Impact of river engineering drivers on denudation processes in a small catchment

Human influence in the form of hydraulic engineering causes disruption to unconfined channel transport in catchments. Hydrotechnical construction includes, among others: weirs, dams, retention reservoirs, river bank supports, bridges, etc. Each of these structures causes disruptions in the flow of trailed and suspended sediments originating from denudation in the river catchment. It is widely known that the dynamic balance of the river bed is disturbed by sediment accumulation upstream of the reservoir's dam, limited sediment outflow from the reservoir, and increased downcutting downstream of the dam. The aim of the study was to investigate the influence of reservoir and additional river regulation facilities on selected rivers in Poland on the structural and functional connectivity of the river channel in terms of sediment transfer from slope denudation in the catchment areas. The study examined a high-resolution digital elevation model together with survey data on sediment accumulation sites along the river. It was shown that a more detailed and realistic connectivity pattern for hillslopes and river channels can be obtained by applying several tools and parameters simultaneously (i.e. field surveys, modelling, etc.).

The study uses the concept of connectivity. A spatial examination of catchment linkages enables a determination of the degree to which a catchment participates in the transfer of sediment in suspended form and the pathways used to facilitate this. A detailed analysis of such pathways is needed to plan appropriate river engineering work and efforts designed to reduce sedimentation in reservoir and downstream erosion as well as efforts to remove dams in situations where this is deemed to be the most appropriate solution.

We observed in the study area, the highest values of the IC were noted for valleys of streams where suspended sediment transfer occurs through multiple pathways found in the vicinity of streams. Conversely, the lowest index values were noted for mouth section areas found near the studied reservoir, where sediment deposition occurs in the form of alluvial fans. In foothill areas sediment transported in stream channels usually comes from lateral bank erosion as well as stream bed erosion in the Wapienica. In foreland basins part of the catchment, the influx of sediment occurs at select sites, mostly via drainage ditches. The presence of levees along rivers and concrete lining of channels effectively prevents sediment influx from area hillslopes. On the other hand, the use of rubble in the channel bed impedes channel floor erosion and channel bank erosion. The largest number of sites where suspended material is supplied to fluvial transport pathways are found in middle mountain areas, followed by foothill areas, and finally foreland basins.