Rainfall, infiltration, and groundwater flow in a terraced slope of Valtellina (Northern Italy): field data and modelling

The aim of this work was to understand and reproduce the hydrological dynamics of a slope that is terraced by dry retaining walls. This approach will help to assess the influence of temporary groundwater perched tables, which can form at the area of contact between the backfill of the wall and the bedrock, on the wall’s stability. The study area is located in Valtellina (Northern Italy) near the village of Tresenda, which was affected by three debris flows that caused 18 casualties in 1983. In 2002, another event of the same type affected this area, but that event only caused the interruption of a major transport road. Direct observations of one of the three flows in 1983 and the reconstruction of 2002 indicated that the most probable triggering cause was the collapse of a dry retaining wall after its backfill was saturated. After field work was conducted to discover the principal hydrological and hydrogeological characteristics of the slope, numerical modelling was performed to determine under what conditions the soil will saturate, and therefore, when the collapse of a dry retaining wall might occur. First, a study of the interaction between pluviometric events and groundwater behaviour was conducted; then, modelling was performed using finite element analysis software that permits the calculation of groundwater flow both for completely and partially saturated conditions. The model was calibrated and validated using the hydrographs of the groundwater table recorded on site. It can be used as a predictive instrument for rainfall events of a given duration and return period.