Impact of river erosion on variances in colluvial movement and type for landslides in the Polish Outer Carpathians

Abstract

The paper deals with the problem of the impact of river erosion on the activity level and type of colluvial movement within landslides in two study regions (Foothills and Beskidy Mts.) in the Polish Outer Carpathians. The principal goal of the study was to investigate how landslides that remain in contact with river channels function and to identify primary colluvial movement types depending on hydrometeorological conditions. Five landslides were studied, with three of them found in the Rożnowskie Foothills and two in the Beskid Niski Mts. The research was performed using terrestrial laser scanning (TLS) in the period April 2014 to November 2017. Ten measurement series were acquired during the study period. A total of 9 DEM of Difference representations (DoDs) were produced that showed both quantitative and spatial changes in the studied landslides. It was determined, based on the DoDs produced, that landslides adjacent to river channels remained continuously active due to the influence of continuously occurring fluvial erosion, although the level of activity varied, as it depended on hydrometeorological conditions. Colluvial material was set in motion within the entire landslide during floods. The recording of spatial and quantitative changes in the body of each landslide allowed to identify two key types of colluvial movement occurring during floods: (i) movement starting at the bottom and subsequently activating parts at the top (landslides: Sękówka, Boczkówka, Żabno), (ii) motion propagating in top-to-bottom and in bottom-to-top directions (landslides: Bodaki, Leszczyny). Then again, in periods between floods, colluvial material was continuously removed from the toe of each landslide, which occurred in all the studied landslides. Colluvial material became activated during these periods through movement starting at the bottom and propagating to the top. The amount of colluvium removed by streams varied depending on hydrometeorological conditions. The colluvial material carried away from the studied landslides by streams during floods comprised 60% to 90% of the volume of material removed during the entire examined period.