The tidal flux in the Firth of Forth

The tidal flow into an estuary controls the export of pollutants and determines the net flux and redistribution of sediments. To make an assessment of the accuracy of model simulations, a ship-mounted acoustic Doppler current profiler was used to measure the flow into the Firth of Forth near the Edinburgh road bridge during neap and spring tide conditions. A pressure gauge was used to record the tidal elevation and concurrent meteorological data were obtained from local weather sensors. The maximum tidal flux into the estuary during spring tides reached 30 000 m3/s. The observations were simulated using nested versions of the DIVAST depth-averaged tidal model. Good agreement was obtained between the observed and simulated tidal elevations, and also in the magnitude of the computed tidal fluxes. There were, however, sometimes large instantaneous differences between the observed and computed fluxes. An analysis of low-pass sea-level data, atmospheric pressure and wind forcing, suggests that the errors in the flux estimates were due to meteorologically induced shifts in the phase of the tide at the open boundary of the model. Such shifts limit the accuracy that can be obtained by a tidally forced model when the impact of meteorological forcing is not included at the open boundary.