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.

Intermittent breaching of sand barriers at temporary open estuaries plays a key role in the functioning of these systems. Breaching events lead to large and rapid changes in the physico-chemical environment which in turn triggers major biological responses. The breaching process can cause significant morphological changes as strong breach outflows can scour large quantities of accumulated sediments from an estuary. Simple laboratory experiments are reported that investigate the temporal evolution of the breach and the scaling of the breach characteristics namely the breach width W, volume Vb, formation time Tf and peak outflow Qp. The experiments were specifically designed to investigate the influence of the outflow volume S, the hydraulic head H and the barrier breadth B on the breach characteristics. The breach width W was found to be proportional to S1/3, whilst the breach volume Vb was found to be proportional to HBS1/ 3. The breach formation time Tf was found to be proportional to (g/S1/3)-1/2 (H/S1/3)-3/2 (B/S1/3)1 and the peak outflow Qp was found to be proportional to (gS5/3)1/2 (H/S1/3)3/2 (B/S1/3)-1. These scalings are also shown to be consistent with observed breach characteristics for actual estuaries and earth dam failures where outflow volumes are typically six orders of magnitude larger than for the models. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.

Salt marshes are some of the most productive ecosystems in the world and have been the centre of attention over the past few decades, due to their decline as a result of global climate change and anthropogenic impacts. The growth of salt marshes is determined by substrate type, soil conductivity and elevation. The permanently open Keurbooms Estuary along the south-east coast of South Africa is subjected to occasional fluvial flooding and its intertidal area lacks well developed salt marshes, with Spartina maritima restricted to the lower reaches of the Bitou tributary and a few sections of the Keurbooms tributary. Presumeably because of fine sediment habitat in the confluence and lower Bitou tributary. The salinity of the estuarine water ranges between 0.1 – 26.9 and 3.2 – 35.3 in the Bitou and Keurbooms tributaries respectively. A typical salt wedge salinity pattern is common in the Keurbooms tributary where saline water often intrudes underneath the freshwater, especially during high river flows. The following hypotheses were developed and tested in this study: The limited spatial distribution of S. maritima in the Keurbooms Estuary is due to limited availability of fine sediment habitat; and the source of the fine sediment in the estuary is the Bitou tributary rather than the Keurbooms tributary or the sea. It was further postulated that after sediment characteristics, floods are the major hydrological driver determining the distribution of S. maritima in the Keurbooms Estuary. The results of the surveys of the estuarine channel bottom sediments showed that the Keurbooms tributary was mostly characterized by the sand-size sediment fraction derived from the feldspathic and sandstone with evidence of fine sediment fractions restricted to the upper reaches at the confluence with Whiskey Creek. The Bitou was almost always composed of coarse sized sediments in the upper reaches, fine sediment deposits in the middle and lower reaches and medium sorted sand with almost no clay or calcium carbonate in the estuarine component below the confluence of the tributaries. These findings were further supported by the surface sediment deposited within the S. maritima intertidal salt marsh, which showed finer sediment deposits in the Bitou marsh compared to the Keurbooms marsh surface. Similar results were also found in the sediment cores, with the Keurbooms marsh sediment becoming finer with increasing depth whereas fine sediments reduced with depth in the Bitou marsh. The results of the sediment mineralogy indicated that the increased concentrations of clay minerals in the S. maritima surface sediments are derived from the Bokkeveld shale, siltstone and clay slate exposed above the N2 Bridge in the Keurbooms Estuary. GIS mapping shows that S. maritima has been declining over the past two decades, with rapid decreases especially evident after big flooding events. The GIS mapping also indicates that the patches of the S. maritima in the Keurbooms tributary are more exposed to big floods than the Bitou marsh. Despite showing an overall decline, S. maritima area coverage remained more consistent in the lower reaches of the Bitou tributary than in the Keurbooms tributary. Despite the larger and more persistent area cover, the S. maritima plants were shorter and less dense than the plants growing in the sandy substrate. The black/grey colouration of soil with increasing depth in the Bitou tributary was an indication of the reduced state of the soil caused by prolonged waterlogged conditions. The roots of S. maritima in both tributaries were mostly restricted to the sub-surface substrate layer (i.e. 0 – 0.25 m), although the Bitou populations showed more vegetative propagation than the Keurbooms populations. This mechanism of reproduction was also demonstrated during the transplant experiment which showed a greater number of new stem production in the fine sediment substrates compared to the sandy silt substrates. Although accretion rates were not determined in this study, the short-term sediment deposition rates revealed that sedimentation is active in the marshes of the Keurbooms Estuary. Therefore, in spite of showing a decline in area cover, the production of viable seed and observed vegetative propagation suggest that the S. maritima is likely to colonize open stable intertidal mudflats / sandflats, thus maintaining its distribution as an intertidal species in the salt marshes of the Keurbooms Estuary.

Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 79-84). / Estuarine density stratification may be controlled primarily by cross-shore processes (analogous to longitudinal control in narrow estuaries), or by both cross- and alongshore processes (typical of coastal plumes). Here field observations and numerical modeling are used to investigate stratification on the low-sloped, periodically inundated Skagit Bay tidal flats. Advection of stratification by the depth-averaged velocity, straining of the horizontal density gradient by velocity shear, and turbulent mixing are shown to be the dominant processes. On the south-central flats (near the south fork river mouth) velocities are roughly rectilinear, and the largest terms are in the major velocity direction (roughly cross-shore). However, on the north flats (near the north fork river mouth), velocity ellipses are nearly circular owing to strong alongshore tidal flows and alongshore stratification processes are important. Stratification was largest in areas where velocities and density gradients were aligned. The maximum stratification occurred during the prolonged high water of nearly diurnal tides when advection and straining with relatively weak flows increased stratification with little mixing. Simulations suggest that the dominance of straining (increasing stratification) or mixing (decreasing stratification) on ebb tides depends on the instantaneous Simpson number being above or below unity. / by Vera L. Pavel. / Ph.D.