A discussion of methods for estimating residual fluxes in strong tidal estuaries

A one-dimensional, time-dependent transport model which includes tidal fluctuations has been developed to simulate the barotropic dynamics and the solute evolution in strong tidal estuaries. The model is used to estimate fluxes in a typical macrotidal estuary, the Scheldt in Belgium and The Netherlands, The results of the model are compared with those obtained with classical flux estimation methods which neglect the nonlinearities of the fluid motion as well as the nonstationary behaviour of solute transport. It is shown that in macrotidal systems, which are generally characterized by fairly long residence times, both contributions are significant. In particular, the residual water flow to the coastal waters differs markedly from the freshwater discharge and the solute dynamics arising from fluctuating conditions at the estuarine boundaries is significantly influenced by the history of the water flow. It is demonstrated that linear property-salinity plots can be obtained in a system characterized by transient (nonsteady-state) conditions. This result allows one to quantify the error associated with the widely used 'apparent zero end-member' (AZE) classical flux estimation methodology. Results demonstrate that the AZE technique, based on the analysis of mixing curves, introduces large errors in the estimation of the long-term residual constituent flux towards the coastal zone. (C) 1998 Elsevier Science Ltd. All rights reserved.