Multidecadal changes of structural sediment connectivity in alpine catchments

Sediment connectivity is defined as the potential of a catchment to route material through itself. It is a system property that regulates the propagation of geomorphic changes through a catchment and is therefore a factor of its sensitivity to climatic change. In well-connected catchments, changes are effectively propagated; where the coupling of hillslopes to channels, or between channel reaches is poor, changes may remain localised. Structural connectivity itself is not a static property; it can be affected by process-response feedbacks, gradual or rapid changes, for example as a consequence of extreme events. In this study, we use a multi-method approach to investigate changes in structural sediment connectivity over time periods of up to 70 years in three alpine catchments.

First, we calculate the Index of Connectivity (IC) and corresponding change maps to identify areas and time periods with major changes in structural connectivity.  The required multitemporal digital elevation models (DEMs) are computed with historical aerial images and Structure-from-Motion Photogrammetry, more recent DEMs are obtained from ALS surveys. The channel networks as targets are manually mapped using the DEMs and orthomosaics.

The second approach for selected areas makes use of multitemporal geomorphological maps, digital elevation models and graph theory. The geomorphological maps were produced based on historical orthomosaics, DEM derivatives and DEMs of Difference. The landforms in the geomorphological maps form the nodes of a graph, and edges connecting the nodes along the direction of flow represent potential or actual sediment transfer between them. The graphs reflect the system structure for a certain point in time; graph metrics can be used to assess the structural connectivity including spatial differences and temporal changes.