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Geomorphic Coupling and Coarse Sediment Connectivity

Research Team: Dr. Markus Thiel, PD Dr. Tobias Heckmann, Prof. Dr. Michael Becht

In a geomorphic system, two components (e.g. specific landforms) are coupled if sediment transfer from one to another is possible. Sediment transfer is effected by geomorphic processes, e.g. mass movements or fluvial sediment transport, along trajectories whose geometric properties depend on the type of geomorphic process, type of sediment and topography. While some sediment pathways end on storage landforms (i.e. sediments are at least temporarily stored), some pathways meet other pathways along which sediments are transported further downslope or downstream. This linkage of two or more sediment pathways signifies geomorphic coupling. The degree to which the components of a geomorphic system (e.g. a catchment) are connected is called connectivity. This system property is thought to influence the effectivity of sediment transport from sediment sources to the catchment outlet and the way the system is reacting to change (sensitivity).

 In our project on coarse sediment connectivity in central alpine geosystems (funded by the DFG, 2008-2011, see entry in KU.FORDOC), we investigated how 'connectivity' can be measured.

Expanding the methods of 'predictive geomorphological mapping', we used GIS-based simulation models to locate (potential) sediment sources, pathways and deposits on a digital elevation model. Sediment pathways belonging to fluvial processes, rockfall and debris flows were combined in a GIS in order to find out where geomorphic coupling may be assumed.

Recently, we have started to use mathematical graph theory to model the "network" of sediment pathways and to quantify the connectivity of the modeled system. We think this is a promising approach, as graph theory is successfully being used to analyse all kinds of networks, also in geosciences e.g. landscape ecology.

Our study areas were located in the Austrian Central Alps (Horlachtal/Ötztal) and the Bavarian Calcareous Alps (Reintal and Lahnenwiesgraben).

The following animation shows the automated delineation of the areas potentially contributing sediment to the channel network. The approach uses a DEM and a set of threshold conditions concerning hillslope gradient, channel gradient and distance to channel network (the parameter values leading to the mapping results can be seen in the table on the right). The sediment contributing area (SCA) grows with decreasing hillslope and channel gradient threshold and with increasing distance to channel network. Research of our working group (Haas et al. 2011) has shown that bedload sediment yield measured in bedload traps highly correlates with the size of the SCA.

 

For graph analysis, an edge is constructed from each member cell of the SCA to the corresponding outlet (which may be the catchment outlet or a local sink).