Channel degradation: sediment transport acceleration of multi-thread gravel bed river after human interventions

Freely migrating and dynamic rivers are strongly affected by changing natural conditions in industrial Europe and altered by increasing human pressure on the landscape. Anthropogenic modification, grade-control structures and channelization resulted in channel narrowing, transformation and incision in many rivers in Europe. The Belá river is the last multichannel river system in Slovakia and during the 20^th century, systematic regulation and human impact on the environment resulted in rapid channel incision. The channel planform evolution and channel bed changes were analysed in the natural, braided-wandering river system of the Belá river. For spatial-temporal analyses of the channel pattern changes and transformation were used long-term historical aerial data (1949 – 2018; 11 horizons) and high-resolution data were collected from UAV (Unmanned Aerial Vehicle) and TLS (Terrestrial Laser Scanning). Both sources were used for detailed topographic models (including bathymetry) and classified point cloud generation. Lidar dataset combined with a floodplain age map was used for tracking incision intensity during the different time periods and in different river sections.  Sediment supply to the channels correlates with the magnitude of flood events and during the TLS survey from March 2016 to November 2018, 25 964 tonnes of fine-grained sediment were delivered into the river channel. Bed incisions achieve 2.5 m in the last 10 years with the propagation of a knickpoint zone by backward erosion upstream conditioned by the system of cross-valley faults and anthropogenic impact. In 2000, a small hydropower plant was constructed in the position of an old abandoned channel. A rapid incision uncovered a bedrock channel bed formed by a clay sequence of Huty formation (inner-Carpathian Paleogene). The abandoned channel is now used as a supply channel for the hydropower plant and the second natural active channel is artificially maintained by heavy machinery to ensure water supply for the hydropower plant. Human impact disturbed the balance of the sediment system leading to the overall degradation of the channel and loss of ecological functions.