Effect of the form of the overhang of a recurve seawall to reduce wave overtopping

Kretschmer, Marcus Malan (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Coastal areas around the world are experiencing a rise in sea level due to the effects of global warming. As a large percentage of the world’s population resides within the coastal zone, the rise in sea level is placing evermore developments and people at risk, consequently increasing the demand for more effective coastal defence structures. One of the most common types of coastal defence structures are seawalls, which reduce or prevent wave overtopping and flooding of the landward side of the structure. These structures are often designed as vertical impermeable walls with high crests to ensure protection against overtopping. However, these designs are not always favourable as the high crest levels often obstruct the view of the sea. Recurve seawalls provide the solution, as they reduce wave overtopping without excessively compromising the sea view. However, existing guidelines for the design and research on the effectiveness of different recurve seawalls are very limited. This project aims to determine the effectiveness of different overhang forms of recurve seawalls on reducing wave overtopping. In order to achieve the objective of this study, the performance of different recurve forms in reducing wave overtopping was evaluated by conducting overtopping tests in a physical model. The model was constructed in a glass wave flume equipped with a piston-type wave paddle. A total number of 147 tests with varying water levels and wave periods were conducted for five different overhang forms, providing a comprehensive set of results which were analysed to determine the most effective overhang form. Analysis of the findings clearly indicated that the shape of the overhang has a strong influence on the overtopping reduction capabilities of a recurve seawall. It was found that the concave shape with a squared overhanging edge offered the most reduction in overtopping, when compared with the performances of the remaining forms. For the high relative freeboard cases (large difference between crest level and water level), small amounts of overtopping were observed as a result of colliding incident and reflected waves. These overtopping events constituted mainly of white water spray which were negligibly small but should be treated with caution as the presence of an onshore wind, which was not modelled in this study, could have a significant influence on the amount of overtopping. In addition, it was found that overtopping generally increases with an increase in the wave return angle of the recurve. When comparing the two different concave recurves tested, the recurve with the rounded overhanging edge produced up to fifty percent more overtopping, due to the adhesion of water along the rounded edge. This finding led to the conclusion that apart from the overhang length, the shape of the overhanging edge also significantly influences the reduction of overtopping. As opposed to the findings of a previous study, increasing the wave period up to 14 seconds consistently led to an increase in overtopping. It is recommended that further model tests should be conducted on concaved recurves, including variations in the vertical dimension of the recurve. The effects of different beach slopes and wave heights on overtopping of recurve seawalls should also be investigated, as these remained constant throughout this study.

AFRIKAANSE OPSOMMING: Kusgebiede regoor die wêreld is besig om ‘n styging in seevlak te ervaar as gevolg van aardverwarming. Aangesien 'n groot persentasie van die wêreld se bevolking binne die kusgebied woon, stel die stygende seevlak al hoe meer ontwikkelings en mense in gevaar, wat gevolglik die vraag na meer doeltreffende kusverdediging strukture verhoog. Een van die algemeenste tipes strukture vir kusverdediging is seemure, wat golfoorslag en oorstroming van die landwaartse kant van die struktuur verminder of voorkom. Hierdie strukture word dikwels as vertikale ondeurdringbare mure ontwerp met hoë kruine om beskerming teen golfoorslag te verseker. Hierdie ontwerpe is egter nie altyd gunstig nie, aangesien die hoë kruinvlakke dikwels die see-uitsig belemmer. Die gebruik van terugkaatsmure voorsien die oplossing, aangesien die golfoorslag verminder word sonder om die see-uitsig so erg te benadeel. Bestaande riglyne vir die ontwerp en navorsing oor die doeltreffendheid van verskillende terugkaatsmure is egter baie beperk. Die doelwit van hierdie projek is om die effektiwiteit van verskillende oorhangvorms van terugkaatsmure op die vermindering van golfoorslag te bepaal. Ten einde die doel van hierdie studie te bereik, is die werking van verskillende terugkaatsvorms in die vermindering van golfoorslag geëvalueer deur oorslagtoetse in 'n fisiese model uit te voer. Die fisiese model was in 'n golfkanaal, toegerus met ‘n suiertipe-golfopwekker, gebou. 'n Totaal van 147 toetse met verskillende watervlakke en golfperiodes is vir vyf verskillende oorhangvorms uitgevoer, wat sodoende 'n omvattende stel resultate verskaf het wat ontleed is om die effektiefste oorhangvorm te bepaal. Ontleding van die bevindings het duidelik aangedui dat die vorm van die oorhang ‘n sterk invloed het op die vermoë van ‘n terugkaatsmuur om oorslag te verminder. Daar is bevind dat die konkawe vorm met 'n vierkantige oorhangrand die grootste vermindering in oorslag bied, in vergelyking met die werking van die oorblywende vorms. Vir die gevalle met relatiewe hoë vryboorde (groot verskil tussen kruinvlak en watervlak), is klein hoeveelhede oorslag waargeneem as gevolg van botsende inkomende en weerkaatste golwe. Hierdie tipes oorslag het hoofsaaklik uit witwaterspatsels bestaan wat weglaatbaar klein was, maar dit moet versigtig geïnterpreteer word, aangesien die teenwoordigheid van 'n aanlandige wind, wat nie in hierdie studie gemodelleer is nie, 'n beduidende invloed op die hoeveelheid oorslag kan hê. Daarbenewens is gevind dat oorslag oor die algemeen toeneem met 'n toename in die golfterugkaatshoek van die terugkaatsvorm. By die vergelyking van die twee verskillende konkawe terugkaatsvorms, het die terugkaatsvorm met die geronde oorhangrand tot vyftig persent meer oorslag veroorsaak as gevolg van die adhesie van water teen die ronde rand. Hierdie bevinding het tot die gevolgtrekking gelei dat behalwe vir die oorhanglengte, die vorm van die oorhangende rand ook ‘n beduidende invloed op die vermindering van oorslag het. In teenstelling met die bevindings van 'n vorige studie, het die verhoging van die golfperiode tot 14 sekondes geleidelik tot 'n toename in oorslag gelei. Daar word aanbeveel dat verdere toetse op konkawe terugkaatsmure uitgevoer moet word, met wisselende afmetings van die terugkaatsvorm se hoogte. Die invloed van verskillende strandhellings en golfhoogtes op die oorslag van terugkaatsmure moet ook ondersoek word, aangesien hierdie veranderlikes konstant deur die studie gebly het.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102850
This item appears in the following collections: