Fundamentals on Ecological Modelling in Coastal Waters Including an Example from the River Elbe

摘要

Modelling dissolved oxygen in water encompasses a range of requirements and process descriptions. Among a good representation of hydraulic, including advection/dispersion, the processes in the ecological part of the model are crucial for a reliable model. A MIKE 21 (DHI 2013) flexible mesh model was used to simulate the hydrodynamic conditions in the River Elbe around Hamburg. The model is based on the numerical solution of twodimensional incompressible Reynolds averaged Navier-Stokes equations subject to the assumptions of Boussinesq and of hydrostatic pressure. Thus, the model consists of continuity, momentum, temperature and density equations and is solved by a turbulent closure scheme. The spatial discretisation is performed using a cell-centred finite volume method. In the horizontal plane an unstructured mesh is used. This hydrodynamic model was coupled to the ecological module (ECO Lab) to form the basis for the water quality model. ECO Lab (DHI 2013) is a numerical lab for ecological modelling. It is an open and generic tool for customising aquatic ecosystem models to describe water quality. The module is mostly used for modelling water quality as part of an environmental impact assessment. In the present case a eutrophication model was used to describe the concentrations of phytoplankton, chlorophyll-a, Zooplankton, organic matter (detritus), inorganic nutrients and oxygen in the river Elbe in Hamburg. Rooted vegetation and macroalgae were not implemented as their influence in this part of the Elbe is negligible. The eutrophication module is integrated within the advection-dispersion module which describes the physical transport processes at each node in the model domain. The model was used to describe the oxygen conditions in the Port of Hamburg, especially the change of dissolved oxygen in the Reiherstieg, a lateral channel which is usually closed with a sluice at the southern end to prevent suspended solids to deposit in the port. Due to plans for a new sluice gate the Hamburg Port Authority tested if a temporary opening of the gate could improve the oxygen conditions in the port. The model showed that resuspension and mineralization of organic material play an important role in this system. The balance between opening the gates to flush oxygen rich water into the port and preventing the introduction of organic material is crucial for better oxygen balance in the Reiherstieg.