Monitoring of landslides in the Brda river valley in Koronowo (Polish Lowlands)

The paper discusses landslides investigated in a Brda river valley in the region of the town Koronowo near Bydgoszcz (Kujawsko-Pomorskie Voivodeship) in the Polish Lowlands. Due to morphologic and geologic setting the area is prone to slope instability. Out of 32 identified landslides, 9 were selected for detailed examination. To register a nature and rate of failures a complex monitoring system was installed on slopes where the selected landslides resulted in damages to municipal infrastructure. The system was facilitated with inclinometers and piezometers, supported by a network of GPS-RTK geodetic benchmarks as well as with a recording station. The performed examination revealed that the most unfavourable arrangement of the layers is, when under a non-cohesive soil lies a cohesive impermeable layer. The landslides are rather shallow ones (except for two incidents) with slow rate of displacement. The landslide-triggering factor is water originating from precipitation and snow melting. Influence of water was especially significant in early spring 2011, due to the combined effect of snow melting and infiltration of thawing water originating from the former, as well as exceptionally high rainfalls. The influence of hydrologic conditions on slope deformations is complex. There is a significant time lag between a movement initiation and unfavourable hydrometeorological conditions. That is exemplified with the landslide that was initiated in February-March 2011 in consequence of atmospheric conditions of November-December 2010. The results formed the background for inventing engineering treatment measures aiming at current remedial stabilisation of slopes and mitigation of possible landsliding in the future.