Update of regional intensity–duration–frequency curves in Denmark: Tendency towards increased storm intensities

The regional model for estimation of extreme rainfall characteristics in Denmark has been updated with data from the augmented rain database 1 January 1997–1 August 2005, corresponding to almost twice the amount of data used in the previous study (1 January 1979–1 January 1997). In general, the analysis shows that the regional statistical extreme value model introduced in the previous analysis is satisfactory. In accordance with the previous study a significant regional variability of extreme rainfall characteristics is observed, which can partly be explained by the mean annual precipitation patterns and a sub-regional division of the country in a western and eastern part. Comparison with the previous regional analysis shows a general increase in extreme rainfall characteristics. For the durations (30 min–3 h) and return periods (~ 10 years) typical for most urban drainage designs the increase in intensity is in the order of 10%. The analysis reveals that the changes are not statistically significant compared with the uncertainties of the regional estimation model, but the increases in design intensities are large and have significant consequences to the costs of engineering designs.