High Intensity Rainfall Design System
HIRDS is a simple online tool that can estimate the magnitude and frequency of high intensity rainfall at any point in New Zealand.
NIWA's High Intensity Rainfall Design System (HIRDS) offers planners and engineers more certainty about the frequency of high-intensity rainfall events, enabling them to better design stormwater drainage, flood defence systems and other vital structures.
HIRDS data is released under the Creative Commons Attribution-NonCommercial (CC-BY-NC) 4.0 License, with copyright to NIWA and New Zealand Regional Councils.
The main purpose of the HIRDS tool is to estimate high intensity rainfall at ungauged locations for a range of return periods and event durations. The depth-duration-frequency (DDF) tables produced can to be used for design storm assessment and for the design of flood protection works and other waterway structures. The DDF tables are also applicable to flood modelling, including flood routing, retention basin design and inundation mapping activities, thereby helping to minimise flood risk within communities across the country.
In certain circumstances, this tool may also be used to estimate return periods for post-event analysis, for example comparing a rainfall gauge event magnitude to the equivalent HIRDS output. However, except for cases where the gauge record is short, it is preferable to estimate the event return period directly from an extreme value analysis of the gauge record itself. That is, the main purpose of this tool to provide reliable estimates of return periods at ungauged locations, not to replace site-specific extreme value analysis at gauged locations.
The current version of the tool (HIRDSv4) was released in August 2018 and replaces HIRDSv3 which was released in 2010. The version 4 update was prepared in consultation with Regional Councils and was funded through an Envirolink Tools Grant. This update endeavoured to utilise all available historical records and the most up-to-date rainfall measurements gathered from multiple agencies. This comprehensive data set enabled more accurate estimates of median annual maximum rainfall and more robust regionalisation of growth curve parameters than was possible in earlier versions of this work. This is particularly true for sub-daily event durations and even more so sub-hourly durations.
The underlying method used in HIRDS is an index-frequency procedure. The index-variable is a relatively common event, such as the median annual maximum rainfall, that can be computed from the available records. The frequency-variable is derived from a regional analysis, where data from several sites are combined to estimate a dimensionless rainfall growth curve which represents the effect of different recurrence intervals relative to the index variable.
Read the HIRDSv4 Technical Report [6MB PDF].
The HIRDS tool also provides estimates of the impact climate change will have on high intensity rainfall events. This has been done using the IPCC 5th assessment framework, focussing on the four representative concentration pathways (RCPs) that are also used in the Ministry for the Environment’s Climate Change Projections for New Zealand report.
The HIRDSv4 Technical Report also provides areal reduction factors (section 5) and temporal storm design profiles (section 6) specific to New Zealand. These can be used when applying results from the HIRDS tool to flood and inundation modelling.
GIS layers containing HIRDS derived rainfall depths for different durations and return periods are available via an ArcGis data service. Each surface contains rainfall depth (in mm) for a given average recurrence interval (in years) and event duration (in hours) on a 2km grid.
A full list of the rain gauges used in the HIRDSv4 analysis along with the data availability for various duration categories are available in spreadsheet form.
View the complete list of gauges used [300KB CSV]
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