engleski

Analysis of a reservoir water level impact on landslide reactivation

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 assessment of a hazard of further landslide movements. The estimated dimension of landslide body are length of 350 m, width of 135 m and 20 to 30 m of depth to the slip surface. The movement of approximately 12 to 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. The landslide is located in the Rječina River Valley which is very well known as an area prone to sliding with a lot of massive and small landslides recorded in the past and recent history (Arbanas et al. 2014). The Rječina River is 18.7 km long, and the mouth is located in the centre of the City of Rijeka. The area of the valley is part of a dominant morphostructural unit along the Rječina River Valley. The Cretaceous and Paleogene limestone are located at the top of the slopes, while the Paleogene siliciclastic rocks and flysch deposits are situated on the lower slopes and the bottom of the valley consisted of silty marl, laminated silt to silty shale and fine- grained sandstones (Benac et al. 2011). 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 (Mihalić Arbanas et al. 2014). It was identified that the recent landslide is a reactivation of the dormant landslide which contours were clearly visible on LiDAR images. Borders of both dormant and reactivated landslides were confirmed by engineering geological mapping on the new topographic map. The reactivated landslide included only part of the dormant landslide area. 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 centre 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. The LS-Rapid software is the first landslide simulation model possible to integrate the whole process of stable state, failure, post-failure strength reduction, motion and deposit of sliding mass (Sassa et al. 2010, 2014). While the foot of the landslide is submerged in the reservoir, the magnitude of motion, run off sliding path and deposition of sliding mass significantly depend on the reservoir water level. The landslide simulations were conducted for four different reservoir water levels correspondent to full reservoir, overflow level, and two lower safety levels. In case of high reservoir water level 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. At the overflow reservoir water level of 229.50 m asl, the total volume of about 300.000 m3 would slide in the reservoir and cause 220 m width landslide dam while the consequent water level rising would reach 2.55 m with additional 5.80 m height water waves. Lowering of a reservoir water level would have an important impact on reduction of sliding mass that would fill the reservoir and consequences caused by the water level rising and landslide caused waves. At the safety reservoir water level of 224.50 m asl, the total volume of about 90.000 m3 would slide in the reservoir and cause 90 m width landslide dam while the consequent water level rising would reach 0.50 m with additional 3.00 m height water waves. 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.

landslide ; reactivation ; simulation ; LS-Rapid ; Valiči Dam ; water leve

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