Understanding the timing and location of extreme rainfall is one of the most important parts of Stormwater Flood Management. In most urban settings in New Zealand, city authorities make use of rain gauges to understand rain events. Often, understanding rain events is accomplished by classifying rainfall according to an Average Recurrence Interval (ARI). Rain gauge networks take point samples of the continuous spatially varying rain field and it is difficult to know if the heaviest rain has fallen on a rain gauge or fallen elsewhere in a catchment. This rainfall variability can lead to large biases in determining ARI statistics for individual events when using rain gauge data, which in turn makes it difficult to assess the performance of infrastructure which may have been designed with a particular recurrence interval in mind.
This paper presents a methodology for preparing rain radar data from the Auckland Metservice radar to allow generation of spatially continuous ARI maps, and demonstrates how the data can be used to gain understanding of the cause of flooding events for a test case.