Maps of flood statistics for regional flood frequency analysis in New Zealand

Abstract

The two-parameter EV1 distribution adequately describes New Zealand's flood series. Contour maps of [Qbar]/A^0.8 and Q₁₀₀[Qbar] are presented, where [Qbar] is the mean annual flood, A is the basin area and Q₁₀₀ is the 1% annual exceedance probability flood. The maps are based directly on measured discharge series from a large sample of river recording stations. Thus when basins are ungauged, or have just a short record, an estimate of a design flood Q_T with specified annual exceedance probability (1/T) can be obtained using map estimates of [Qbar]/A^0.8 and Q₁₀₀[Qbar], without having first to estimate rainfall statistics for the basin, a particularly difficult task in sparsely instrumented mountainous areas. These maps succinctly summarize a great deal of hydrological information and permit improved flood frequency estimates.