Estimation of mean annual flood in New Zealand
The Rational Method of flood estimation is used in an exploratory study to calculate values of mean annual flood in New Zealand basins. To improve applicability of the method and accuracy of the results a power law formula for time of concentration as a function of length, slope and average Manning roughness coefficient of the main channel is derived. It is calibrated employing data from 25 basins with widely differing areas, rainfall intensities and catchment characteristics. The formula explains 88% of the variance between the logarithms of measured and predicted values. A table for calculating the value of the catchment factor (ratio of runoff to rainfall) is compiled which is also designed for use in New Zealand. Rainfall intensities with twoyear return periods are determined using the HIRDS(V3) computer programme. With these new estimators for time of concentration and catchment factor, application of the Rational Method to calculate mean annual flood yields a root mean square error (RMSE) of +/-27% between measured and predicted values for the 25 basins. Performance of the Rational Method is compared to that of the McKerchar-Pearson estimator for mean annual flood using data from a further 10 basins, giving RMSE values of +/-34% and +/-90% for the two approaches respectively. While further testing, calibration and development of the formulae is required, particularly regarding the computation of catchment factor values, application is relatively simple and the Method should result in a substantial improvement in both the accuracy of estimates of mean annual flood and the reliability of use of the index flood technique in ungauged New Zealand basins.