Morphodynamics of headland bypass dunefields, with special reference to the Cape St Francis headland, Eastern Cape, South Africa

Burkinshaw, Jennifer Ruth

Title: Morphodynamics of headland bypass dunefields, with special reference to the Cape St Francis headland, Eastern Cape, South Africa
Creator: Burkinshaw, Jennifer Ruth
Subject: Sand dunes -- South Africa -- Eastern Cape -- Cape St Francis
Date Issued: 1998-01
Date: 1998-01
Type: Doctoral theses
Type: text
Identifier: http://hdl.handle.net/10948/52814
Identifier: 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.
Description: Thesis (PhD) -- Faculty of Science, 1998
Format: computer
Format: online resource
Format: pdf
Format: 1 online resource (384 pages)
Publisher: University
Publisher: Faculty of Port Elizabeth
Language: English
Rights: University of Port Elizabeth
Rights: All Rights Reserved
Rights: Open Access

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