Calculating exchange times in a Scottish fjord using a two-dimensional, laterally-integrated numerical model

In order to assess the potential impact of pollutants, particularly soluble wastes discharged by the mariculture industry, on the fjordic sea loch env ironment in Scotland, simple management models have been developed which es timate steady-state concentrations based on the quantities of effluent rele ased and the residence time of such material within a loch. These models ma ke various simplifications about the hydrodynamic characteristics of Scotti sh sea lochs, the most important of which is the concept of an exchange tim e which parametrizes the rate at which pollutants are removed from the syst em. Exchange times for individual lochs are calculated using the tidal pris m method, which has some well-known shortcomings. In this paper, a two-dime nsional laterally-integrated circulation model is used to investigate the e xchange characteristics of Loch Fyne and its sub-basins. By simulating the transport of a passive, conservative tracer, the turnover times for the loc h, two sub-basins and various depth layers are calculated. By varying the s tarting time of the tracer simulations, the variability in the exchange tim es is examined. The results from the circulation model are compared with th e estimates given by the tidal prism method. The results show that the tida l prism method consistently underestimates the exchange times, although the predicted times tend to lie within the range of the simulated times. Inclu ding a simple return flow factor into the tidal prism estimate leads to sig nificant improvements in the comparison.