The impact of climate change effects on the planform of a headland-bay beach on the southern coast of South Africa

[摘要] ENGLISH ABSTRACT: The various consequences of climate change pose a significant threat to developments near thecoast. These threats include saltwater intrusion, coastal erosion and flooding. In the coastal context,the climate change effect often raising the most concern is that of sea-level rise. Much work hastherefore been done on the linear setback caused by a rise in sea-level. In order to get the fullpicture of possible changes caused by sea-level rise, the secondary effects of a rising sea-level alsoneed to be considered. Sea-level rise could cause changes to the nearshore wave climate and couldhave impacts such as coastal erosion and changes to the coastline shape. The primary objective ofthis study was therefore to investigate the effects of sea-level rise on the nearshore wave climateand, consequently, the coastline stability.Other consequences of climate change considered in this study include increasing average waveheights and a rotation of offshore wave directions. The many headland-bay beaches on the SouthAfrican coastline are generally in a state of dynamic equilibrium and find their planforms based onthe local wave climate. Changes to the wave climate may therefore disrupt the equilibrium shapes ofthese bays. This study was therefore also aimed at investigating the effects of the changes to thewave climate on the stability of headland-bay beaches.The three consequences of climate change expected to affect the nearshore wave climate wereidentified as (1) sea-level rise; (2) an increased wave height; and (3) changing offshore wave angles.Although changes to storm frequency and intensity are also possible, the impacts of these changeswere not studied.In order to assess the impacts of the three considered changes on a typical headland-bay beach, twonumerical models were set up for Mossel Bay – a headland-bay beach on the southern coast ofSouth Africa. The modelling approach included a wave transformation model to calculate nearshorewave climates from offshore data and a coastline model to assess the stability of the bay under thechanged nearshore wave climates.The model results indicated that the rising sea-level alone would cause changes in the nearshorewave direction. These changes were shown to alter the longshore sediment transport regime suchthat rotations are expected in the south-western corner and eastern end of Mossel Bay. These rotations do not include the cross-shore effects of inundation and erosion, as suggested by modelssuch as the Bruun Model.The results for an increased offshore wave height were inconclusive. The southerly rotation inoffshore wave angles was shown to affect the nearshore wave angles. These changes affected the longshore transport regime such that the outward sediment transports were reduced. A minoraccretion resulted in the centre of the bay for a 1° southerly rotation in offshore wave angles. For a2° rotation, the extent of accretion increased and shifted towards the eastern end of the bay,primarily due to the dominance of south-westerly waves in the local wave climate.A valuable observation was made regarding the current stability of Mossel Bay. Inter-tidal reefs arepresent along three sections of the bay. These reefs protect the coastline such that the current bayshape contains sharp bends between the reefs. Under a rising sea-level, however, the effect of thereefs will become less pronounced. If a water level should be reached where these reefs becomeless significant, the planform of the bay is expected to smooth out through a significantredistribution of sediment. This smoothing effect was shown to cause erosion of the coastline in theorder of 80m near the town of Klein Brak River.

[发布机构] Stellenbosch University