AIRCRAFT AND SEA-TRUTH OBSERVATIONS OF A SMALL-SCALE ESTUARINE INTRUSION FRONT

Results are presented from an unusually detailed survey of the tempora l and spatial behaviour of an estuarine tidal intrusion front (Simpson , J. H., Nunes, R. A., 1981. Estuar. Coast. Shelf Sci. 13, 257-266). T he front was observed as it propagated through the inlet and moved int o the main channel of a small estuary (Tweed Estuary, UK) during a flo oding spring tide. High salinity coastal waters plunged beneath low sa linity estuarine waters, the plunge lines defining a ''V'' shaped fron t for part of the flood. The data were obtained using aircraft and sea -truth measurements as part of a two-day field programme in September 1994. Hydrographic surveys undertaken during similar environmental con ditions in September 1993 also are used to aid interpretation. A conve x-seaward front was observed in the seaward section of the inlet at LW + 2.1 h (LW = low-water). By LW + 2.7 h, approaching maximum flood cu rrents at mid-water, a ''V'' shaped tidal intrusion front was observed at the neck of the inlet, with the ''V'' pointing up-estuary. These d ata are consistent with measured salinity in the inlet at LW + 2.1 h a nd the occurrence of a critical inflow Froude number in the inlet neck at LW + 2.7 h. At this time a tongue of colour-difference waters was observed which extended about 400 m into the estuary and emanated from the intrusion front inside the inlet. The front moved through the nec k as flood currents increased, entered the estuary main channel at LW + 3.1 h and collapsed within the channel at LW + 3.3 h. The plunge poi nt then transferred to the tip of the colour-demarcation tongue, which subsequently developed a ''V'' head and migrated up-estuary with an a pparent speed that was approximately equal to the estimated difference between tidal and buoyancy current speeds (0.2 m s(-1)). Measured sal inity showed the horizontal, near-surface structure of the intrusion f ront when it was located approximately 1 km inside the estuary at LW 3.9 h. A lens of less saline surface water was observed slightly up-e stuary of this front, effectively trapped between the main channel and shoals. The front then remained effectively stationary until LW + 4.6 h, at which time it dissipated. Pronounced frontal structures that de fined the boundary of the low salinity lens, on the channel margins an d over the inundated shoals, remained however, and were surveyed at LW + 4.8 h. These frontal structures, and a diminished lens of fresher w aters, were observed remotely by aircraft at LW + 5.6 h, within 30 min of high-water. The effects of mixing, basal current circulations and variable channel depths, especially the occurrences of particularly de ep areas, are discussed. Centrifugal effects are shown to be important . (C) 1997 Elsevier Science B.V.