Adaptation to the impacts of climate change and human intervention in the Elbe estuary: model-based identification of possible trade-offs

The Elbe estuary ensures the connection between one of the largest ports in Europe, Hamburg, and the North Sea. The need to adapt the navigational channel to the increasing demands of ship transport as well as the necessities of coastal protection and of ensuring the functioning of the ecosystem are leading to contradicting demands. In order to decide on possible trade-offs, a detailed understanding of present and possible future interactions between local and global processes is necessary. In this study, we use realistic modeling and observations to show the functioning of the current hydrodynamic and biogeochemical estuarine system. We demonstrate the increasing importance of the formation of stratification in the low-salinity reaches, which is related to an increase in salt intrusion, sedimentation and the risk of hypoxia. The model results reveal that dry and hot summers are leading to dangerous oxygen minima in the artificially deepened tidal Elbe. To examine the effect of a more natural expansion of the navigational channel, we further use an idealized model of the estuary. In this exercise, we study the influence of sinusoidal meanders on the tides, the thermohaline dynamics and the estuarine ecosystem. It turns out that this deregulation of the shipping channel leads to a reduction of the tidal range and of silting in the port area. Furthermore, the channel curvature enhances oxygen levels in the area of the estuariane oxygen minimum zone in comparison with the straight channel. Sensitivity experiments demonstrate the sustainability of such an adaptation strategy towards the dominant trends of climate change such as sea level rise and global warming. Finally we assess the plausibility of the measure in terms of its potential to conciliate contradicting demands in the intensively used estuarine environment.