Altered near-bed turbulence: insights and negative impacts of inland navigation on benthic habitats

Aquatic ecosystems in rivers are currently exposed to a large number of stressors, causing a massive decline in abundance and biodiversity. One of these impacts is the wave action induced by inland navigation on riverbanks. The wave impact causes increased remobilization and erosion of the bed substrate and thus a recurring disturbance of the benthic habitats. The focus of this study on impacts of ship waves was on altered near-bed turbulence and its effects on a riverbank on the Danube in Austria. The natural and ship-induced wave action at a well-exposed gravel bar upstream of Vienna was recorded over the course of a year. Near-bed flow velocities were measured, which were used to calculate the turbulence and bed shear stresses. The results of this study reveal differences in the wave impact caused by the different types of ships. Here, the shape of the ship, its speed and the distance to the riverbank have a particular impact on the benthic habitats. To improve the aquatic habitat quality, speed limits and minimum distances to the river banks should be introduced based on further measurements. A possible restriction of recreational boating during ecologically sensitive seasons could also improve the quality of the habitats. The derived data also provide insights into the effects of strong winds compared to the waves caused by inland navigation. The observed wind-induced redistribution of sediments occurs sporadically, whereas the ship-induced resuspension depends on the type of ship and occurs regularly with shipping traffic on the Danube. However, seasonal effects must continue to be investigated and researched in the future. The presented data are also used to discuss the need for research into natural and anthropogenic wave impacts on river morphology and ecology.