Efficiency of tandem breakwater in reducing wave heights and damage level : a Mossel Bay case study

Thesnaar, Eldre (2015-03)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: In recent years, breakwater design has been governed not only by structural stability, but by cost effectiveness as well. Breakwater designers are constantly trying to find the perfect balance between low-risk design and low-cost design. The combination of a main rubble mound breakwater and a submerged offshore reef, that are designed to function together, is known as a tandem breakwater. The reef structure is responsible for dissipating some of the energy by causing wave breaking. Thereafter, the area between the reef and the main structure - the tranquillity zone - allows for natural energy dissipation. The combination of the effects of the reef and tranquillity zone results in reduced significant wave heights at the main rubble mound structure, which allows it to be designed with lighter armour units. This study investigates the application of a tandem breakwater, based on the conditions at the port of Mossel Bay, by achieving the following set of objectives: (1) to determine the influence of the tandem breakwater‟s submerged reef crest elevation on the damage level of the main rubble mound structure, (2) to determine the relationship between the relative wave attenuation distance and the percentage wave attenuation, and (3) to compare the abovementioned parameters for rock and geotube reefs. A physical model test series was conducted to gain the data required for achieving the objectives. A rubble mound structure that makes use of dolos armour units, resembling the one at Mossel Bay, was constructed inside a concrete flume equipped with a single-paddle wavemaker. Two reef structure types (rock and geotube) were tested at three crest elevations (below-LAT, LAT and ML), against combinations of two significant wave heights (2.5 m and 3 m) and two peak periods (8 s and 12 s), at one water level (ML) and one offshore reef distance (50 m). From the model test results, it is evident that the presence of a reef structure significantly affects the wave conditions that reach the main structure. When comparing significant wave heights measured at a prototype distance of 20 m in front of the main breakwater, a reduction of as high as 42% can be observed for a reef structure made from rocks and 54% for a geotube structure. In all cases, the geotube structure causes more wave attenuation due to its lower permeability, which enables it to reflect more wave energy. However, it should be noted that the stability of the geotube reef was not considered during testing. Generic graphs are presented, that aim to provide guidance in the design process of such a tandem breakwater system. The graphs are produced for a case where dolos armour units are used and might not be exactly the same when a different type of armour unit is used. One graph shows the relationship between the damage reduction at the main breakwater and the relative reef submergence. The other shows the relationship between wave attenuation and the relative wave attenuation distance. Unfortunately, the implementation of geotube reefs of the nature described in this investigation is not likely in the South African context at present. This, however, does not eliminate the possibility of future applications. As geotextile technology develops and greater operational experience and equipment is gained, tandem breakwaters that incorporate geotube reefs could provide an alternative that is both cost-effective and more environmentally friendly with regards to transport emissions. Until then, tandem breakwaters that incorporate rock reefs may be able to provide a desired alternative design for certain scenarios.

AFRIKAANSE OPSOMMING: In die afgelope jare, word breekwater ontwerp nie net beheer deur strukturele stabiliteit nie, maar ook koste effektiwiteit. Ontwerpers poog alewig om die perfekte balans tussen lae-risiko ontwerp en lae-koste ontwerp na te streef. Die kombinasie van 'n hoof ruklipgolfbreker en 'n sekondêre onderwater rif breekwater, wat ontwerp is om as 'n eenheid te funksioneer, staan bekend as „n tandem breekwater. Die rif struktuur is verantwoordelik vir die verlies van 'n gedeelte van die golf energie deur golf breking te veroorsaak. Daarna veroorsaak die area tussen die rif en die hoof struktuur – die kalmeringsone – verdere natuurlike energie verlies. Die gekombineerde effek van die rif en kalmeringsone veroorsaak dat kleiner branders die hoof breekwater bereik, wat toelaat dat dit ontwerp kan word met kleiner pantser eenhede. Dié studie ondersoek die toepassing van 'n tandem breekwater, gebaseer op die kondisies by die Mosselbaai hawe, deur die volgende doelwitte te bewerkstellig: (1) om die invloed van die onderwater rif kruinhoogte op die vlak van skade aan die hoof breekwater te bepaal, (2) om die verhouding tussen die relatiewe golfhoogte-verminderings-afstand en die golfhoogte vermindering te bepaal, en (3) om die bogenoemde parameters vir rots en geo-buis riwwe te vergelyk. 'n Fisiese model toets reeks is uitgevoer sodat die benodigde data ingesamel kan word om die doelwitte te bereik. „n Rotsvul breekwater wat gebruik maak van dolos pantser eenhede, soortgelyk aan dié by Mosselbaai, is gebou in 'n beton kanaal wat toegerus is met 'n enkel-spaan golfmasjien. Twee tipes riwwe (rots en geo-buis) is getoets met drie kruin hoogtes (onder-LAG, LAG en GV), teen kombinasies van twee beduidende golfhoogtes (2.5 m en 3 m) en twee spitsperiodes (8 s en 12 s), by een watervlak (GV) en een sekondêre breekwater afstand (50 m). Uit die model toets resultate is dit duidelik dat die teenwoordigheid van 'n rif struktuur, die golfkondisies wat die hoof breekwater bereik, beduidend beïnvloed. Wanneer beduidende golfhoogtes, gemeet op 'n prototipe afstand van 20 m voor die hoof breekwater, vergelyk word, word 'n vermindering van so hoog as 42% waargeneem vir 'n rif bestaande uit rots en 54% vir 'n rif bestaande uit geo-buise. In alle gevalle veroorsaak die geo-buis struktuur meer golfhoogte vermindering, as gevolg van sy laer deurlaatbaarheid, wat dit in staat stel om meer golfenergie te reflekteer. Die stabiliteit van die geo-buis struktuur is egter nie in ag geneem tydens die toetse nie. Generiese grafieke word weergegee, met die doel om leiding te gee tydens die ontwerpsproses van só 'n tandem breekwater struktuur. Die grafieke hou verband met die geval waar dolos pantser eenhede gebruik word, en mag verskil vir ander tipes pantser eenhede. Een van die grafieke dui die verhouding tussen skadevermindering aan die hoof breekwater en die relatiewe posisie van die onderwater rif se kruinhoogte aan. Die ander grafiek dui die verhouding tussen die golfhoogte vermindering en die relatiewe golfhoogte-verminderings-afstand aan. Huidiglik is die toepassing van die tipe geo-buis riwwe soos beskryf in hierdie ondersoek, ongelukkig onwaarskynlik in die Suid-Afrikaanse konteks. Dit skakel egter nie die moontlikheid van toekomstige toepassings van dié aard uit nie. Soos geo-tekstiel tegnologie ontwikkel en meer operasionele ervaring en toerusting bekom word, kan die effektiewe implementasie van geo-buis riwwe 'n alternatief bied wat beide koste effektief en omgewingsvriendelik is met betrekking tot die vrystelling van uitlaatgasse tydens die vervoer van materiale. Tot dan, kan tandem breekwaters wat van rots riwwe gebruik maak, moontlik die gewenste alternatiewe ontwerp bied vir sekere situasies.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96896
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