Towards prediction of extreme floods in New Zealand

摘要

Prediction equations for non-dimensional flood frequency as a function of average annual number of floods at a site are derived for return periods of 10 to 500 years. These equations may be used to provide a reasonably reliable and rapid check on extreme flood peak estimates derived using other methods. Separate equations apply to the North Island and South Island, each treated as a single region, and require knowledge of the mean annual flood value at a site. Envelope curves for recorded extreme flood peaks as a function of catchment area are presented based on five significant flood peaks. World maxima are about 1.25 times greater than New Zealand values for areas exceeding 90 km~2 and from 1.25 to 12 times greater for smaller basins. It is likely that New Zealand maxima will be exceeded in the future. Problems with employing estimates of probable maximum flood are reviewed. Probable maximum flood use in design is not recommended as the estimates are subjective and not free from risk of exceedance. A probable maximum flood estimate for the Waimakariri River near Christchurch is likely to have a return period of the order of 10~4 years. Allowance for the effects of human-induced climate change should be made when using the prediction equations and envelope curves derived in this study. Further work is needed employing a much larger sample of catchments to provide more accurate prediction equations for non-dimensional flood frequency for return periods exceeding 500 years.