Model-based assessment of sustainable adaptation options for an industrialised meso‑tidal estuary

• We use a unstructured coupled physical-biochemical model on the elbe estuary. • Based on model results, we assess estuarine adaptation options. • The measures aim to improve the state of a heavily dredged and degraded estuary. • Model results indicate improvement of key physical and biogeochemical parameters. • Coupled modelling identifies the adaptation options’ major trade-offs for society. Human-shaped estuaries play a vital role in supporting a range of economic, ecological and social functions. Such cultural landscapes often require enormous services, which may be provided at the expense of the ecological status and the ability to provide ecosystem services. This is exemplified by the estuaries of the German North Sea coast, of which the Elbe estuary is the most prominent and stands out as the largest and most consistently developed. The port of Hamburg, which is the primary economic driver in the region, has shaped the morphology of the surrounding water body. This has resulted in a number of hydrodynamic effects and sedimentological and ecological consequences, which have been well documented and subject to extensive debate. Despite this understanding, however, there is a tendency to propose solutions that are limited to the smallest local scales and are unable to mitigate the consequences of human interventions that have taken place or continue to take place at the estuarine and catchment scales. The lack of illustrative and quantitative scenario simulations and holistic assessments also hinders the ability to implement ambitious adaptation measures. To step forward, this study presents a model-based assessment including scenario simulations of four prototypical adaptation measures that are potentially capable of mitigating the problems of high turbidity, sedimentation and oxygen minimum without compromising coastal protection. The experimental design comprises a two-month morphodynamic simulation for each adaptation scenario and a one-year simulation of coupled hydrodynamics and ecology. The model simulations demonstrate that the proposed measures have the potential to reduce the siltation of the upper estuary, thereby reducing the need for extensive and costly maintenance dredging. Furthermore, the simulated measures also reduce the tidal range in the densely populated upper estuary, albeit to varying degrees. This also applies to mitigating the consequences of eutrophication, such as the oxygen content in the navigation channel. These differences, as well as the differing scale and effort associated with the four measures, form the basis of a final comparative evaluation based on universal sustainability criteria.