Impact of landslides activity on changes in the composition of alluvia and morphometric changes in stream beds on the example of the Polish Outer Carpathians

Abstract

An important aspect of the function of slope and fluvial processes is their interaction. Riverside landslides are more active than other landslides due to an additional activating factor in the form of fluvial erosion of landslide fronts. Another type of interaction is the change in particle size composition of the alluvium through colluvium supplied to riverbeds. This paper considers the effect of active landslides on the change in particle size composition in the alluvium below the landslides. The effect of active landslides on the change in channel morphometry at the site of the resulting landslides was also determined. Five sections of the channels of small streams were selected for the study: three were in low mountain areas - Rożnowskie Foothills, and two in middle mountain areas - Beskid Niski Mts. Material from the riverbeds and landslides was sampled twice, first immediately after a flood and second a year later after a period without a flood. The material was analysed using laser diffraction. The volume of colluvium supplied to the riverbed was measured using terrestrial laser scanning. The results show a significant effect of colluvium influx on the particle size composition of the alluvium by increasing the proportion of the very fine fraction: clay and silt. The highest proportion of material from landslides in the composition of alluvium was noted immediately below each given landslide. Further downstream this proportion decreased and then declined to zero after 190 to 450 m. Differences in the particle size composition of alluvium depending on the time of sampling were also observed. In the material sampled immediately after the landslide occurrence, significantly more of the fine fraction component was registered, i.e. the impact of landslides was higher.