Multi-staged development of landslide slope after extreme hydro-meteorological event in 1997 (Beskid Wyspowy Mts, Polish Flysch Carpathians)

The study is focused on a multi-staged development of slopes affected by landslides in Beskid Wyspowy Mountains, Polish Flysch Carpathians. The role of extreme 1997 hydro-meteorological event as well as successive events in years 1998-2017 in landslide triggering was particularly taken into account.   

The flysch Carpathians are characterized by large percentage of an area affected by landslides. Landslides occupy 30-70% of slopes in this area. Meanwhile, this region has high population density (approx. 130 people per km2), which combined with low inclination of slopes encourages people for settlement and agricultural land-use on slopes, including landslide slopes.  

Development of slopes in flysch Carpathians is currently progressing mainly during extreme hydro-meteorological 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 slope sections ranging from the foot to the top of the ridge were selected for the research. All these sections were transformed by landslides during the 1997 event. They are all located in the Beskid Wyspowy Mts., which is one of the regions most affected by landslides in Polish Flysch Carpathians in 1997.  

The degree of activation of landslide slopes in 1997 and thereafter was defined based on field research and the analysis of DTM and orthophotomaps. A number of parameters of the relief of activated landslides were analyzed in detail, including: their slope, network of drainage. Later these parameters were compared to the relief parameters in inactivated parts of landslides. 

In one of the studied slope sections no landslides older than 1997 were detected. In other studied slope sections older landslides were present, covering up to 65% of the section’s area. The rejuvenation of landslides covered from 8 to 26% of the area of landslides existing before 1997. Main landslide activity was the formation of numerous secondary scarps, fissures and a fresh accumulation zones of colluvia. Most of landslides that occurred in 1997 occurred within the older landslide forms. As a result, the total area of the slopes affected by landslides increased by only 1.0-1.7% of the studied slopes’ area.  

The further development of the landslide slopes in the period 1997-2017 was investigated. 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 cover of 13-52% in activated parts of the landslides in 2017 compared to 1997. An increase in the diversity of the relief of landslide slopes and a greater mosaic of land use, especially a decrease in agricultural land for the benefit of the forest were found.