Estimating the flood frequency distribution

The goal of this study is to develop an objective methodology for estimating design floods, i.e. the flood peak discharge with return period T , at ungauged river catchments in Iceland. First, a regional flood frequency analysis is presented for estimating the T -year flood peak discharge with fixed duration D, Q(T,D), for poorly gauged and ungauged catchments. This is done by scaling a regional flood frequency distribution by the so-called index flood of the catchment. The regional flood frequency distribution is a dimensionless probability distribution function (PDF) estimated by pooling together the individual flood frequency distributions of a group of homogeneous gauged catchments. The index flood is defined here as the mean annual maximum flood peak discharge. For gauged catchments, the mean annual maximum flood discharge is estimated from the measured streamflow series. For ungauged catchments, the mean annual maximum flood is estimated by linear regression using physiographic and climatic catchment descriptors. Then, the method is evaluated for ten catchments located in the Tröllaskagi region and in the West-fjords using annual maximum instantaneous (D=0) and daily (D=24h) streamflow series. The results indicate that the method looks promising for the estimation of the T -year flood peak discharge and the associated confidence interval, along river channels, as part of the design of bridges or dams for instance and in other hydrological applications such as reservoir management and analyses of dam safety. This general methodology should be further developed and also tested in other regions.