Sand dunes and ventifacts on the coast of South Africa

Abstract

The Indian Ocean coast of South Africa is characterized by strong seasonal alongshore winds and high sediment availability, resulting in formation of different types of active and vegetated sand dunes landward of sandy beaches. This study considers the dynamics of windblown sand systems along part of this coastal stretch by presenting geomorphological evidence from ventifacts and sand dunes, which can inform on integrated sand transport and depositional system dynamics, respectively. Crestline positions of supratidal translational dunes were mapped from aerial images (2003-2018); results show a range of responses over different time windows, including systematic migration in particular to the northeast, and crestline rotation. Changes in dune geometry are linked closely to changes in synoptic climate patterns, determining seasonal wind regimes. Exceptionally well developed ventifacts are present on clasts within deflation lags located between translational dune ridges. Based on morphological analysis of ventifacts (n=49), the dominant geomorphically-effective wind direction is from west or northwest, oblique to regional prevailing winds. An important conclusion of this study is that, although both ventifacts and dunes at this site reflect the same wind forcing, they show quite different morphological responses and dynamical behaviours. A reason for such differences may be the role of microtopographic effects on near-surface wind patterns. This means that neither dunes nor ventifacts alone can be used as unambiguous indicators of wind regime, and instead should be viewed as integrated elements of coastal sediment systems.

The morphodynamics of transverse dunes on the coast of South Africa

“…Tides are in the high microtidal/low mesotidal range throughout, and a strong swell wave regime (Hs >5 m) reflects wind forcing from the Southern Ocean (>5 m) (Wepener and Degger, 2019). Several studies have examined sandy beach and dune processes and dynamics in South Africa, and these link their morphodynamics to wind and wave regimes, including episodic storms (La Cock et al, 1992;Olivier and Garland, 2003;Mitchell et al, 2005;Corbella and Stretch, 2012;Guastella and Smith, 2014;Knight and Burningham, 2019).…”

“…As such, transverse dunes can be clearly distinguished from foredunes that are broadly aligned parallel to the shoreline and that are located at the back of the beach. Many studies have examined the nature of windflow over transverse dunes, from in situ field measurements and numerical modelling (van Dijk et al, 1999;Reffet et al, 2010;Melo et al, 2012;Araújo et al, 2013;Jiang et al, 2014;Jackson et al, 2020), but there are fewer studies of the morphometry and morphodynamics of transverse dunes (Hunter et al, 1983;Miguel and Castro, 2018;Knight and Burningham, 2019). This includes calculations of dune migration rates, based on field or remote sensing data, which can be linked to the development of the dune bedform with respect to wind forcing (Tsoar and Blumberg, 2002;Miguel and Castro, 2018;Knight and Burningham, 2019).…”

Characterisation of Coastal Sediment Properties from Spectral Reflectance Data

“…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).…”

Formation of the Wahiba Sand Sea in the Sultanate of Oman: implications of change in wind energy

“…By this mechanism, Hereher (2018) interpreted the formation of the Great Sand Sea in the Western Desert of Egypt by analyzing the trend of DP in relation to the dune forms of the desert. Many other studies elaborated the geomorphology and dune forms with wind energy of some dune fields (Fryberger et al 1984;Embabi 1986;Al-Awadhi et al 2005;Tsoar 2005;Yao et al 2007;Hereher 2014;Hamdan et al 2016;Louassa et al 2018;Knight and Burningham 2019). Wasson and Hyde (1983) provided a comprehensive interpretation to correlate drift potential, dune type, and sand supply.…”