Sand dunes and ventifacts on the coast of South Africa

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.