and measure fluvial and sediment grain characteristics. Comparing model output with field data enables determination of sediment transport potential and the maximum sediment quantity that can reach the main river. Depending on the connectivity threshold, the results of sediment reaching the main river for a critical rainfall event can vary between 250000 to 10000 m³.

Assessing sediment at the river cross-section helps in defining the principal coarse-grained production phenomena, such as shallow landslides, rock falls and debris flows. Grain characterization of sediment is necessary to study sediment mobilization through a hydrological-driven module. The main objective is to track coarse-grained sediment until it reaches the water reservoir and identify the meteorological and physical factors that trigger the process.

A historical baseline of sediment production has been determined for the Saldes River basin, based on historical landslide inventories, previous triggering events, and meteorological scenarios for the current climate. The assessment considers the impact of climate change in Spain at different timelines based on return periods. The rate of sediment production is determined by analyzing critical climate change scenarios, resulting in values below and above the baseline. This analysis places special emphasis on extreme climate events and the projection of mean annual precipitation.