Aircraft and sea-truth observations of a small-scale estuarine intrusion front
Uncles, R.J.; Stephens, J.A.; Murphy, R.J. (1997). Aircraft and sea-truth observations of a small-scale estuarine intrusion front. J. Mar. Syst. 12(1-4): 199-219. https://dx.doi.org/10.1016/S0924-7963(96)00098-X
In: Journal of Marine Systems. Elsevier: Tokyo; Oxford; New York; Amsterdam. ISSN 0924-7963; e-ISSN 1879-1573
Also appears in:
Ruddick, K. (1997). Processes in regions of freshwater influence (PROFILE): selected papers from the 27th International Liège Colloquium on Ocean Hydrodynamics, held in Liège, Belgium, on May 8-12, 1995. Journal of Marine Systems, 12(1-4). Elsevier: The Netherlands. 1-326 pp., more
Results are presented from an unusually detailed survey of the temporal and spatial behaviour of an estuarine tidal intrusion front (Simpson, J.H., Nunes, R.A., 1981. Estuar. Coast. Shelf Sci. 13, 257-266). The front was observed as it propagated through the inlet and moved into the main channel of a small estuary (Tweed Estuary, UK) during a flooding spring tide. High salinity coastal waters plunged beneath low salinity estuarine waters, the plunge lines defining a “V” shaped front 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 conditions in September 1993 also are used to aid interpretation. A convex-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 currents at mid-water, a “V” shaped tidal intrusion front was observed at the neck of the inlet, with the “V” pointing up-estuary. These data are consistent with measured salinity in the inlet at LW + 2.1 h and 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 neck 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 point then transferred to the tip of the colour-demarcation tongue, which subsequently developed a “V” head and migrated up-estuary with an apparent speed that was approximately equal to the estimated difference between tidal and buoyancy current speeds (0.2 m s−1). Measured salinity showed the horizontal, near-surface structure of the intrusion front 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-estuary 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 defined the boundary of the low salinity lens, on the channel margins and over the inundated shoals, remained however, and were surveyed at LW + 4.8 h. These frontal structures, and a diminished lens of fresher waters, 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 deep areas, are discussed. Centrifugal effects are shown to be important.
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