Contrasting styles of swell-driven coastal erosion: examples from KwaZulu-Natal, South Africa

Abstract

During 2006During -2007, the KwaZulu-Natal coast of South Africa was exposed to several large swell events (H o > 3 m), near the peak of the lunar nodal cycle, causing shoreline recession. The largest swell (H s = 8.5 m) struck the coast on the March equinox (18th-20th) and generated a strong storm-return flow. Observations made before, during and after record dramatic coastal erosion (shoreline recession of up to 40 m and substantial property damage). This swell event removed the semi-continuous nearshore bar system and 'conditioned' the coast such that lesser subsequent swell events accomplished much greater amounts of coastal erosion than expected (up to 100 m at certain erosion hotspots) because waves reached the coast without significant energy dissipation. Subsequent bar generation rebuilt the inshore bars within six months. The styles of erosion during the March '07 event and other 2007 swells were markedly different. Lesser swells are focused by headlands and result in megarip development and activation of erosion hotspots. The March '07 event still-water level was raised (equinoctial spring high tide and a storm surge of 0.33-0.45 m) to a level that rendered most headlands (and erosion hotspots) ineffective and resulted in laterally extensive erosion of soft shorelines. Results record cumulative effects of successive swell events on coastal behaviour that proved to be critical in enabling erosion to proceed at rapid rates after the coast had been initially destabilized. Unlike hurricanes and tsunamis, surges associated with swell events are relatively minor and therefore extensive erosion is linked with high lunar tides. There is circumstantial evidence that swell-induced erosion follows the broad 18.6 yr lunar nodal tidal cycle when the chances of large swells coinciding with high water levels are increased.

Extreme erosion on high-energy embayed beaches: Influence of megarips and storm grouping

“…Clérigo and Amoreira beaches by megarips was presumably transported to depths between 10 to 15 m, based on schematic mapping (Figs. 13-15), and spread out on the lower shoreface, as observed elsewhere by Coutts-Smith (2004) and Smith et al (2010), resulting in net loss of sediment from beaches. There were, however, significant differences between the study beaches.…”

“…Nevertheless, under the action of the January-February 2009 storm group the relationship is remarkable. Storm groups, or sequences of storms without significant recovery in between (Ferreira, 2006), have long been recognised as an important mechanism for extreme erosion of embayed beaches (Thom, 1974), driving dramatic shoreline retreat on erosion hotspots (Smith et al, 2010), or triggering erosion-dominated periods of several years (Thom and Hall, 1991). Considering that the most significant beach cut on short embayed beaches is produced by megarips (Wright, 1981), their maintenance during storm groups would be expected to produce extreme and continued erosion.…”

“…Sediment removed from the beach by megarips has been shown to be deposited at depths of 10-25 m (Coutts -Smith, 2004;Smith et al, 2010), owing to seaward extensions of approximately 1.5 to 4 times the surf zone width (Coutts- Smith, 2004;Dalrymple et al, 2011), or between 1 and 2 km (Short, 2010). Sand withdrawn from Arrifana, Mt.…”

“…During storm groups, sediment lost from the beach and the upper shoreface is expected to be even greater than for single storm events (Cowell et al, 1999), due to inability to reach a storm equilibrium profile during storm groups (Inman et al, 1993) and continued conditions for megarip-driven erosion as a result of coastal disequilibrium (Smith et al, 2010). While megarips developed during the first year of monitoring caused moderate erosion, even with the January 2008 extreme single storm, no significant net volumetric loss was observed.…”

“…Thom, 1974;Short and Hesp, 1982), and offshore sediment transport from beaches (e.g. Coutts -Smith, 2004;Smith et al, 2010).…”