Properties of extreme point rainfall II: Parametric data interpretation and regional uncertainty assessment

As an alternative to the traditional non-parametric method the partial duration series approach with exponentially distributed exceedances is used to model extreme values of depth and maximum 10 min intensity per rainfall event, measured at gauges placed at different locations in Denmark. A statistically significant regional variation is documented and shown to be of importance to engineering application. The apparent variability is divided into sampling uncertainty and uncertainty caused by true regional variability. Further, a method for assessing the total inherent design uncertainty, taking into account a possible intersite dependence structure, is developed. Adding one total design standard deviation to the regionally averaged T-year events yields increased design values between 5 and 17%. This result brings up a number of questions with respect to application of historical rainfall data in urban storm drainage design and analysis.