Assessment of Use of At-site and Regional Flood Data for Flood Frequency Estimation

The Standard error of flood quantiles estimated by three different methods were obtained by simulation. The methods were based on (a) at-site, (b) at-site and regional data combined and (c) regional data alone. Data were generated from GEV distributions. Since method (c) involves a regional regression relation an element of physical realism was preserved by basing the generating distributions on those estimated from real data of rivers in S. E. England and by using the corresponding catchment characteristic values in regional regression relations. The generated regional data set included 17 sites of 20 years and one site of variable record length n=1, 3, 6, 10 years. Three sites (low, medium, high Cv) were investigated separately. The at-site/regional method is considerably better than either (a) or (c) especially at high return periods. For large T(c) can be better than (a) especially if n is small. In general, the mean annual flood obtained by regional regression is less precise than $$\overline {\text{Q}} $$ obtained from one year of record.