Landslides on river banks in the western part of Podhale (Central Carpathians, Poland)

The authors studied landslides in alluvial sediments on undercut steep banks of the rivers flowing in the western part of the Podhale region. The landslides are of rotational type. Landsliding processes are complex and they differ from those on solid rock slopes because of the heterogeneous lithology of the alluvial sediments in the banks (muds, sandy gravels, clays) and because of uneven degree of their consolidation. Their course depends on the mode rate of water infiltration into the sediments. Multiannual observations of changing landslide surfaces and measurements of scarp surface with erosion pins indicate that each type of sediment behaves in a different way during the sliding. The layers of mud in the highest parts of the banks were sliding down in blocks and soon became fragmented, soaked and washed away. Sandy gravels were sliding as whole layers or separate fragments. The more compact the alluvium, the more compact was the resulting colluvium. The thick layers of massive claystones underlying the alluvial sediments at the bank bases were mobilized by landsliding only to the depth to which they became plastic. The claystones appeared the most resistant to landsliding among the bank materials. The surfaces of rupture were shallow and uneven where poorly consolidated layers of alluvium lied horizontally (landslides at Chochołów and Ludźmierz). In areas where alluvial sediments were more consolidated and inclined, and the layers of clay alternated with sands and gravels, the surface of rupture occurred deeper and was smooth (Stare Bystre landslide). Landslides in undercut river banks are an important source of debris in fluvial channels.