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

A one-dimensional, time-dependent transport model which includes tidal fluc tuations has been developed to simulate the barotropic dynamics and the sol ute evolution in strong tidal estuaries. The model is used to estimate flux es in a typical macrotidal estuary, the Scheldt in Belgium and The Netherla nds, The results of the model are compared with those obtained with classic al flux estimation methods which neglect the nonlinearities of the fluid mo tion as well as the nonstationary behaviour of solute transport. It is show n that in macrotidal systems, which are generally characterized by fairly l ong residence times, both contributions are significant. In particular, the residual water flow to the coastal waters differs markedly from the freshw ater discharge and the solute dynamics arising from fluctuating conditions at the estuarine boundaries is significantly influenced by the history of t he water flow. It is demonstrated that linear property-salinity plots can b e obtained in a system characterized by transient (nonsteady-state) conditi ons. This result allows one to quantify the error associated with the widel y used 'apparent zero end-member' (AZE) classical flux estimation methodolo gy. Results demonstrate that the AZE technique, based on the analysis of mi xing curves, introduces large errors in the estimation of the long-term res idual constituent flux towards the coastal zone. (C) 1998 Elsevier Science Ltd. All rights reserved.