Evaluation of sediment yield from valley slopes: a case study

Abstract

Hydro-geological hazards in alpine areas is a really common problem. Many calamitous phenomena (such as debris flows, landslides, and others) are related to the sediment yield from the slopes of the valleys. Sediment yields are far from being fully understood and predictable, due to a lack of knowledge of the physical mechanisms underlying these processes and to the variability of the peculiar geomorphologic characteristics of river basins. Key unknowns are the medium- and long-term average sediment production, the recharge time of the sediment sources (and consequently the frequency of the yields), the triggering factors and the thresholds for activation. The manuscript documents the results of the estimation of sediment production for the basin of the Tartano valley in northern Italy. The basin is characterized by a significant presence of weak rocks (cataclastic, mylonitic), that makes considerable amounts of loose sediments available. In this work, semi-quantitative models were applied to evaluate the basin-scale, yearly sediment yield. Estimates sediment volumes were compared to records of sediment volumes extracted from an artificial reservoir located at the downstream section of the catchments. In addition, the spatial distribution of the sediment instability level was obtained, highlighting a significant heterogeneity of the river basin. Therefore, the relevance of the basin-scale modelling of sediment yields for off-site and on-site processes was discussed. The dependency of the sediment yield regime on the spatial and temporal scale supporting the evaluations was analyzed and discussed.