Research Letter KwaZulu-Natal coastal erosion: A predictive tool? Page 1 of 4 KwaZulu-Natal coastal erosion events of 2006/2007 AUTHORS: and 2011: A predictive tool? Alan Smith' Lisa A. Guastella^ Severe coastal erosion occurred along the KwaZulu-Natal coastline between mid-May and November 2011. Andrew A. Mather^ Analysis of this erosion event and comparison with previous coastal erosion events in 2006/2007 offered the Simon C. Bundy" opportunity to extend the understanding of the time and place of coastal erosion strikes. The swells that drove Ivan D. Haigh* the erosion hotspots of the 2011 erosion season were relatively low (significant wave heights were between AFFILIATIONS: 2 m and 4.5 m) but ot long duration. Although swell height was important, swell-propagation direction and 'School of Geological Sciences, particularly swell duration played a dominant role in driving the 2011 erosion event. Two erosion hotspot types University of KwaZulu-Natal, were noted: sandy beaches underlain by shallow bedrock and thick sandy beaches. The former are triggered Durban, South Africa by high swells (as in March 2007) and austral winter erosion events (such as in 2006, 2007 and 2011). ^Oceanography Department, University of Cape Town, Cape The latter become evident later in the austral winter erosion cycle. Both types were associated with subtidal Town, South Africa shore-normal channels seaward of megacusps, themselves linked to megarip current heads. This 2011 ^Ethekwini Municipality, Durban, coastal erosion event occurred during a year in which the lunar perigee sub-harmonic cycle (a ±4.4-year South Africa cycle) peaked, a pattern which appears to have recurred on the KwaZulu-Natal coast. If this pattern proves "Sustainable Development true, severe coastal erosion may be expected in 2015. Evidence indicates that coastal erosion is driven by Projects CC, Ballito, South Africa the lunar nodal cycle peak but that adjacent lunar perigee sub-harmonic peaks can also cause severe coastal ^Ocean and Earth Science, erosion. Knowing where and when coastal erosion may occur is vital for coastal managers and planners. University of Southampton, National Oceanography Centre, Southampton, United Kingdom Introduction CORRESPONDENCE TO: The KwaZulu-Natal coastline (Figure 1 ) is subject to coastal erosion - an ongoing process that has occurred Alan Smith throughout the latest Quaternary transgression. The sea level has risen 130 m since the Last Glacial Maximum at 18 000 years BR'^^ Sea-level rise,^" increasing storminess^'^ and increasing coastal urbanisation' will certainly EMAIL: contribute to increased future coastal erosion and the increased infrastructural costs thereof. [email protected] POSTAL ADDRESS: 29 Brown's Grove Sherwood, Durban 4091, South Africa S^ Richards Bay DATES: Received: 31 Oct. 2011 Revised: 25 Sep. 2012 Accepted: 09 Oct. 2012 _ /^ /Sheffield KEYWORDS: /^ ^ (»LittleMaritzburg coastal erosion: megarip C^ ^^^Ä^ Willards currents; lunar nodal cycle; lunar perigean subharmonic; erosion hotspots; coastal ^ .:: ^^^^^^^V^Umhlanga erosion prediction Y. ^^^^^^^^r Durban <-. i^^^^^^Anstey's HOW TO CITE: Smith A, Guastella LA, Mather V^r^ic^ jii ^r Amanzimtoti AA, BundySC, HaighlD. ? Sm KwaZulu-Natal coastal erosion f ^W Umkomaas events of 2006/2007 and 2011; / . Tscottburg A predictive tool? S Afr J Sei. ^f •• f'Submarine Bay 2013:109(3/4), Art #0025, 4 y^ /sezela / ^Mtwalume pages. http;//dx.doi.org/10.1590/ sais.2013/20120025 ^^^^P «^PortShepstone ^^^^•^ y St Michael's *v~j_ «FUvongo ^ /Margate ¿ / V« Port Edward ^^_ 1^ 100 Kilometres / ^^ Figure 1 : Map showing the location of erosion hotspots along the KwaZulu-Natal coast. ©2013. The Authors. Published under a Creative Knowledge of where and when erosion will strike is vital to coastal managers." Recent work^ has highlighted the Commons Attribution Licence. KwaZulu-Natal coast's vulnerability to erosion. However, this work did not address the reasons for this vulnerability. South African Journal of Science Volume 109 I Number 3/4 http://www.sajs.co.za 72 March/April 2013 Researeh Letter KwaZuiu-Natai coastai erosion: A predictive tooi? Page 2 of 4 nor did it assess the oeeurrenee, future iikelihood and prevaienee of the Austral winter erosion event of 2011 phenomenon, in this paper, we aim to assess eoastal erosion aiong the KwaZuiu-Natai coast from these perspectives. Austrai winter erosion aiong the KwaZuiu-Natai coastiine is typicaiiy compiete by September, but the 2011 erosion event continued to the end The KwaZuiu-Natai coastline is subject to a net south-to-north of November Beach webcam imagery'* and fieidwork indicates that the iittorai drift.'"-'^ The northeriy fiow dominates but reversais occur onset of erosion can be piaced in the iate austrai autumn - mid-May for (see 15-19 Juiy 2011 sweii account), especially in summer as a resuit of Amanzimtoti and Ansteys Beach (south of Durban) and eariy June for the greater number of easteriy winds and sweiis. The iittorai drift is driven Umdioti, Umhianga and Westbrook (north of Durban) (Figurei). by south to southeasteriy sweiis which are generaiiy higher in winter*'^ This seasonaiity drives a net beach rotation" within the topographicaiiy The highest significant wave heights (HJ of the 2011 coastai erosion bound bays (smaii bays created between two headiands), resuiting in peaked at 2.9 m offshore"= and 4.5 m inshore" on 23 June 2011. During the 2011 austrai winter the sweii remained between 2 m and beach thinning in the south and thickening in the north during the austrai 3 m (Hj) for significant periods.'^ These sweiis consistently arose from a winter, a pattern which reverses in the austrai summer" southeasteriy direction, with periods varying from 12 s to 16 s." During Aiong urbanised coastiines, the coast's naturai abiiity to repair itseif this time, eoastai erosion was dominant at the known erosion hotspot is compromised by the destabiiising effects ot buiit structures and (EHS) ioeations and, aithough the stiii-water position (ievei of the sea on the dysfunctionai coastai dune cordon (Figures 2 and 3).' Protracted which waves are superimposed) remained reiativeiy iow, sediment ioss erosion in 2006 and 2007 ied to some coastai reaches being artificiaily and infrastructure damage was reported. Some EHS coastal defences, defended to protect the adjacent infrastructure. Because of net sediment constructed in the aftermath of the 2006/2007 erosion event, were iosses during this erosion event, this coastai butter was piaced 5-30 m exposed, damaged or breached (Figure 2). These effective sweiis were shoreward of its pre-2006/2007 position.'" This placement has resuited generated by a series ot eoid frontai systems moving trom west to east, in the urbanised coastai reaches being more vuinerabie than they were passing to the south of KwaZuiu-Natai. Several distinct erosive events prior to 2006. were recorded. Recent research" suggests that strong erosion events have occurred at A sweii period which peaked between 17 and 19 Juiy 2011 is of particuiar or near the peak of the 18.6-year iunar nodai cycie (LNC) when the moon interest as the sweii came from a very unusuai direction (ENE: ±65°) is ciosest to Earth and spring high tides are unusuaiiy high. for that time of year This sweii resuited in a iongshore drift reversai (north-to-south). Under this reversai many EHS iocaiities underwent significant temporary deposition (Figure 3). The storm system whieh formed this sweii had aii the characteristics ot a tropicai iow pressure system. These are summer weather systems expected in December- March'" and are very unusuai in winter Notabiy, the 2011 austrai winter erosion event did not fail on the LNC peak Í3ut did occur in a year coinciding with the iunar perigee sub- harmonic (LPS) peak, a ±4.4-year tidai cycle that is aiso typified by unusuaiiy high tides. The iast LNC and LPS peaks were 2006 and 2007, respectiveiy - a period noted for strong coastal erosion." The 2011 EHS iocaiities were associated with prominent megarip currents, themselves controiied by coastai geomorphoiogy and bathymetry" Megarip currents (identified by iarge megacusps on the beach) are typiealiy associated with deeper ottshore-directed subtidai channels (to the north of headlands). This situation is caused by the compiex buiid- up of water caused by wave action, which is then reieased by an ottshore directed megarip current flow. These deep channels are used as ski boat iaunch sites. Figure 2: (a) Umdioti, (b) Littie iVlaritzburg Road, (c) Ansteys and (d) Within most topographicaiiy bound bays, deposition occurred to the iJmhianga beaches during a iarge sweii. These locations are south of points and megarip head-cutting erosion to the north. Beach known erosion hotspots. rotation is generaiiy seasonai" but during austrai winter 2011 it was observed to be the net product ot multipie shorter-duration, sweii events. These beach rotation events were rapid, occurring on the scaie of a few days, and couid reverse just as quickiy with a change in sweii direction (Figure 3). The faet that beach rotation was not universai indicates that iocai factors, such as bathymetry and coastai orientation and configuration, must have piayed a role. AN the 2011 EHS ioeations underwent severe erosion during the high swell of March 2007' (Figure 1; Tabie 1). The 2011 austrai winter erosion event was driven by sustained sweiis. initiaily, the pattern seemed to foilow that ot the March 2007 high-sweii erosion event. At tirst, thin beaches (1-3 m) overiying shaiiow bedrock underwent severe erosion,''" whereas deeper sandy beaches escaped erosion. As the season progressed, the wide and deep sandy beach EHSs became extensiveiy eroded as a resuit of eontinuing megarip current activity. From this evidence, two types of EHS are recognised: Type 1 (EHS-1) are beaches underlain by shaiiow bedrock such as Umdioti, Umhianga and Westbrook and Type 2 (EHS-2) are wide, sandy deep beaches such as Amanzimtoti, Submarine Bay, Umkomaas and Trafaigar (Figure 1; Tabie 1).