Water mass mixing in the estuary of the River Don and its associated coastal waters
Grant, Peter H.
PublisherUniversity of Aberdeen
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The River Don discharges into the North Sea by way of a small estuary on the northern outskirts of the city of Aberdeen. This thesis aims to establish the patterns of mixing of river and seawater within the estuary and its associated coastal waters, and to identify those factors governing this pattern. The estuary was found to be highly stratified under mean conditions, a distinct saltwedge penetrating upstream beneath seaward flowing freshwater during the flooding tide. During the ebb tide saline waters are completely flushed from the estuary, except where retained at depth in 'potholes'. Under particularly high river flows saline penetration of the estuary is prevented at all stages of the tide. Mixing during the flooding tide was by entrainment of fresh water down into the advancing saline waters. Only during the ebbing tide does a high level of mixing occur, as a result of increased turbulent diffusion, causing a rise in surface salinity. Under most conditions, therefore, fresh or brackish water spreads seaward from the estuary mouth, forming a thin bouyant plume. Mixing of plume waters is initially rapid due to turbulence induced by inertia and bouyant spread. However, brackish waters spread beyond this zone, forming a thin surface layer slowly mixing with the coastal waters. These brackish waters may maintain their identity for several kilometres. The direction of spread of this bouyant discharge is chiefly determined by tidal currents, flowing north on the ebb and south on the flood. Wind and wave induced currents may restrict or enhance plume spread. Storm wave activity, causing rapid mixing, precludes the formation of a bouyant plume. The plume was often demarcated by a sharp thermohaline front; a zone of strong surface convergence marked by a collection of foam and debris.