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

Abstract

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.

Precipitation Intensity Effects on Groundwater Recharge in the Southwestern United States

“…IDF analysis employing the Gumbel distribution provides a statistical graphical representation of rainfall intensity for a given exceedance probability [50,51]. IDF curves have been used to evaluate groundwater recharge [61][62][63][64] which may be impacted by changes in precipitation patterns as a result of climate change [35]. Of these studies, only Dourte et al [61] evaluated IDF changes which may be related to deviations in groundwater recharge.…”

Modeling the stability of terraced slopes: an approach from Valtellina (Northern Italy)

“…For this reasons, many studies were conducted on this subject (Azzola and Tuia 1983;Cancelli and Nova 1985;Blahut et al 2012;Camera et al 2012;Quan Luna et al 2014). In 1983, three soil slip/debris flow events caused the death of 18 people in Tresenda and in the near village of Valgella.…”

“…The choice of the physical, mechanical, and hydrogeological properties (see list in Table 1) to assign to the involved materials (backfill soil, bedrock, and dry-stone wall) was supported by field and laboratory tests and based on the experience of previous studies (Camera et al 2012.…”

“…4) were recognized in the study area and used to assign variable inputs in different areas of the slope. Porosity (n) and saturated hydraulic conductivity (k s ) were adjusted for calibration respecting the ranges of values obtained from field and laboratory tests (Camera et al 2012). In urban areas, k s values were considered equal to that of open soil, but a very low infiltration capacity was assigned (100 times lower than open soil).…”

“…However, all these studies have not developed a complete physically based approach that takes into account the hydrogeological and geotechnical processes that can affect terraced slopes. A first step in this direction is represented by the studies of Camera et al (2012Camera et al ( , 2014, who showed, working at the single terrace scale, how the main cause of failure in these environments is related to the building-up of saturated horizons in the backfill of the dry-stone wall. Due to the overpressure, the wall bulges at toe and then collapses, leading to the subsequent failure of the whole terrace.…”