Multi-annual nearshore evolution of a mega-nourishment in the southern coast of Namibia

Increasing pressures on coastal systems due to rising sea levels, extreme events, and human modification of coastal areas have led to accelerating erosion around many of the world’s coasts. Climate change and expanding coastal populations will worsen coastal pressures, creating a strong demand for engineering solutions that safeguard assets and infrastructure. Beach nourishment – the artificial placement of sand on the coast to increase the width and volume of a beach – is increasingly used as an engineering solution for sandy coasts, with  engineers and managers now focusing on mega-nourishments involving the placement of tens of millions of cubic meters of sand. As scale and complexity of nourishments increases,  the evolution of these highly modified coasts becomes more uncertain, particularly in areas where nourishments create strong disequilibria. In the southern coast of Namibia, diamond mining operations rely on the continuous seaward advance of the shoreline, achieved by artificial placement of large volumes of sediment along the coast. This can be characterised as a continuously prograding mega-nourishment, which over the last decades created the largest sandy beach accretion hotspot in the world. The complexity of such unprecedented shoreline advance, which is achieved through sediment discharge of unprecedented volumes of sediment in multiple locations using different methods, is further compounded by the very energetic wave climate that characterises the coast of southern Namibia, the variable bathymetric configuration of the nearshore that is masked by an apparent large-scale uniformity along the coast, and the notable variation in sediment characteristics, which range from fine sand to very coarse gravel and boulders. Analysis of regular nearshore bathymetric surveys performed since 2013 revealed variable morpho-sedimentary dynamics in this coastal system, with three main sectors with characteristic nearshore morphological change patterns. These range from progressive nearshore accretion in the northern section, despite significant local variation linked to morphological changes in nearshore bar systems, a strongly erosive nearshore in the central section where coarser sediment has been introduced, leading to changes in nearshore seabed composition that enhanced sediment transport potential due to hiding and exposure effects, and a southern section characterised by accretion in the upper nearshore and erosion in the lower nearshore, consistent with the redevelopment of an outer bar system, promoted by offshore-onshore transference of sediment. The variable morpho-sedimentary change patterns identified in the Namibian mega-nourishment provide novel insights regarding the nearshore evolution of highly dynamic and modified sandy coasts.