Water mass mixing in the estuary of the River Don and its associated coastal waters
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 interpret those factors that affect these patterns. Fieldwork consisted mainly of boat borne surveys of salinity, temperature and currents. This programme was augmented with air based visual observations, and a photogrammetric survey of surface water movement using fluorescent dyes. River flow data was acquired from the North East River Purification Board, and wind records from Total Oil Marine Ltd., Altens. Estuarine mouth configuration was monitored via bimonthly tacheometric surveys. The estuary was found to be highly stratified under mean conditions, a distinct saltwedge penetrating upstream beneath seaward flowing freshwater during the flooding tide. The extent of this saline intrusion depends upon the freshwater head, as represented by river flow; and the tidal head, as represented by the height of high water. During the ebbing tide the saline waters are completely flushed from the estuary, except where retained within the depths of 'potholes'. Under particularly high river flows saline penetration is prevented at all stages of the tide. Mixing during the flooding tide was by entrainment of freshwater down into the advancing salt wedge. However, 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 seawards from the estuary mouth, forming a thin, highly buoyant plume. As a result of buoyant spread and inertia induced turbulence, mixing of these waters is intense in the zone immediately seaward of the mouth. However, mixing is not complete and the waters form a thin surface layer which may preserve its identity for several kilometres beyond this initial zone. The direction of spread of this buoyant discharge is chiefly determined by the tidal stream, although initially by the geometry of the estuary mouth. During the ebbing tide the plume forms to the north of the mouth, and during the flood, to the south. Wind and wave induced currents may enhance or restrict the spread of these surface waters. Storm wave activity, causing mixing of estuarine discharge as soon as it leaves the outlet, precludes the formation of a buoyant plume. Restricted estuarine discharge during the flood tide, and enhanced discharge during the ebb, result in the ebb plume being a considerably more extensive feature. The plume was often demarcated by a sharp thermohaline front marked by a distinct colour change. The front was a zone of strong surface convergence, and, consequently, was often demarcated by a collection of foam and floating debris.