Sediment and bedload continuum at alpine hydropower plants – case studies from Tyrol (Austria) and suggestions for monitoring and management

TIWAG is operating 10 large (>10 MW) and 26 small (<10 MW) hydropower plants, with an installed capacity of about 1,650 MW and a mean annual production of 3,400 GWh. Long-term monitoring reveals insights into suspended sediment loads as well as bedload transport. This includes abiotic (suspended sediment measurements as well as the analysis of bedload transport) and biotic (biological quality elements, i.e., phytobenthos, macroinvertebrates and fish) monitoring. A characteristic of alpine run-of-river hydropower plants is that there are usually no dams along the impoundment. Thus, during an annual flood, the weir is opened completely, resulting in free-flowing conditions and free bedload transport. This is exemplified with data from the Inn River, e.g., at HPP Kirchbichl the annual bedload transport is about 120.000 m3 and the weir is opened at about 800 m³/s (HQ1) for 5 to 10 days. Regarding alpine reservoirs, mainly suspended sediments from glaciated catchments cause sedimentation. Generally, sedimentation in reservoirs causes a decrease in the storage volume and affects hydropower production. Thus, in alpine reservoirs, different sediment management strategies are applied: partly mechanical excavation (e.g. reservoir Längental, reservoir Tassenbach) or suction dredging and dislocation within the reservoir (e.g. reservoir Längental) was carried out. Currently, we are testing innovative systems to ensure continuous transport of suspended sediments to the penstock (transport via the turbines). This includes a system using air bubbles during the summer half of the year (when the inflow is high in suspended sediments) to avoid settling of fine sediments in the reservoir Längental. Another example deploys a small automated suction dredger for continuous dredging of material into the penstock at HHP Amlach (Tassenbach reservoir). Our case studies exemplify strategies for monitoring and sustainable management of sediments and bedload at alpine hydropower plants, to facilitate sediment continuum.