Extremewaterlevelisanimportantconsiderationwhendesigningcoastalprotectionstructures.However,frequencyanalysisrecommendedbystandardcodesonlyconsiderstheannualmaximumwaterlevel,whereaswaterlevelsshouldactuallyberegardedasacombinationofastronomicaltideandstormsurge.Thetwoimpactingfactorsarebothrandomvariables,andthispaperdiscussestheirdependencystructuresandproposesanewjointprobabilitymethodtodetermineextremedesignwaterlevels.Thelognormal,Gumbel,Weibull,Pearsontype3,traditionalmaximumentropy,andmodifiedmaximumentropydistributionsareappliedtofitunivariatedataofastronomicaltidesandstormsurgesseparately,andthebivariatenormal,Gumbel-Hougaard,FrankandClaytoncopulasarethenutilizedtoconstructtheirjointprobabilitydistributions.Toensurethatthenewdesignmethodissuitableforusewithtyphoondata,theannualoccurrencefrequencyoftyphoonprocessesisconsideredandcorrespondingbivariatecompoundprobabilitydistributionsareproposed.BasedonmaximumwaterleveldataobtainedfromHengmenhydrologicalstationinthePearlRiverBasin,China,theseprobabilitymodelsareappliedtoobtaindesignsforextremewaterlevelsusingthelargestsumoftheastronomicaltideandstormsurgeobtainedunderfixedjointreturnperiods.Thesedesignvaluesprovideanimprovedapproachfordeterminingthenecessaryheightofcoastalandoffshorestructures.
