Multidisciplinary study of a 40-year landslide history: who or what caused the reactivation?

During the construction of the power plant Tušimice 50 years ago, a sludge reservoir was built as one of its auxiliary structures. The sludge pit, barred by 50 m high earthen dam filled fully a valley of Vysočany brook, left-side tributary of Hutná brook. As the sludge pit was gradually filled up during the operation of the powerplant (1980ies-2004). However, already since 1980ies, the activity of landslides at the N slopes of Hutná valley increased. Numerous springs and marshes developed on the slope, and fresh reactivations of the landslides damaged part of the Hořetice village and the local road between Hořetice and Vysočany. The persisting activity of the landslides was documented by numerous reports and papers throughout 1980ies and 90ies (Rybář 1984, Rybář 1987, Nováková 1990, Zika et al. 1991, Rybář 2001).

            Since 2020, the team of dpt. of Engineering geology has performed a complex study on the site trying to document the lasting activity and reveal its causes even almost 20 years after the closing of the power plant. The employed methods included geomorphological and engineering geological mapping, site documentation, vast geophysical survey, aerial photogrammetry using both visible spectrum and IR cameras, hydrological measurements and documentation, and study of borehole and geological documentation.

The mapping was aiming to observe the current activity of the landslide, document spatial changes of the individual landslides compared to previous research, locate the springs and swamps, using GPS devices to ascertain the position of the landforms. Photographic documentation of landforms (scarps, drainage objects, damaged road, etc) and occasional soil probes were dug to observe the bedrock composition. Geophysical survey consisted of 12 profiles across and along the slope, covering its whole extent. The aerial survey allowed construction of a detailed, actual DEM (though occasionally obscured by high vegetation) and orthophotomap, and the IR aerial photos were used for observing wetter areas on a special orthophotomap. Finally, the hydrological measurements of the discharge of Hutná brook were performed to observe water inflow from the studied slope.

The study confirmed that landslides are still active. They have obviously changed and developed since the last mapping in 2001, and numerous findings indicate persisting activity, including documented fresh scarps, inclined trees, broken and dislocated vertical drainage elements, etc.

The cause of the persisting activity of the slope processes is the combination of valley slope, structural-lithological settings and seeping of sludge basin water through permeable sandy layers in the underlying sedimentary complex. As the underground water level increased with the filling of the dam, the water could reach the N slope of the Hutná valley. This was anticipated already before the operation of the sludge basin and confirmed during operation (Nováková 1990), which lead to construction of a drainage system in part of the slope. However, the drainage is slowly ceasing to work due to silting and deterioration, thus allowing the water seep into the slope again.