SALT INTRUSION IN THE TWEED ESTUARY

Results are presented from a 2-week field programme in the Tweed Estua ry, U.K. Maximum values of the empirically based Estuarine Richardson Number, Ri(E), occurred during neap tides, and minimum values occurred during spring tides. Estimated values of Ri(E) varied between 0.3 and 2.3, suggesting the occurrence of partially mixed to stratified condi tions, depending on tidal state and freshwater inflow. These relativel y large values of Ri, were consistent with both observed strong salini ty stratification and large salt fluxes due to vertical shear transpor t. Low values (< 0.25) of the estimated gradient Richardson Number, Ri , generally occurred close to the bed on the flood, suggestive of tida l mixing there, and higher (> 0.5) values in the halocline. A velocity maximum occurred within the halocline during the early flood. Wave-li ke spatial oscillations of the halocline occurred on the ebb. The osci llation troughs were situated above deep holes located just down-estua ry of the rail and old road bridges. There was an indication that the constricted flow between the bridges' arches resulted in enhanced mixi ng of near-surface waters and a thickening of the halocline. It is als o possible that these wave-like structures were stationary, near-criti cal internal lee waves, triggered by the deep holes. Trapping of high- salinity waters occurred on the ebb. Saline pools were isolated within a deep hole or deeper section of bed by the falling halocline. When t he salt wedge moved further down-estuary, the 'trapped' waters were su bjected to strongly ebbing, overlying freshwater, and were subsequentl y entrained and flushed. The salinity intrusion was a strong function of spring-neap tidal state and a weaker function of freshwater inflow. The estimated salinity intrusion varied from about 4.7 to 7.6 km duri ng the fieldwork period. The strong dependence on tidal range followed from the comparable lengths of the tidal excursion and salinity intru sion. Long excursion lengths were also partly responsible for the shor t residence (or flushing) times and their strong dependence on spring- neap tidal state. For typical summer freshwater inflows of 40 m(3) s(- 1), the estimated residence times varied from one tide at large spring tides (removal on one ebb) to two tides (removal over two ebbs) at ne ap tides. Computed residence times were shorter for typical winter inf lows. (C) 1996 Academic Press Limited