Synthesis of extreme wave climate for the Canterbury Bight, New Zealand

Numerical models of the marine wind field and wave generation offer a techn ique for estimating the severity of extreme wave events. Measured and model led waves in the Canterbury Eight, New Zealand, were used to examine the hy pothesis that extreme conditions can be more confidently estimated by model ling the wave climate over a decade than by analysing shorter measured reco rds. The meteorological conditions causing high waves in the Canterbury Eig ht were examined by inspecting wind measurements at Christchurch Airport an d wave buoy measurements made in 1983-85 near the Rakaia River mouth. Altho ugh the arrival of high waves was sometimes associated with wind changes at the airport, peak wave heights generally occurred at least several hours a fter such wind changes and were associated with south-westerly or southerly winds in the Bight and a depression south-east of the South Island. Wave c onditions at the buoy site for 1980-89 were modelled using marine winds obt ained from a global circulation model, running a wave generation model for the seas around New Zealand, and then applying a refraction model. The deca de modelled included two El Nino events and one La Nina event, and the ocea n wind climate should therefore have been reasonably representative of the long-term range of conditions. The model results were verified against the buoy measurements. Estimates of extreme wave heights were then made from 10 years of modelled record and also the measured buoy record. Application of the Gumbel distribution to the monthly maxima gave a significant wave heig ht of 9.5 m for the wave event with an annual exceedance probability of 1 i n 50. Model predictions indicate that the most severe waves arrive in the B ight from due south. Estimates of extremes from the measurements and model hindcast were close, indicating that the hindcast represents a population o f events similar to those measured.