Bewertung des potenziellen Strandungsrisikos für Gewässerorganismen bei Schwall und Sunk hinsichtlich langfristiger morphologischer Veränderungen von alpinen Flüssen

Hydropeaking is one of the largest hydropower-related disturbances to natural processes in river (eco) systems. The artificial flow fluctuations caused by on-demand electricity generation are known for their impact on aquatic ecosystems. This particularly affects those species and life stages that are unable to adapt their habitat selection to rapid rises and falls in discharge. To date, the risk of stranding has been investigated, both experimentally and numerically mainly using variable hydropeaking scenarios over stable or non-changing river sections/structures. There is therefore a lack of knowledge about how individual, specific hydropeaking events differ in terms of their impact on stranding risk when river morphology changes over the long term. The present work addresses precisely this knowledge deficit by examining morphological changes over a period of 20 years and the associated variability in lateral ramping velocity as a parameter for the risk of stranding. Two alpine gravel-bed rivers affected by hydropeaking for decades were tested using a one-dimensional unsteady modeling approach. The risk of stranding showed high variability for both rivers in relation to individual cross sections. However, at the section level, no significant changes in stranding risk due to changing morphology were calculated for either river section. Thus, the present study shows that the quantified stranding risk of aquatic organisms is a function of the already general morphological (gravel bar) characteristics of the affected river and that both the morphological development and the development of grain sizes have an influence on the potential stranding risk of aquatic organisms. This is particularly important when re-licensing hydropower plants in river systems subject to multiple demands.