A simple model for water level and stratification in Ringkøbing Fjord, a shallow, artificial estuary

Ringkøbing Fjord is a shallow estuary connected to the North Sea through a canal and a sluice. A simple model is presented with which the water level and the stratification in the estuary may be calculated from observations of wind, input of freshwater, water level in the North Sea, and data for the operation of the sluice. Two main components are the use of open-channel, hydraulic theories to determine the exchange through the canal and sluice and a potential energy concept to calculate the vertical mixing. One major simplifying assumption is to disregard the baroclinic dynamics, which works well except in case of weak or strong winds. The model is run for 1999, and the results compare reasonably well with observations of water levels and salinity profiles at several locations. The primary objectives of the model are to be able to estimate, as a function of the operational practice of the sluice, a) the salinity of the main body of water and b) the amounts and the lives of the intermittently inflowing volumes of seawater, which could lead to oxygen depletion in the deep parts of the estuary due to eutrophication. This is demonstrated in a period in September 1999 during which a volume of seawater is first admitted to the estuary and then gradually eroded, while the oxygen in it is rapidly consumed. The present model provides an efficient and computationally inexpensive tool to study some estuarine properties, other alternatives being three-dimensional models of high spatial and temporal resolution and complex turbulence description. The hydraulic part of the model, describing the flow through the canal and sluice, can be modified, and so the model can be adapted to other estuaries that are characterized by a large degree of horizontal homogeneity and a well-defined connection to the sea.