Preliminary Investigations and Numerical Simulations of a Landslide Reactivation

Abstract

A large landslide occurred near the Grohovo Village in outback of the City of Rijeka, Croatia, on 13 February 2014 after long term period of heavy rain. The preliminary surface observation was carried out immediately after sliding appearance enabled estimation of site condition, dimension of the landslide so as an assessment of a hazard of further landslide movements. The estimated dimension of landslide body are length of 350 m, width of 135 and 20–30 m of depth to the slip surface. The movement of approximately 12–15 m down the slope caused the complete damage of the local road over the landslide body. The toe of the landslide reached the bank of the Valići Reservoir 250 m away from the downstream located Valići Dam. Based on hazard assessment of further landslide movements, a lowering of the water level in reservoir and surface drainage from the landslide body were conducted as emergency landslide mitigation measures but the main reason for further landslide movements reduction was the end of heavy rain. Although the detailed filed investigations were not carried out, the analysis of possible further development of sliding was conducted. To establish the engineering geological model of the landslide, the analyses of existing LiDAR imagery and engineering geological mapping were carried out. It was identified that the recent landslide is a reactivation of the dormant landslide. Since a slip surface position was not identified by field investigation, a numerical analysis of a slope using the strength reduction method was introduced to determine the shape of a zone of rupture which would be used in landslide simulation. As a main hazard of further landslide movements, the filling of the Valići Reservoir, forming a landslide dam so as possible overflow of the dam and forming the wave that can reach the center of the City of Rijeka downstream the Rječina River were identified. To determine possible scenarios those could be realized in case of new long term rainy period and raising of ground water level in the landslide body, a numerical simulations of further landslide development were conducted using LS-Rapid simulation software. In case of high reservoir water levels correspondent to the dam overflow, the sliding mass would significantly fill the reservoir and cause the landslide dam, while the water level rising and landslide caused waves (tsunamis) would overflow the Valići Dam and cause significant damage downstream the Rječina River channel. Conducted LS-Rapid simulation results enabled a selection of relatively safe reservoir water level at which, in case of further landslide movements, no current harmful consequences would be realized. The reservoir filling by sliding mass would cause significant reduction of reservoir volume, disturbances in the Rječina River and Valići Reservoir flow regimes so as long term disruption of hydro power plant work that imply on necessary landslide remediation before further landslide reactivation.