Analysis of Extreme Events in the Coastal Engineering Environment

Stander, Cornel (2015-12)

Thesis (MSc)--Stellenbosch University, 2015

Thesis

ENGLISH ABSTRACT : Coastal zones are subject to storm events and extreme waves with certain return periods. The return period of such events is defined as the average time interceding two independent, consecutive events, similar in nature, i.e., with the same return level. Coastal structures have to be designed to provide sufficient protection against flooding or erosion to a desired return level associated with a particular return period, for example 100 years. Statistical analyses of measured wave data over a time series are used for these estimations. In this study, wave data, measured by a Datawell Waverider buoy, is analysed by means of extreme value analyses. This dataset covers only approximately 18 years. Extreme value theory provides a framework that enables extrapolation in order to estimate the probability of events that are more extreme than any that have already been observed. It can, for example, be used to estimate wave return levels over the next 100 years given only an 18 year history. Different methods for making these estimations are implemented and evaluated. Datasets containing periods where data values are absent (i.e., gaps in a dataset), as well as the effects these missing values have on the estimation of extreme values, are also investigated. Methods for the treatment of gaps are evaluated by using NCEP (National Centre for Environmental Prediction) hindcast data, containing no missing values, and creating incomplete datasets from this data. Estimations are then made based on these incomplete sets. The resulting estimations are compared to the estimations made based on the complete NCEP dataset. Finally, recommendations are made for conducting optimal extreme value analyses, based on this study.

AFRIKAANSE OPSOMMING : Kusgebiede is onderhewig aan storms en ekstreme golwe met sekere terugkeer-periodes. Die terugkeer-periode van 'n gebeurtenis word gedefinieer as die gemiddelde tyd tussen twee onafhanklike, opeenvolgende gebeurtenisse, gelyksoortig van aard, met ander woorde, met dieselfde terugkeer-vlak. Kusstrukture moet ontwerp word om genoegsame beskerming teen oorstromings of erosie tot 'n spesifieke terugkeer-vlak, geassosieer met 'n spesi eke terugkeer-periode, byvoorbeeld 'n 100 jaar, te bied. Statistiese analise van gemete golfdata oor 'n tydsreeks word gebruik vir hierdie benaderings. In hierdie studie word golfdata, gemeet deur 'n Datawell Waverider boei, geanaliseer deur middel van ekstreemwaarde analise. Hierdie datastel dek slegs ongeveer 18 jaar. Ekstreemwaarde-teorie bied 'n raamwerk wat ekstrapolasie moontlik maak om die waarskynlikheid van gebeurtenisse te bepaal wat meer ekstreem is as enige gebeurtenisse wat reeds waargeneem is. Dit kan byvoorbeeld gebruik word om golf-terugkeer-vlakke oor die volgende 100 jaar te voorspel, gegewe slegs 'n 18 jaar geskiedenis. Verskillende metodes om hierdie beramings te maak, word ge-implementeer en ge-evalueer. Datastelle met periodes waar datawaardes afwesig is (ook genoem gapings in 'n datastel), asook die uitwerking van hierdie afwesige datawaardes op die beraming van ekstreemwaardes, word ondersoek. Metodes vir die hantering van gapings word ge evalueer deur onvolledige datastelle te skep uit volledige (met ander woorde, sonder enige afwesige waardes) NCEP (National Centre for Environmental Prediction) data. Beramings word dan gemaak gebasseer op die onvolledige datastelle. Hierdie resulterende beramings word vergelyk met die beramings gemaak gebasseer op die volledige NCEP datastel. Uiteindelik word aanbevelings gemaak vir die optimale uitvoering van ekstreemwaarde analise, gebasseer op hierdie studie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97939
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