The Salt fluxes in a tidally-energetic estuary

Time series measurements of velocity and salinity have been used to examine the salt flux components in the tidally-energetic regime of the Conwy Estuary in North Wales. The instantaneous volume transport, up to 400 m3 s−1, was deduced from a current meter located in mid-channel using a frictional model with a correction for the observed mean velocity shear. Transport estimates obtained in this way have been compared with independent measurements of integrated cross-section transport using a boat-mounted ADCP. The two determinations of the flow are found to be in satisfactory agreement confirming the validity of the frictional model. Estimates of the covariance of the tidally varying transport Qtand salinity Stindicate that a consistently upstream transport of salt occurred over the 9 day period of the observations. This upstream tidal pumping flux (≈60 kg s−1) was greater than the downstream transport (≈27 kg s−1) estimated from the freshwater river discharge Qf(≈3 m3 s−1). The difference in the salt fluxes implies a net upstream movement of salt and was consistent in sign with the observed general increase in mean salinity which occurred in the upper estuary prior to a rapid salinity decrease due to a large increase in freshwater discharge at the end of the measurement period. The observations have been interpreted to illustrate the problems involved in trying to determine the net flux of a dissolved constituent from such as estuary. Using estimates of the net inflow from river gauging to determine the volume dischargeQ0 , estimates of the net flux are realistic but, in the absence of river gauging, large errors are likely to arise from the uncertainties in Q0, which is the small difference between large flood and ebb transports.