Extreme Rainfall Events : Too Small and Too Few?

As a local authority North Shore City Council needs to know how frequently extreme events are likely to occur within the city.

Extreme events described by Average Recurrence Interval are useful for specific locations within a city but not for the management of a city, region or country.

This paper contains an assessment of NSCC’s rain gauge network and details the number and severity of recorded rainfall events across the city.

Current extreme event methodology underestimates the frequency/depth of extreme rainfall events of less than 5 years Average Recurrence Interval. In many instances the period of recorded data is sufficient to determine the magnitude of extreme events without resorting to extreme value theory.

Rainfall depths for frequent extreme events are underestimated by up to 20% using standard methods.

Coupled with underestimation of event size is the misunderstanding that Average Recurrence Interval can be applied to a large geographic area. From North Shore records it is apparent 2 year ARI storms occur within North Shore at least twice a year – four times more frequently than most people expect. Areal extents of an extreme event can vary from <1% up to 60% of the city area.

This paper details how often North Shore City Council and cities in general should expect extreme storm events to occur.