SEDYN-X - Interdisciplinary sediment flux research in the Johnsbach valley

Sediment transport in river systems is a recurrent problem for geomorphological sediment flux studies, natural hazard assessment, river ecology and river engineering. While each discipline has developed its own tools and models of sediment budget evaluation, complex issues of sediment management might be better served by an interdisciplinary cooperation between them. In our proposed project we aim at establishing a close teamwork between geomorphology (Institute of Geography and Regional Science, Karl-Franzens University, Graz) and river engineering (Institute of Water Management, Hydrology and Hydraulic Engineering, University of Natural Resources and Life Sciences (BOKU), Vienna) to work out sediment management strategies in the Johnsbachtal catchment, Nationalpark Gesäuse. In this area, costly renaturation measures were carried out; however, their positive effects are endangered by a too low sediment supply. The Sedyn-X project will investigate the Johnsbachtal sediment budget in an integrative way, including (1) geomorphic mapping and ALS evaluation to derive a conceptual model of the sediment cascade, (2) quantifying important geomorphic processes using repeated TLS surveys and further techniques, and assessing the volume of sediment storage in the system, (3) quantifying transport in the river system using different techniques of bedload sampling and (4), working out possible measures and future sediment management strategies, also under the viewpoint of ongoing climate change. Work packages 1 and 2 will be mainly carried out by the geographers and no. 3 mainly by river engineers (even if the work packages are interlocked), while the management strategies can only be worked out in close cooperation and under consideration of local expert knowledge. By our novel interdisciplinary approach we want to work out strategies for future river management in the study area considering the divergent interests of local actors like National Park authorities, hydro power stations and road maintenance authorities; but also deliver guidelines for similar problems in other regions.

Our project results suggest that sediment transport will increase in the future in our study area (Johnsbachtal, Austria) due to national park policy and the reduction of man-made impacts. Bedload transport in rivers is highly important for sustainable river management and river ecology and sufficient sediment flow needs to be maintained to enable renaturation measures. In the SEDYN-X project we combined expertise from geomorphology and river engineering to investigate the pathways and amounts of sediment moving through the river catchment and to assess the impact of climate change and anthropogenic disturbance on sediment yields, as well as to provide basic data for sediment management. Among others, we applied (a) repeated laser scans and aerial photo assessments (back to 1954) to create surface models and monitor surface changes, (b) a bedload measurement station to measure the sediment output from the Johnsbach catchment into the receiving river, the Enns. The bedload station, consisting of a metal sill equipped with geophones and a sediment trap, is a new development designed for use in small alpine torrents without disturbance of the aquafauna. Bedload monitoring over a period of three years at the outlet of the Johnsbach showed that transported gravel does not solely depend on runoff. For individual events, we observed very different bedload rates at comparable flow conditions which we assign to different sediment availability e.g. between spring and summer. The highest rates of bedload transport in 2016 took place in July (42% of the annual yield). We found that the transported sediments mainly derive from erosion cuts near the valley bottom and from some steep side valleys. Most of the sediment transfer in the catchment was triggered by rainstorm events in summer. We found that the side trenches deliver very different amounts of sediments (between 0 and approx. 2000 m per year). The determined flux rates allow to set up a sediment budget model. The determined sediment fluxes in the catchments coincide well with the output measured at the bedload station. Two important side valleys are currently seperated from the main torrent because of decade-long gravel mining which was only recently terminated due to national park policy. The repeated laser scans allowed us to estimate how long it will take to fill the resulting sinks. We expect that these side valleys will again be coupled to the river within 10-20 years which will considerably increase the amount of gravel transported by the Johnsbach. This heightened sediment transport along the river is positive from the viewpoint of river ecology but could potentially cause management problems in downstream hydropower stations.