The morphodynamics of transverse dunes on the coast of South Africa

Abstract

Transverse sand dunes located within the supratidal zones of beaches are a significant geomorphic feature along sand-dominated coasts worldwide, and are generated by strong alongshore winds in areas of high sediment availability. Transverse dunes are present along the South African coast and these are known to migrate dynamically in response to wind forcing. However, the detailed dynamics of individual dune systems along the same coastal stretch have not been compared to one another, and the relationship of transverse dunes to their hosting beach systems has also not been examined. This study examines the properties and dynamics of transverse supratidal dunes from three systems along the coast of South Africa, using remote sensing methods. Results show that, although the underlying beach system appears to be relatively stable over the time period of analysis, there is a dominant aeolian-driven migration of transverse dunes towards the northeast, following prevailing wind direction, countered by less dominant movement to the southwest. There is also considerable variations in calculated annual dune migration rates between adjacent systems, between summer and winter seasons, and between dunes within a single site. This highlights that, although beach and dune landforms can be conceptually considered as part of the same sediment system, there is not a clear relationship between phases of beach aggradation and phases of dune aggradation. Instead, a primary control appears to be beachface erosion by waves that reduces beach width and influences dune morphodynamics, independent of sediment supply.

Characterisation of Coastal Sediment Properties from Spectral Reflectance Data

“…Bedrock headlands to the east and west define the overall shape of the bay. An extensive supratidal zone is present, containing parallel-aligned transverse dunes with crests that are 40-50 m apart, similar to those found elsewhere along the South African coast [39]. Dune migration periodically blocks off the mouth of the incoming Klipdrift River.…”

“…The presence of a wave-dominated shoreface and wide supratidal zone with migrating transverse dunes (Figures 1 and 4) is typical of the south-and southeast-facing South African coast, in which the dunes have a net eastward migration rate of some 3-12 m yr −1 [39,48,49]. Sediment grain size analysis of field samples from Oyster Bay shows that overall they are fairly texturally uniform but with some significant differences in properties between different beach-dune sub-environments (Table 2) and a wide range of carbonate contents (Table 1).…”

“…The relatively limited textural and compositional differences mean that it is sometimes difficult to distinguish between such coastal samples, especially in wave-dominated shoreface environments and wind-affected supratidal environments, where sediments are relatively well sorted e.g., [2,4,11,52]. This is certainly the case with beach-dune sediments along the South African coast, e.g., [39,45,49,51,53]. However, detailed statistical analysis shows that different landforms and beach-dune settings at Oyster Bay have different sediment properties (Table 2).…”

“…African coast [39]. Dune migration periodically blocks off the mouth of the incoming Klipdrift River.…”

The Green Infrastructure of Sandy Coastlines: A Nature-Based Solution for Mitigation of Climate Change Risks

“…Along coastlines, understanding of climate forcing-response relationships are of particular relevance for several reasons. They can: (1) help in conceptualizing and predicting the future evolution of coastal systems under different climate change scenarios; (2) enable The forcing-response relationships between coastlines and climate have been examined in many different contexts, including with respect to patterns and rates of coast erosion; changes in beach, dune and barrier morphology; sediment flux calculation; flood inundation risk; and coastal ecosystems [16][17][18][19][20]. The co-variability between coastal change and the different climatic forcing factors that impact the coastline (sea level, storms, waves) has been used to construct empirical models that, in theory, can then be used to predict coastline responses under changes in coastal forcing [21][22][23][24].…”