Shoreline changes and longshore transport at the Port of Ngqura

Rutherford, Kyle (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The fixed sand bypass system at the Port of Ngqura was designed to match the natural longshore transport rate. Several factors, such as natural pebble presence and pump downtime, have resulted in the bypass system not matching the proposed transport rate. This shortfall has transpired to pressure from parties with environmental interests arguing that, by missing the target, erosion would result along the northern beaches. Concern was also expressed by Transnet National Port Authority, the operators of the system, that the target was not a fair representation of the available sand for the bypass system to pump. An investigation into the longshore transport rate and the Port of Ngqura’s effects on the surrounding shoreline thus became necessary. A review of existing literature revealed that the shoreline surrounding the port consists predominantly of sandy beaches with underlying pebbles. The wave climate can be described by a dominant southwesterly swell and a less frequent, but not insignificant, easterly swell. It was established that transport rates passing the Port Elizabeth Harbour are approximately 180 000 m³/year to 230 000 m³/year. Progressing northward, the stabilised shoreline resulted in limited sand supply from the northern beaches of PE and erosion has been witnessed at New Brighton Pier, amounting to approximately 160 000 m³/year. At the Port of Ngqura it was concluded that the net transport rate can be estimated to 160 000 m³/year to 200 000 m³/year in the northward direction, the latter being the chosen target rate. Studies of historical images and beach survey data determined that erosion in close proximity to the port was limited, likely due to the proximity to the discharge point and a high level of pebbles. Progressing northward, erosion became more visible with the decrease of pebbles acting as a beach stabiliser. On the southern beaches, accretion was evident against the breakwater and extended to approximately 1.8 km south of the port. The shoreline northward of the Swartkops River exhibited evidence of steady erosion, believed to be as a result of the sand supply deficiency caused by the PE Harbour and stabilised shoreline north of PE Harbour. Studies of the accretion concluded an approximate transport rate of 180 000 m³/year past the Port of Ngqura. Transport rates were also calculated along the southern shore as calibration data for the MIKE Littoral Process FM model. Longshore transport model simulations concluded an average annual net transport rate of 225 000 m³/year in a northward direction. Considering all the results it was concluded that a transport rate of 200 000 m³/year accurately represented the transport rate at the Port of Ngqura. Studies into the variability determined that, in order to match the natural transport rate, the average pumped rate should fall between 180 000 m³/year and 220 000 m³/year after five years, and after 10 years, between 190 000 m³/year and 210 000 m³/year. Furthermore, the cross-shore transport distribution was studied. The original and accreted profiles were modelled and the results compared. For the original profile, describing the shoreline prior to construction, it was determined that 92% of the transport occurred within the reach of the bypass pumps. It was then established that for the accreted profile, this amounted to only 74%. An investigation into pump priority was conducted and described the shift in importance to seaward pumps due to the accretion. Finally, recommendations were provided in accordance with the shoreline monitoring. Further topics of study were also included.

AFRIKAANSE OPSOMMING: Die vaste sandverbyvoeringstelsel by Ngqura-Hawe is ontwerp om die natuurlike langsvervoertemo na te boots. Verskeie faktore soos die teenwoordigheid van klippies en pomphersteltyd, het daartoe gelei dat die stelsel nie die voorgestelde verbyvoeringstempo (die teiken) kon behaal nie. Hierdie tekortkoming het druk van omgewingsbelanghebbers veroorsaak deur die argument dat, deur die teiken te mis, erosie langs die noordelike strande sou plaasvind. Bekommernis is ook deur Transnet Hawe-owerheid wat die stelsel bedryf, uitgespreek dat die teiken nie ’n billike verteenwoordiging van die beskikbare sand vir die verbyvoeringstelsel is nie. ’n Ondersoek aangaande die langsvervoertempo en die effek van Ngqura-Hawe op die omliggende kuslyn word dus benodig. ’n Hersiening van bestaande literatuur het getoon dat die kuslyn rondom die hawe hoofsaaklik uit sanderige strande met hoë vlakke van onderliggende klippies bestaan. Die golfklimaat kan deur ’n oorheersende suidwestelike deining met ’n minder gereelde, maar nie onbelangrike, oostelike deining beskryf word. Daar is vasgestel dat die langsvervoertempo’s verby Port Elizabeth-Hawe ongeveer 180 000 m³/jaar tot 230 000 m³/jaar is. Verder noordwaarts, het die gestabiliseerde kuslyn tot ‘n beperkte beskikbaarheid van sand aan die noordelike strande van PE gelei. Erosie kan by die New Brighton-seehoof gesien word waar die langsvervoer ongeveer 160 000 m³/jaar is. By Ngqura-Hawe is die gevolgtrekking dat die langsvervoertempo op 160 000 m³/jaar tot 200 000 m³/jaar in ‘n noordelike rigting beraam kan word. Laasgenoemde is die gekose langsvervoertempo. Studies van historiese beelde en strandopmetings het bepaal dat erosie in die nabyheid van die hawe beperk was, waarskynlik as gevolg van die nabyheid van die stortingspunt en ’n hoë vlak van klippies. Noordwaarts het erosie toegeneem met die afname van blootgestelde klippies en ’n groter voorraad sand. Op die suidelike strand was opbou teen die golfbreker duidelik en het dit tot ongeveer 1.8 km suid van die hawe gestrek. Die kuslyn noord van die Swartkopsrivier het bestendig geërodeer wat vermoedelik ’n resultaat van ‘n gebrek aan sand was as gevolg van die gestabiliseerde kuslyn noord van PE-hawe. Studies van die opbou het ’n benaderde langsvervoertempo van 180 000 m³/jaar verby die van Ngqura- Hawe getoon. Vervoertempo’s is ook langs die suidelike strande bereken en as ykingsdata vir die MIKE Littoral Proses FM-model gebruik. Simulasies het ’n gemiddelde netto langsvervoerstempo van 225 000 m³/jaar in ’n noordwaartse rigting vasgestel. Deur al die resultate in ag te neem, is die gevolgtrekking gemaak dat 200 000 m³/jaar die langsvervoertempo by Ngqura-Hawe akkuraat verteenwoordig. Studies aangaande die wisseling het gevind dat die gemiddelde pomptempo tussen 180 000 m³/jaar en 220 000 m³/jaar na vyf jaar moet val, en na tien jaar, tussen 190 000m³/jaar en 210 000 m³/jaar. Voorts is die dwarsstrandse verdeling bestudeer. Die oorspronklike en opgeboude profiele is gemodelleer en die resultate is vergelyk. Vir die oorspronklike profiel, bedoelende die kuslyn voor konstruksie, is bepaal dat 92% van die vervoer binne die bereik van die verbyvoeringspompe plaasgevind het. Daar is toe vasgestel dat, vir die opgeboude profiel, dit slegs 74% beloop. ’n Ondersoek oor pompvoorkeur is gedoen wat die belangrikheid van die gebruik van die seewaartse pompe te danke aan die opbou, bevestig het. Ten slotte is aanbevelings oor kuslynmonitering gemaak. Verdere studie-onderwerpe word ook aanbeveel.

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