The Effects of Severe Rainfall on the Stability of Natural Slopes A numerical analysis of a slope in Hjärtum – Lilla Edet

Abstract

The effect of global warming tends to cause more frequent extreme weather events across the planet. These types of extreme weathers may present themselves as more intense and long-lasting rainfalls which have proved to be critical in terms of slope stability. As a rainfall occurs, the negative pore pressure of an unsaturated of partially saturated slope reduces hence the slope loses part of its stability capacity. The negative pore pressure, known as suction, is directly correlating to the safety factor of a slope. Ultimately, this means that when the suction reduces, the safety reduces and vice versa. The aim of the study is to examine how future rainfalls will affect the stability of a natural slope during an entire month. The goal is for the report to act as a framework for future studies regarding slope stability as a component of evolving precipitation conditions. To exemplify this framework, an existing slope in Hjärtum, Lilla Edet, adjacent to Göta älv has been investigated. Prior to this study, an analysis has been conducted by SWECO to evaluate the stability at different sections of the slope. To evaluate how future precipitation will evolve, existing studies for Västra Götaland have been utilized and adapted to the specific case Hjärtum, Lilla Edet. To determine whether the month with the single most extreme rainfall or the month with the most total precipitation was more critical for the slope stability, two months in the reference period 1961-2010, matching those descriptions, were identified. The rainfalls from the reference period were adjusted to correspond with each scenario. The slope was modelled in the software PLAXIS 2D and the precipitation was defined as an external infiltration subtracted with the evapotranspiration. In total, four calculations for each scenario were performed, namely the reference case, followed by the time-period intervals: 2011-2040, 2041-2070, and lastly 2071-2100. It was found that the precipitation will increase significantly until the year 2100 with short duration extreme rainfalls and precipitations during the winter being the most affected. Furthermore, the safety factor of slopes similar to the one in Hjärtum, are in great risk of being significantly impacted by the increasing severity of the situation. Lastly it was concluded that the most critical future scenario is one where more prolonged periods of rain unfold.