Mapping of tidal currents in the vicinity of an offshore sandbank, using remotely sensed imagery

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Abstract
Localized flow patterns in the vicinity of a headland-associated linear sandbank (the Scarweather Sands, northern Bristol Channel), cannot be detected in temporally averaged observations using conventional oceanographic measuring techniques. Airborne Thematic Mapper (ATM) data has been used previously to identify such patterns at one particular stage of a tidal cycle, using suspended sediments in the surface waters as passive tracers. The present contribution describes tidally varying flow patterns around the sands, based upon the interpretation of ATM imagery over a large proportion (8 hours) of a tidal cycle. Comparisons are made between flow patterns identified from the imagery and (a) surface vector current measurements using high frequency (HF) radar, (b) predicted tidal currents and (c) field observations made at the times of the aircraft overpasses. There is shown to be good correlation between the results obtained using the different methods. The information derived enhances the understanding of the mechanisms of formation and maintenance of this and other similar linear sandbanks. During the latter stages of the tidal cycle (towards low water) over which data were collected, the airborne imagery was affected by Sun-glitter, The presence of such a feature introduces errors in the quantitative evaluation of suspended sediment and chlorophyll in surface waters, in this and other investigations elsewhere.