A synthesis of the coastal geophysical characteristics of sandy beaches along the South African coastline

Van der Merwe, Colette (2017-03)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The purpose of this thesis was to evaluate the relationships between the coastal geophysical characteristics of the coastal regions of South Africa and to provide an overall, systematic characterization of the features of the South African coast and the physical coastal processes affecting them. Geophysical parameters include grain size/characteristics of beach sediment, beach width, surf zone width, beach slope, beach exposure, beach orientation and beach morphodynamic type. Physical coastal drivers include waves, wind, current and tides. The focus of the research was on the nearshore characteristics of sandy beaches on the West, South-West, South and East Coasts of South Africa. Study locations were selected to represent the coastal regions of South Africa, namely Saldanha Bay, Jakkalsfontein, Table Bay, False Bay, Mossel Bay, Algoa Bay, East London, the KwaZulu-Natal south coast and Richards Bay. Nearshore waves in particular, affect the shoreline in various ways. The mean significant nearshore wave conditions at -15 m to MSL were determined by converting wave data at -15 m to MSL for a return period of one year. The mean nearshore significant wave height at -15 m to MSL was converted to an equivalent mean deep-sea significant wave height by applying ‘reverse shoaling’ (dividing by the shoaling coefficient). It should be kept in mind that waves refract and diffract as they enter a bay, whereas on open linear coastlines, high-energy waves reach the shoreline. The deep-sea wave conditions are therefore not representative of the conditions in the nearshore of the bays. Two methods were used to derive the breaker wave heights at each of the sites along the nine South African study locations. It can be concluded from the calculated breaker heights that the linear exposed coastlines are clearly subject to more severe wave conditions than the calmer conditions found along the coastlines in the bays. The distribution of the average nearshore significant wave heights along the South African coast indicates that of the selected study locations, Jakkalsfontein has the most energetic coastline and Saldanha Bay (according to breaker heights) and Table Bay have the least energetic coastlines. The wave heights at Mossel Bay, Algoa Bay and along the KwaZulu-Natal coast were found to be similar to each other. It can be concluded that the wave climate along the South African coast is swell dominated due to the generally long wave periods. The tidal range along the South African coast does not vary much (SANHO, 2012) and can differ from about 2 m during spring tide to about 0.5 m during neap tide. Sand grains were classified according to the Wentworth scale (Wentworth, 1922). Overall, it was found that the study areas along the South African coast have predominantly medium-grained sand. The coarsest sand was found at Saldanha Bay (0.68 mm) and the finest at Orient Beach in East London (0.21 mm), which also has the mildest beach slope (0.024). The steepest beach slope occurs at Richards Bay (0.195). The Iribarren number was calculated to determine the beach type of each study location according to Battjes (1974) and Short (1999). The majority of study areas are classified as dissipative. Some beaches at Richards Bay have steeper slopes, hence are classified as reflective. In general, South African beaches are predominantly intermediate, followed by dissipative beach types. Reflective beaches in South Africa occur mostly along the KZN coast. According to the criteria for beach exposure by Theron et al. (2010), Jakkalsfontein, the KwaZulu-Natal south coast and Richards Bay have the most exposed beaches as they are linear and open coastlines. It was found that Saldanha Bay has the most protected beaches as it is bounded by headlands and breakwaters occur in the bay. Regression analyses and correlation graphs of the coastal geophysical parameters and wave height data generally did not show clear trends. Therefore, the data points of all the study locations along the South African coast were compared to existing correlations of Komar (1976) based on various coasts of America, which showed the low and high-energy boundaries for beaches. Wiegel (1964) and Komar (1976) state that beaches exposed to high-energy waves tend to have larger grain sizes than beaches exposed to low-energy waves and that beach slope increases with grain size and decreases with increasing wave energy. Overall, the South African study areas with smaller (thus finer) grains were plotted close to the low-energy boundary, and the South African study areas with larger (thus coarser) grains were plotted close to the high-energy boundary, which is in accordance with Wiegel (1964) and Komar (1976). The majority of the data points were plotted close to the low-energy boundary. The measured beach slopes at all the South African study locations were compared to the numerical formulations for predicting slopes of Sunamura (1984), Wiegel (1964) and Swart (1986). Overall, a good similarity was observed. It was concluded that the beach slope tends to decrease with an increase in the value of the dimensionless parameter, Hь/√gd₅₀T of Sunamura (1984); and increase as the breaker wave height decreases and the median grain size increases. Reis and Gama (2009) and Coelho et al. (2009) concluded for the Portuguese Atlantic coast that for the same wave height, the equilibrium beach face slope increases with sand grain size and decreases with increasing wave height, therefore becoming more dissipative. From the South African data covered in this thesis, the data points at Table Bay showed the clearest correlation between beach face slope and median grain size. A trend of increasing beach slope with increasing grain size was observed for Table Bay, thus in accordance with Reis and Gama (2009). The five coastal regions of South Africa, namely the west coast, south-west coast, south coast, east coast and the north-east coast, were characterised in terms of coastal features. The west coast is an extremely diverse coast including coastal ecosystems like rocky cliffs, mixed beaches, exposed linear sandy beaches and small, partially sheltered bays. The coastline of the Western Cape (on the south-west coast) is also very diverse – with sandy and rocky beaches and mountainous areas. The east coast has a linear, high-energy coastline that is interspersed with small pocket beaches at river mouths. The southern portion of the east coast shoreline is rocky and quite irregular due to small headlands and rocky points. Sandy or rocky shores are found on the north-east portion of the east coast. The far north-east coastline is linear and open with no major headlands or capes. This high-energy shoreline consists of sandy beaches with high dunes, beach rock, small headlands and small bays.

AFRIKAANSE OPSOMMING: Hierdie tesis handel oor die verhoudings tussen die geofisiese eienskappe van die Suid-Afrikaanse kus en die effek van fisiese faktore daarop. Die doel van die tesis is ook om die kenmerke van die Suid-Afrikaanse kus te karakteriseer. Geofisiese eienskappe van sanderige kusgebiede sluit in die korrelgrootte van die strandsediment, die wydte van die strand en die brandersone, die helling van die strand, die blootstelling en oriëntasie van die kuslyn en die tipe strand. Fisiese faktore sluit in golwe, wind, strome en getye. Die tesis fokus op die kenmerke van sandstrandkuslyne langs die weskus, suid-weskus, suidkus en ooskus van Suid-Afrika. Studiegebiede sluit in Saldanhabaai, Jakkalsfontein, Tafelbaai, Valsbaai, Mosselbaai, Algoabaai, Oos-Londen, die KwaZulu-Natalse suidkus en Richardsbaai. Nabystrandse golwe het verskeie invloede op die kuslyn. Die gemiddelde nabystrandse golftoestande by -15 m tot GSV (gemiddelde seevlak) is bepaal deur die golfdata by -15 m tot GSV vir ‘n herhaalperiode van een jaar. Die gemiddelde nabystrandse golfhoogtes by -15 m tot GSV is omgeskakel na ekwivalente diepsee golfhoogtes deur die toepassing van ‘n metode wat beskryf word in US Army, Corps of Engineers (1984), naamlik om te deel met die vervlakkingkoeffisiënt (KS). Dit moet ingedagte gehou word dat diep-see golwe wat baaie binnekom buig en refrakteer, waar hoë-energiegolwe reguit en oop kuslyne bereik. Die diepsee golftoestande kan dus tegnies nie gebruik word om die nabystrandse kondisies in baaie te beskryf nie. Twee metodes is gebruik om die brekerhoogtes by elk van die nege studiegebiede langs die Suid-Afrikaanse kus te bepaal. Daar kan afgelei word uit die afgeleide brekerhogtes dat die golftoestande wat voorkom by reguit, blootgestelde kuslyne is meer hewig as dié by kalmer kuslyne in die baaie. Daar is afgelei vanuit die verspreiding van gemiddelde nabystrandse kuslyngolfhoogtes by die studiegebiede dat Jakkalsfontein die meeste golfaksie ervaar en Saldanhabaai (op grond van brekerhoogte) en Tafelbaai die minste. Die golfhoogtes by Mosselbaai, Algoabaai en langs die kus van KwaZulu-Natal is eenders. Daar kan afgelei word dat die golfklimaat van Suid-Afrika hoofsaaklik oorheers word deur deiningsgolwe, as gevolg van die lang golfperiodes. Die getye is redelik konstant om die kus van Suid-Afrika (SANHO, 2012) en verskil vanaf ongeveer 2 m tydens springgety tot ongeveer 0.5 m tydens dooiegety. Sandkorrels is volgens die Wentworthskaal (Wentworth, 1922) geklassifiseer. Daar is tot die gevolgtrekking gekom dat die studiegebiede langs die Suid-Afrikaanse kus meestal bestaan uit medium-grof sandkorrels. Die grofste korrels is gevind by Saldanhabaai, met ‘n korrelgroottevan 0.68 mm, en die fynste korrels is gevind by Orientstrand in Oos-Londen, met ‘n korrelgrootte van 0.21 mm. Die platste strandhelling (0.024) is ook gevind by Orientstrand. Daar is gevind dat Richardsbaai die steilste strandhelling het (0.195). Die Iribarren nommer is bereken om die strandtipe by elke studiegebied te bepaal volgens Battjes (1974) en Short (1999). Die meerderheid studiegebiede se strande is geklassifiseer as dissiperend, alhoewel sommige strande by Richardsbaai steiler hellings het, dus reflekterend is. Oor die algemeen word Suid-Afrikaanse strande geklassifiseer as intermediêr, gevolg deur dissiperende strandtipes. Reflekterende strande kom meestal voor langs die KwaZulu-Natalse kus. Die strandblootstelling is geklassifiseer volgens Theron et al. (2010) se kriteria. Daar is bevind dat Jakkalsfontein, die KwaZulu-Natalse suidkus en Richardsbaai die mees blootgestelde strande het aangesien hierdie gebiede reguit en oop kuslyne het. Saldanhabaai het die mees beskermde strande as gevolg van skiereilande aan weerskante van die baai en golfbrekers binne die baai. Regressie-ontledings en korrelasiegrafieke van die geofisiese eienskappe en golfhoogtedata het oor die algemeen nie duidelike korrelasies aangedui nie.Daarom is die data van al die Suid-Afrikaanse studiegebiede eerder vergelykmet korrelasiessoos bepaal deur Komar (1976), gebasseer op die Amerikaanse kus. Wiegel (1964) en Komar (1976) stel dat strande wat blootgestel is aan hoë-energie-golwe, groter korrelgroottes het as strande wat blootgestel is aan lae-energie-golwe, en dat die strandhelling toeneem met korrelgrootte en afneem met golfenergie. Oor die algemeen het die Suid-Afrikaanse studiegebiede met kleiner (dus fyner) sandkorrels voorgekom naby die lae-energiegrens en die Suid-Afrikaanse studiegebiede met groter (dus grower) sandkorrels voorgekom naby die hoë-energiegrens, wat in ooreenstemming is met Wiegel (1964) en Komar (1976). Die meerderheid studiegebiede se datapunte het voorgekom naby die lae-energiegrens. Die gemete strandhellings van al die studiegebiede langs die Suid-Afrikaanse kus is vergelyk met numeriese formulerings vir die bepaling van hellings van Sunamura (1984), Wiegel (1964) and Swart (1986). Oor die algemeen is ‘n goeie ooreenkoms waargeneem. Daar is gevind dat die strandhelling afneem met ‘n toename in die waarde van die parameter Hь/√gd₅₀T, van Sunamura (1984) end at die strandhelling toeneem met ‘n afname in brekerhoogte en toename in sandkorrelgrootte. Reis en Gama (2009) en Coelho et al. (2009) het bevind vir die Portugese Atlantiese kus dat dat die strandhelling toeneem as die korrelgrootte toeneem, met ‘n konstante golfhoogte, en afneem met ‘n toename in golfhoogte. Dus sal die strand meer dissiperend word. Die beste en duidelikste korrelasie tussen strandhelling en korrelgrootte is waargeneem by Tafelbaai, waar die strandhelling toeneem met ‘n toename in korrelgrootte. Hierdie tendens is in ooreenstemming met Reis en Gama (2009). Die kusstreke van Suid Afrika sluit in die weskus, suid-weskus, suidkus, ooskus en noord-ooskus. Hierdie streke is gekarakteriseer in terme van hul kustelike eienskappe. Die weskus is baie divers. Kustelike ekosisteme soos rotsagtige kranse, gemengde strande, oop sandstrande en klein baaie word gevind langs hierdie kus. Die kuslyn van die Weskaap (wat geleë is op die suid-weskus), is ook baie divers, met sanderige en rotsagtige strande. Die hoë-energie kuslyn van die ooskus is meestal reguit, met klein strande by riviermondings. Die suidelike deel van die ooskus is rotsagtig as gevolg van klein skiereilande en rotsagtige gebiede. Sanderige en rotagtige strande kom voor langs die noord-oostelike deel van die noord-ooskus. Hierdie hoë-energie kuslyn van die Noord-ooskus is meestal reguit en blootgestel, met sanderige strande, hoë duine, klein skiereilande en klein baaie.

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