Determining the effects of nearshore conditions on rip current behaviour using hydrodynamic numerical modelling

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dc.contributor.advisorTheron, Andre Karl
dc.contributor.authorAbel, Nicolaas Cornelius
dc.contributor.otherStellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.en_ZA
dc.date.accessioned2023-02-28T13:20:22Z
dc.date.accessioned2023-11-16T09:18:10Z
dc.date.available2023-02-28T13:20:22Z
dc.date.available2023-11-16T09:18:10Z
dc.date.issued2023-02-28
dc.descriptionThesis (MEng)--Stellenbosch University, 2023.en_ZA
dc.description.abstractENGLISH ABSTRACT: At many beaches around the world, narrow seaward flowing currents of water, known as rip currents, are commonly found. Unfortunately, rip currents, which are vital for the transport and cross-shore mixing of nutrients, sediment, biological species, heat as well as pollutants, can easily entrap unsuspecting bathers and rapidly carry them to deeper water. In these circumstances, a combination of exhaustion, panic and fear can result in drowning. The focus of this research is not on the positive aspects of rip currents as vital transport mechanism, but rather on their dangers to bathers. Rip currents are the leading cause of sea rescues among beachgoers, resulting in more than 65% of all nearshore rescues worldwide. Thus, there is a need to develop an operational rip current forecasting model. The purpose of this research is to use numerical modelling to gain a better understanding of the behaviour and severity of rip currents for a range of nearshore conditions, specifically nearshore wave heights, tidal elevation and intermediate beach states. Using hydrodynamic numerical modelling software packages such as MIKE21 Coupled FM, the behaviour of rip currents concerning varying nearshore wave conditions, tidal elevation, and intermediate beach states can be studied. Such studies add to the existing body of knowledge on rip current behaviour and can ultimately have a significant positive outcome on bather safety. Rip currents are primarily driven by wave breaking and are influenced by water levels, originating within the surf zone and extending beyond the breaker line. Several rip current types exist, defined by their various forcing mechanisms. Bathymetry-controlled channel rip currents are the focus of this research. Thirty five hydrodynamic numerical simulations were conducted comprising of various combinations of nearshore wave-, bathymetry-, and water level characteristics. The results showed that wave-breaking and wave-bathymetry interaction are equally important in rip current behaviour. A rip current cannot form if the wave-bathymetry interaction does not allow for wave breaking. Ultimately the bathymetry controls the location and existence of the rip current. Rip current behaviour is controlled by the water level (tidal elevation) and the wave energy acting on the bar-rip morphological template. Higher rip flow velocities are present at lower water levels (≤ Mean Sea Level), with larger wave periods (≥ 12 s) and greater incident wave heights (≥ 2 m). Adult bathers experiencing a moderate 0.2 m/s flow velocity at a still water depth of 1.6 m are exposed to a significant danger of being overpowered by the rip current. Subsequently, a safety threshold is identified and defined as the cross-shore distance from the shoreline to the location where a bather is exposed to significant danger. The smallest safety threshold (30 m) was achieved on a transverse bar and rip bathymetry, imposed with a 2 m incident wave height with a 12 s wave period at a water level of -1 m MSL. The rip current flow velocity alone is not dangerous. Combining higher flow velocities (≥ 0.2 m/s) with significantly deep water (~≥ 1.6 m) is what makes rip currents treacherous. Further studies are required on the behavioural characteristics of rip currents. More tests are recommended on possible wave characteristics combinations and to validate the results by conducting physical modelling. Future studies should also include nearshore morpho dynamics.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: By baie strande regoor die wêreld word small seewaarts vloeiende waterstrome, bekend as sleurstrome, algemeen aangetref. Hierdei strome is noodsaaklik vir die vervoer en vermenging van voedingstowwe, sediment, biologiese spesies, hitte, sowel as besoedelingstowwe. Ongelukkig kan hierdie seewartsgerigte vloei niksvermoedende baaiers maklik vasvang en vinnig na dieper water dra, waar ‘n kombinasie van uitputting, paniek en vrees meer dikwels as nie ‘n verdrinking tot gevolg kan hê. Die fokus van die studie is nie op die goeie vervoer en vermenging aspekte van die sleurstroom nie, maar eerder op die gevaar wat dit die strandganger aan blootstel. Sleurstrome is die hoofoorsaak van seereddings onder strandgangers, wat lei tot meer as 65 % van alle nabykus-reddings wêreldwyd. Daar is dus ‘n groot behoefte om ‘n operasionele sleurstroomvoorspellingsmodel te ontwikkel. Deur gebruik te maak van hidrodinamiese numeriese modelleringsagtewarepakkette soos MIKE21 Coupled FM, kan die gedrag van sleurstrome met betrekking tot wisselende nabykusgolftoestande, getyhoogte en verskillende strandtoestande bestudeer word. Sulke studies dra by tot die bestaande kennis oor sleurstroomgedrag en kan uiteindelik ‘n beduidende positiewe uitkoms op baaierveiligheid hê. Sleurstrome is small seewaarts-gerigte vloeie van water, aangedryf deur golfbreking, afhanglik van spesifieke watervlakke, wat binne die brandersone ontstaan en verby die brekerlyn strek. Verskeie sleurstroomtipes bestaan, gedefinieer deur hul verskillende drywende meganismes. Batimetrie-beheerde kanaalsleurstrome is die fokus van hierdie navorsing. Fyf-en-dertig hidrodinamiese numeriese simulasies is uitgevoer wat bestaan uit verskeie kombinasies van nabystrandse golf-, batimetrie- en watervlakke. Die resultate het getoon dat golfbreking en golf-batimetrie-interaksie ewe belangrik is in sleurstroomgedrag. ‘n Sleurstroom kan nie vorm as die golf-batimetrie-interaksie nie golfbreking toelaat nie. Uiteindelik beheer die batimetrie (of-te-wel die bodemvorm) die ligging en bestaan van die sleurstroom. Sleurstroomgedrag word beheer deur die watervlak (getyhoogte) en die golfenergie wat op die sandbank-sleurstroom morfologiese bodemvorm inwerk. Hoër sleurvloeisnelhede is teenwoordig by laer watervlakke (≤ Gemiddelde Seevlak), met groter golfperiodes (≥ 12 s) en groter invals-golfhoogtes (≥ 2 m). Volwasse baaiers wat ‘n matige vloeisnelheid van 0.2 m/s in ‘n stilwaterdiepte van 1.6 m ervaar, is blootgestel aan ‘n aansienlike gevaar om deur die sleurstroom oorweldig te word. Vervolgens word ‘n veiligheidsdrempel geïdentifiseer en gedefinieer as die afstand loodreg seewaards vanaf die kuslyn na die punt waar ‘n baaier aan beduidende gevaar blootgestel is. Die kleinste veiligheidsdrempel (30 m) is bereik op ‘n Transversale Sandbank en Sleurstroom-batimetrie, onderhewig aan ‘n 2 m invallende golfhoogte met ‘n 12 s golfperiode by ‘n watervlak van -1 m GSV. Dit is belangrik om op te let dat die sleurstroomvloeisnelheid alleenlik nie gevaarlik is nie. Die kombinasie van hoër vloeisnelhede (≥ 0.2 m/s) met aansienlike waterdiepte https://scholar.sun.ac.za v (~≥ 1.6 m) is wat sleurstrome gevaarlik maak. Verdere studies word benodig oor die gedragskenmerke van sleurstrome. Meer toetse word aanbeveel op moontlike golfkombinasies en om die resultate te bevestig deur fisiese modellering uit te voer. Toekomstige studies moet ook nabystranse-morfodinamika insluit.en_ZA
dc.description.versionMastersen_ZA
dc.format.extentxv, 121 pages : illustrationsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/128712
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshRip currents -- South Africaen_ZA
dc.subject.lcshHydrodynamics -- South Africaen_ZA
dc.subject.lcshWave resistance (Hydrodynamics) -- South Africaen_ZA
dc.subject.lcshSeashore -- Search and rescue operationsen_ZA
dc.subject.lcshShorelines -- Monitoringen_ZA
dc.subject.lcshCoastal morphodynamicsen_ZA
dc.titleDetermining the effects of nearshore conditions on rip current behaviour using hydrodynamic numerical modellingen_ZA
dc.typeThesisen_ZA
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