Effect of the nearshore profile on wave overtopping at a recurve seawall

De Bondt, Franco Hibbard (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Infrastructure located close to the shoreline is generally highly valuable, even though it is often exposed to significant flood risk. With an increased wave attack caused by climate change and the consequent rise in sea levels, the risk of economic damage or loss of life due to violent overtopping is constantly increasing. Reduction of overtopping at coastal defensive structures is therefore crucial in terms of the design, management and adaptation of coastal structures, especially when existing coastal defensive structures are evaluated for suitability in future conditions. In places where space does not allow for the construction of dikes or rubble mound breakwaters, vertical seawalls are often built to decrease the risk of flooding and damage to landward infrastructure, assets and injury to people, especially pedestrians. As some of the existing vertical seawalls are now being exposed to increased wave attack, more sudden, severe overtopping events are taking place which is dangerous to both property and humans. In order to decrease overtopping volumes, often vertical defensive structures incorporate a recurve seawall. The recurve shaped overhang reduces overtopping by deflecting water that is forced upwards by the vertical face of the structure, back seaward. By adding a recurve section onto the crest of a vertical seawall, the crest height of the structure can be reduced. Although recurves are often incorporated into seawall design, literature offers limited guidance for the design of such structures. This study, therefore, investigates the effect that the nearshore profile has on the overtopping of an optimised recurve seawall. A secondary objective was to determine the effect that the nearshore profile has on the increase or reduction of the incoming wave height. In order to achieve the objectives, 2D physical modelling tests were performed in a glass wave flume equipped with a piston type wave paddle that is capable of active wave absorption. The tests were performed on four different nearshore profiles, three which were typical to the Southern African coastline, and the other an average beach profile used in preceding studies. The three typical Southern African profiles included a steep, flat and mild nearshore profile. Tests were performed with 3 different water levels and 5 different wave periods, while the seawall design, crest height and wave height were kept constant. Analysis of the findings indicated that the nearshore profile had a significant influence on the overtopping of a recurve seawall. Results show that wave overtopping increases with a decrease in the steepness of the nearshore profile. Consequently, the gentlest nearshore slope led to the highest overtopping volumes of the recurve seawall. However, the mild profile with an offshore berm also produced large overtopping rates, mostly due to colliding reflected and incident waves causing large individual overtopping events. It is recommended that further model tests be conducted on the effect that beach erosion or accretion has on the overtopping of recurve seawalls. Another possible study can be done on the effect of different wave heights on the overtopping of recurve seawalls. The wave forces caused by different wave conditions should also be investigated, as forces on a structure is a necessary component for the structural design of recurve seawalls.

AFRIKAANSE OPSOMMING: Infrastruktuur geleë naby die kuslyn is dikwels uiters waardevol, selfs al is daar ‘n groot risiko vir oorstromings as gevolg van stygende seevlakke en verhoogde golfoorslag. Met klimaatsverandering wat lei tot verhoogde seevlakke en meer intense storms, gaan kusverdedigingstrukture gebuk onder ‘n verhoogde golfaanval en meer kragtige golfoorslag. Hierdeur neem die risiko vir ekonomiese skade en lewensverlies voortdurend toe. Dit is waarom die vermindering van golfoorslag by kusverdedigingstrukture so uiters belangrik is, vanaf die ontwerpfase, tot die bestuur en aanpassing van bestaande strukture vir hulle geskiktheid in toekomstige seetoestande. In plekke waar spasie vir die konstruksie van dyke en rotsgolfbrekers ontbreek, is vertikale seemure dikwels die beste opsie om die risiko van oorstromings en skade aan landwaartse infrastruktuur, bates en menselewens te verminder. As gevolg van die verhoogde golfaanval word verskeie bestaande vertikale seemure al meer gereeld blootgestel aan skielike, hewige golfoorslaggebeurtenisse wat gevaarlik vir beide eiendom en mense is. Ten einde hierdie groot volumes golfoorslag te verminder, word vertikale seemure dikwels aangepas om op te tree as terugkaatsseemure. Terugkaatsseemure verlaag golfoorslag deur die opstuwende waterkolom, veroorsaak deur die botsing van inkomende golwe en die vertikale deel van die see muur, terug seewaarts te weerkaats as ‘n byna-horisontale waterstraal. Alhoewel terugkaats-oorhange dikwels in die ontwerp van seemure ingesluit word, verskaf literatuur beperkte leiding omtrent die ontwerp van sulke strukture. Daarom ondersoek hierdie studie die effek wat tipiese Suider-Afrikaanse nabystrandse profiele het op die oorslag van terugkaatsseemure, asook die invloed van die nabystrandse profiele op die verandering in die inkomende golfhoogte. Om hierdie doelwitte te bereik, is 2D-fisiese modellering in ‘n glas-golfkanaal, toegerus met aktiewe golf-absorpsie, uitgevoer. Toetse is uitgevoer met vier verskillende nabystrandse profiele, waarvan drie van die profiele tipies is aan die Suider-Afrikaanse kus en die ander profiel ‘n gemiddelde strandhelling gebruik in voorafgaande studies. Die drie nabystrandse profiele wat tipies is aan die Suider-Afrikaanse kus het ‘n steil, plat en gemiddelde helling ingesluit. Toetse is uitgevoer vir 3 verskillende watervlakke en 5 verskillende golfperiodes. Die inkomende golfhoogte, asook die terugkaatsseemuur se ontwerp en -kruinhoogte is egter konstant gehou. Ontleding van die resultate dui daarop dat die nabystrandse profiel ‘n groot impak het op die oorslag van ‘n terugkaatsseemuur. Resultate wys dat oorslag toeneem met ‘n afname in die helling van die nabystrandse profiel. Dus het die nabystrandse profiel met die platste helling gelei tot die grootse golfoorslagvolumes. Die terugkaatsseemuur op die nabystrandse profiel met ‘n seewaartse sandbank het egter ook groot oorslag tempo’s ervaar, grotendeels as gevolg van die botsing van inkomende en weerkaatste golwe.Daar word aanbeveel dat toekomstige studies oor terugkaatsseemure fokus op die effek wat stranderosie of -opbou op die golfoorslag het, asook die effek wat ‘n verandering in golfhoogte op die oorslagvolumes het. Verdere moontlike navorsing kan ook fokus op die kragte wat inkomende golwe uitoefen op die terugkaatsseemuur, aangesien dit ‘n belangrike aspek is wat in die ontwerp van nuwe terugkaatsseemure oorweeg moet word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107789
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