A study of the airflow on the windward slope of a transverse dune in the Alexandria coastal dunefield
- Authors: Burkinshaw, Jennifer Ruth
- Date: 2021-04
- Subjects: Port Elizabeth (South Africa) , Eastern Cape (South Africa) , South Africa
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/52734 , vital:43883
- Description: Our understanding of the evolution of dune morphology has been hampered by a lack of empirical observations of airflow behaviour over dune forms. Sand dunes intrude into the atmospheric boundary layer and convergence of streamlines results in an acceleration of airflow up the windward slopes of dunes. This study examines the airflow structure and corresponding bedform development on the windward slope of a 7 m high transverse dune on the edge of the Alexandria coastal dunefield, Algoa Bay, South Africa. The Alexandria dunefield is subjected to a trimodal wind regime, consisting of the dominant south-westerly which blows all year round, summer easterlies and winter northwesterlies. The morphology of the study dune, Dune13, is controlled by the easterlies and north-westerlies, and reverses seasonally with respect to these two winds. Seven section lines 30 m apart and normal to the dune crest were surveyed regularly over the period of a year to monitor the reversal process. Three detailed topographic surveys were also done during this period. Airflow behaviour was monitored during the year. Wind speed profiles on the windward slope of the dune were measured using 4 to 5 vertical arrays of anemometers positioned from the base of the dune to the crest on a 1 selected section line. Usually 4 to 5 anemometers were deployed in each vertical array, from a height of 6 to 10 cm above the surface, up to a height of 150 cm above the surface. Initially 8 microanemometers were available; ultimately 28 anemometers were run simultaneously. An independent weather station at an elevation of 6 m recorded the unaccelerated flow. Local gradient measurements and erosion and deposition rates were recorded along selected section lines. Strong summer easterly winds (14 m/sec at 1.4 m above the dune crest) were measured on a dune slope in the process of being transformed from a slipface to a stoss slope. The following winter, light north-westerly winds (typically B m/sec at 1.6 m above the dune crest) were measured on the new windward slope already reversed by the prevailing winter wind. Airflow data confirm the compression of airflow against the windward slope resulting in a non-logarithmic wind speed profile. Compression results in an increased shear velocity within 30 cm of the dune surface, and the dune slope is eroded. Higher up in the wind speed profile, shear velocity decreases to 0.1 m/sec. It is not known at what height the wind speed profile recovers from the intrusion of the dune into the boundary layer. High values of shear velocity (1.6 m/sec) above the rounded crestal area of the dune record the recovery of the wind speed profile from flow divergence, which is a response to the rapid reduction of dune gradient and is accompanied by deposition of sand in this region. 2 The erosion pin data act as a simple and sensitive test for changes in gradient, reflecting the dune's response to changes in the airflow regime. The shape of the dune plays a major role in determining the extent of the compression and the distribution of shear velocity up the slope. Increased shear velocity is experienced on that part of the slope which is nonaerodynamic with respect to the prevailing wind. Under unidirectional conditions, feedback between flow and form results ultimately in a slope with a curvature such that shear velocity increases systematically upslope. The survey data and erosion pin data record the reversal process as the dune achieves a new steady state during each wind season. The existence of a non-logarithmic wind speed profile makes it difficult to know what relevant measure of shear velocity is to be used in sand transport equations. Future work should include wind speed measurements within 10 cm of the surface. An ideal study modelling aeolian bedform development would utilise wind tunnel measurements, combined with field measurements such as obtained in this study, for comparison with numerical modelling. The study needs to be extended to 3-dimensional airflow measurements. , Thesis (MSc) -- Faculty of Science, School of Environmental Sciences, 2021
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- Date Issued: 2021-04
Morphodynamics of headland bypass dunefields, with special reference to the Cape St Francis headland, Eastern Cape, South Africa
- Authors: Burkinshaw, Jennifer Ruth
- Date: 1998-01
- Subjects: Sand dunes -- South Africa -- Eastern Cape -- Cape St Francis
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/52814 , vital:44206
- Description: The temperate Cape south coast, South Africa, is characterised by a series of log-spiral bays bound by low-relief rocky headlands. In South Africa corridors of transverse dunes crossing the headlands are referred to as headland bypass dunefields (HBDs). Sand blown off upwind beaches is transported across the headlands towards the downwind bays, bypassing the littoral route around the headlands. The key to the development of the HBDs is the presence of sandy beaches upwind of the headlands which lie athwart the prevailing winds. The low relief of the headlands and their shoreline configuration, coupled with their exposure to the marine wind regime, are such that the predominant high energy, westerly winds blow directly onshore. The local longshore drift system plays a critical role in replenishing beach sand deflated by the wind. The "type" HBDs occur in the Eastern Cape on the prominent Cape St Francis and Cape Recife headlands where HBDs have been active since at least the Mid to Late Pleistocene. The dunefields have length/width ratios ranging from 3 to over 10. Sand coverage within HBDs is metasaturated, i.e., the transverse dunes are separated by well-defined interdune areas. Average dune height in the larger dunefields in the Eastern Cape is 9-10 m. The HBDs are commonly bound by sidewalls that develop along the interface between the dunefields and vegetation. The stabilised sidewalls provide the most prominent record of former HBD activity and can influence subsequent HBD morphology. The Oyster Bay dunefield on the Cape St Francis headland was chosen for a study of the aeolian morphodynamic processes that operate within these systems. The dunefield currently operates over a distance of about 14 km and a width of 500 - 1200 m; it is at present cut off from its sand source. Shallow lakes develop in the interdune areas after periods of high rainfall. A 2 week airflow study recorded wind speed along the prevailing wind axis of the Oyster Bay dunefield during spring 1990. Wind speed increased significantly downstream until at least the mid-dunefield area during both the prevailing westerly and seasonal easterly winds. The downwind wind speed increase can be partially attributed to topographic acceleration of airflow as it is compressed over the low cape. The extent of the wind speed increase can vary depending on the land/sea temperature contrasts and the temperature structure of the lower atmosphere. Wind speed recorded across the width of the dunefield was slowed down along the margins by vegetation, while a faster core moved down the centre of the dunefield. Rates of dune movement along the prevailing wind axis show a general correspondence between the volume of sand transported eastwards in different areas of the dunefield and the airflow pattern down the length of the dunefield. Sand is transported most efficiently in the centre of the dunefield where westerly flow is at its maximum. Sand transport calculated from dune migration rates is about 50% of potential sand transport calculated from wind data, as flow is metasaturated. Flow off sandy beaches may be naturally below saturation owing to the effect of intertidal moisture and salts binding the sand. In the Eastern Cape metasaturation can be further enhanced by the downwind entrapment of sand by vegetation along the dunefield margins and within wet interdune areas. The 2-dimensional model for bedform development within HBDs focuses on the downwind extension of the dunefields and transmission of sand through the systems. The combination of the high transport capacity of the wind downwind and metasaturated flow maintains the transport of sand across headlands. HBDs operate aerodynamically as erosional or sediment bypassing systems although deflation of the wet interdune areas is limited. Fully functional short HBDs are the most likely to operate with neutral sediment budgets. Long systems are more susceptible to spatial and temporal effects that modify the sandflux through the systems and hence the outflux from the systems. The Oyster Bay dunefield operates with a mildly positive sediment budget because of sediment entrapment downwind. The morphology and morphodynamic behaviour of HBDs is in contrast to that of the tabular transgressive dunefields that form parallel to the shores of the large log-spiral bays of Algoa Bay and St Francis Bay. The existing morphological classification of transgressive dunefields needs to be revised to include headland bypass dunefields. Keywords: headland bypass dunefields, transgressive coastal dunefields, aeolian morphodynamics, Cape St Francis, Cape south coast. , Thesis (PhD) -- Faculty of Science, 1998
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- Date Issued: 1998-01