On the influence of sample size and recentness for the definition of design rainfall.

The definition of the design storm is of pivotal importance in hydrological studies, such as for the design of urban drainage networks and for flood risk management. The design storm is usually defined throughout frequency analysis of the observed annual maxima of rainfall depth, by defining the probability distribution that best fits the sample of observations. Due to the recent increasing frequency of urban floods and to the gained interest in climate change effects on precipitation features, many local water authorities are asking water engineers to employ not just long time series, but also recent samples to estimate design storm values. However, although it is well recognized the role of sample size in frequency analysis, it has not been investigated yet the influence of the recentness of samples in the estimation of design rainfall. In this study we analyze the influence of both sample features on the estimation of a 100 years return period and 1-hour duration design storm, by using rainfall time series of rain gauges located in central Italy. The study reveals that in most of cases the time frame from which samples are extracted is relevant only for short sized samples, while for opportunely large samples the values of design rainfall computed using recent observations is similar to those obtained with older observations. [ABSTRACT FROM AUTHOR]