NIWA’s High Intensity Rainfall Design System (HIRDS) offers planners and engineers more certainty about the frequency of high-intensity rainfalls, enabling them to better design stormwater drainage systems and other structures.
How to use HIRDS
a) click on the map, or enter an address or site coordinates
b) enter a site name for the location
c) if required, enter the temperature rise for the climate change scenario you are planning for
d) select a format for the output table
e) hit the "generate tables" button to get the results
Entering site Coordinates
There are several ways to enter the site coordinates.
1. Enter an address of interest into box above the map and then press “Find Address”
2. On the map, click on the location of interest.
Both these methods use Google Maps to determine the latitude and longitude of the identified location. For specific information about the exact coordinate systems used, see the section at the bottom of this page.
3. Input the site coordinates manually, either as latitude and longitude or northing and easting into the appropriate boxes beside the map. Click on the colour tab that matches the coordinate system you are entering.
NZTM – easting and northing, based on the NZGD2000 datum, obtained from the NZTM2000 coordinate system
WGS84 – latitude and longitude based on the WGS84/NZGD2000 datum, or directly from Google Maps
NZMG – easting and northing, based on the NZGD1949 datum, and obtained from New Zealand Map Grid coordinate system
NZGD1949 – latitude and longitude based on the NZGD1949 datum.
for more information see "Specific information about the coordinate system" below
Assessment of extreme rainfalls with climate change
New to version 3 of HIRDS is the option to generate tables of high intensity rainfall for given climate change scenarios. By entering projected temperature changes for the location for up to three periods, extreme rainfall tables for the given scenario can be generated.
Projected temperature changes for the site can be obtained by contacting NIWA or by obtaining the climate change guidance manual from the Ministry for the Environment website http://www.mfe.govt.nz/publications/climate/preparing-for-climate-change....
Choosing an Output Table Format
Depending on the option selected, tables will be prepared for the following two formats:
1. Depth-Duration-Frequency. A table of rainfall depths (in mm) for the given storm durations (in minutes and hours) and given recurrence intervals (in years).
2. Intensity-Duration-Frequency. A table of rainfall intensities depth (in mm/hour) for the given storm durations (in minutes and hours) and given recurrence intervals (in years).
Press the “Generate Tables” button to produce tables of design rainfalls. When the output table appears on the screen, the user will have an option to save the table as a CSV file onto their PC.
About the Output Table
The generated table of high intensity rainfall estimates contains the following:
1. A table of either rainfall depths or rainfall intensities for given storm durations and recurrence intervals (ARI). The table also provides the annual exceedance probability (aep) which is the probability of a given rainfall being exceeded in any one year.
2. For the depth-duration-frequency (DDF) table option, a table of coefficients is provided, that will allow the user to calculate rainfall depths for any given set of recurrence intervals and durations. For tables of intensity-duration-frequency (IDF) the coefficients can be used along with an adjustment for the number of hourly units within the event duration. Details are given this PDF.
3. A table of estimates of the standard errors associated with the rainfall depths or intensities, from which confidence intervals can be computed.
4. For each of the projected temperature changes entered, a table of rainfalls is produced using adjustment factors provided in the Ministry for the Environment’s climate change guidance manual. http://www.mfe.govt.nz/publications/climate/preparing-for-climate-change...
Specific information about the coordinate system
Google Maps uses a Universal Transverse Mercator Projection based on the World Geodetic System (WGS84). This datum coincides almost exactly with the New Zealand Geodetic Datum 2000 standard (NZGD2000). The coordinate system in HIRDS is based on the NZGD1949 datum, and default datum on the web-page is the New Zealand Grid Coordinate System (NZMG). The latitude and longitude from Google Maps are transformed into NZMG coordinates, unless one of the three other coloured coordinate system boxes beside the map is first clicked (see also item 3 below map), in which case that coordinate system will be displayed. Land Information New Zealand (www.linz.govt.nz) provides formula and transformation parameters that allow conversion between the two datum’s (NZGD1949 and NZGD2000).