Estimation of mean annual flood in New Zealand

摘要

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 two-year return periods are determined using the HIRDS (V3) computer programme.