Modelling the Effects of a Pumping Program for Increasing Water Circulation in a Semi-Enclosed Bay in the Stockholm Archipelago

Brunnsviken in the inner Stockholm archipelago, close to the City of Stockholm and popular for recreational activities, is a semi-enclosed bay with a very narrow passage to the adjacent archipelago and consequently has a limited water exchange. Various attempts have been made over the past three decades to improve the water quality in the bay. Since 1986, the drinking water authorities of Stockholm withdraw bottom water by pumping it from one of the deep basins of the bay through a pipe to be eventually discharged into the nearest embayment of the archipelago. There are, however, some questions regarding the cost-benefit aspect of this strategy. In particular, it is unclear if the location and the rate of pumping are well chosen in order to increase the ventilation of the bay at large. In addition, it should be possible to eventually optimize the pumping schedule, so that these energy-demanding and thus costly efforts are concentrated to times when they act in concert with natural forcing to increase water exchange. To help improve the pumping program, a modelling project was started in spring 2004. First, a one-dimensional (1-D) layer model that resolves the bay into one basin with multiple-layer stratification was attempted. Second, a three-dimensional (3-D) model with the capacity to be run under non-hydrostatic assumptions was set up at high resolution to study a number of scenarios. Both models were forced by wind, river discharge, surface temperature and the exchange driven by density fluctuations across the boundary to the adjacent archipelago. In the 3-D model, the pumping is included as a virtual divergence of the flow at the location of the pipe. The main results are that the 1-D model performs considerably better than the 3-D model in simulating the measured salinity profiles, even though it does not resolve the basin in the horizontal direction. The poor performance of the 3-D model with regard to salinity is however mainly due to boundary problems: too little inflow of saline water through the narrow and shallow entrance channel, and underestimation of the freshwater supply. The dynamics of the thermocline is better captured by the 3-D model but can certainly also be improved.