SEHAG - Sediment connectivity, delivery, and the propagation of changes in high Alpine geosystems

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In the second phase of the SEHAG project, a landscape evolution model (LEM) will be used to study past, present and future erosion, transport and deposition of sediments from hillslopes to channels, within the channel network, and towards the catchment outlet. These sediment cascades, and thus the transmission of changes (an element of catchment sensitivity), are influenced by sediment connectivity, a system property that represents the degree to which subsystems of the study areas are coupled with respect to sediment transfer. The model is driven by precipitation and/or discharge data and will be calibrated and evaluated against numerous digital elevation models (DEMs) generated in phase one for the period 1953-2020. While DEMs of difference (DoD) document surface changes only for inter-survey periods, the model will support a quasi-continuous analysis of surface changes and sediment fluxes.SP8 has three main objectives: (1) To continue and deepen the investigations started in phase one. This includes the continuation of annual surveys of channel reaches, and above all the multitemporal evaluation of system structure (landforms and their configuration, landcover) and functioning (channel morphodynamics, hillslope-channel coupling, sediment yield). Already in phase one, oriented and referenced historical photos (esp. time slice I, 1850-1920), are used for “back-dating" geomorphological and landcover maps. Together with DEMs, these maps form the basis for a graph-theoretic analysis of toposequences and sediment cascades. (2) To explain past and to project future changes in fluvial morphodynamics and sediment transfer. To this end, we use the LEM to explicitly model erosion, transport and deposition of sediment at the spatial scale of channel reaches. A more comprehensive quantification of total sediment infill of reservoir lakes by echosounding will aid model calibration on sediment yield at the catchment outlet. Finally, we use the LEM to explore the process-response system of hillslopes characterized by gullying and undercutting by adjacent river channels. (3) To take a dynamic, model-based approach on geomorphic coupling and sediment connectivity. First, the ratio of sediment yield to gross erosion is calculated for different spatial units from sequential DEMs output by the LEM; it represents a metric of functional connectivity and will be analysed (i) in response to forcing magnitude and (ii) in comparison to (static) connectivity indices. The inverse approach to morphological sediment budgeting is used to compute sediment fluxes between landforms or larger spatial units from DoD. The results lead to the set-up and analysis of area-wide networks (=weighted graphs) based on the geomorphological maps. The LEM, carefully calibrated on observed hillslope and channel morphodynamics, will finally be employed to explain past (time slices I-III, 1850-2022) and to project future (2023-2050) morphodynamics and sediment yield.