Changes in the relief and land use of landslide slopes during the 20-year period after an extreme hydrometeorological event (case study from the Polish Flysch Carpathians)

The research is focused on the development of landslide slopes in areas with mixed agricultural and built-up land use in the middle mountain region. The multi-stage studies of the development of landslide slopes were carried out in the 20-year period after na extreme 1997 hydrometeorological event in Beskid Wyspowy Mountains, Polish Flysch Carpathians.

Landslides occupy 30-70% of slopes in the Flysch Carpathians. Meanwhile, this region has high population density (approx. 130 people per km²), which combined with medium inclination of slopes encourages people for settlement and agricultural land-use on landslide slopes. Development of slopes in the Flysch Carpathians occurs most often during extreme hydrometeorological events. One of the most significant events took place in 1997 and it was unique one in terms of both the rainfall total and its intensity. After this event approximately 20,000 landslides were created or reactivated in the Polish Carpathians.

Six areas in the immediate vicinity of the Łososina River and its tributaries were selected for the study. All areas were transformed by landslides during the 1997 event. The degree of activation of landslides was determined on the basis of field studies carried out after the extreme event in 1997 and the analysis of DTM and orthophotos (1997-2019). A number of relief parameters of activated landslides were analyzed in detail, including: slope, density of drainage network, shape (W/D), type of landslide.

In five studied areas older landslides were present, covering up to 65% of the area. In one of the studied slope no landslides older than 1997 were detected. Most of landslides that occurred in 1997 occurred within the older landslide forms. As a result, the total area of the slopes affected by "new" landslides increased by only 1.0-1.7% of the studied slopes’ area. The rejuvenated landslides covered 8 to 26% of the area of landslides existing before 1997. Principal landslide activity was the formation of numerous secondary scarps, fissures and a fresh accumulation zones of colluvia. Subsequent development of the landslide slopes during the 1997-2017 period was investigated. Extreme precipitation events occurring during that period, especially in 1998, 2001, 2010, 2014, were analyzed in terms of their efficiency in transformation of the studied landslides.

The changes in land use in activated parts of landslides were also analyzed. It was found that there was a significant increase in forest area of 13-52% in activated parts of the landslides in 2017 compared to 1997. As a result a complex land use mosaic developed in the study area with forest patches and different agricultural areas (pastures, orchards, arable lands and abandoned fields).