HIRDS: A tool for evaluating storm rainfalls
Estimates of high intensity rainfalls are used by scientists and engineers in a wide range of environmental studies, engineering construction work, and design flood estimation. They are also an important tool for assessing the rarity of observed rainfall amounts. Design rainfalls are used in climatological analyses by meteorologists and hydrologists, as well as in the insurance industry, to assess risks of inundation and set premiums. They can also help city and district council planners and engineers to evaluate the intensity of extreme events which storm-water drainage systems and other infrastructure should be designed to handle.
One such tool, used in New Zealand to provide design rainfall estimates, is NIWA’s HIRDS (High Intensity Rainfall Design System) software. For any site in New Zealand, HIRDS will estimate the expected rainfall over a range of storm durations and recurrence intervals. A typical screen shot of the output from HIRDS is shown above.
The underlying science behind HIRDS is based on a regional frequency analysis of annual extreme rainfalls, from around 2000 sites in New Zealand, from NIWA and regional council data archives. A very brief summary of the regional frequency analysis procedure involves:
- the mapping of the median annual maximum rainfall (or index storm rainfall)
- the development of regional growth curves that relate rainfall at different recurrence intervals to the index rainfall.
When these two components are combined, design rainfalls are calculated.
The southern North Island storm of 14-17 February 2004 produced widespread flooding and extensive infrastructure damage. So how rare an event was this storm? The map to the right shows the average recurrence interval of the 24-hour maximum rainfall. A large part of the Horizons Regional Council area had storm rainfalls that were likely to occur less frequently than once every 150 years.
Aerial view of Matata following the extreme rainfall events of May 2005. Storm rainfalls and their likelihood of occurrence can be estimated by HIRDS for any location in New Zealand. Cover photo: Alan Blacklock