Estimating single-site design flood variance using a generalised logistic distribution

An easy-to-use equation is presented for calculating the variance of a design flood estimated using a generalised logistic distribution with model parameters estimated using single-site analysis, as described in the UK Flood Estimation Handbook. The equation is applicable for return periods in the range 2–1000 years and for annual maximum flood series with L-skewness of −0.45 to 0.45, which is considered representative of most UK flood data and practical uses.