Dependence of Flood Peak Magnitude on Catchment Area

The relationship between flood peak and catchment area is examined and exceedance probabilities of an envelope line for these variables is derived for Westland, New Zealand. In particular, dimensional and Rational Method analysis, together with statistical evidence, is used to show that flood peak magnitude is proportional to catchment area raised to the power of 0.75. The analyses also support the mathematical form of the empirical TM61 formula and its recommended use in New Zealand for checking estimates of flood peak magnitude in small, ungauged catchments. Using flood records from Westland, an envelope line is derived from a logarithmic plot of maximum recorded flood peak magnitude against catchment area for Westland. The expected exceedance probability of the envelope and the exceedance probability of the expected envelope are estimated. Values obtained are consistent with assessments provided by station year and index flood methods. Consequently, where envelope lines can be derived, they provide a useful preliminary estimate or a quick check on other estimates of flood peak magnitude and its exceedance probability obtained, for instance, from rainfall-runoff models or flood frequency analysis.