By Martin Doyle
The rainstorm near Nelson on 21 April, 2013, was one of the most intense ever measured in New Zealand, and caused considerable flooding through urban Stoke and Richmond. Just 18 months prior, another storm also caused significant damage to the same area. It would be tempting to draw parallels between the two storms, but in fact they provide an interesting contrast.
The figure to the right shows the hourly rainfall from the onset of rain for the two storms. The total rainfall over the 24-hour periods was 192 mm for the 2011 storm and 191 mm for 2013. Despite this similarity, in December 2011 the rainfall was much less intense.
The 2011 storm caused land slippage, debris flows and choking of riverbeds. The flooding that occurred was often because rivers channels were filled with sediment and were unable to carry the flows. Much of the damage that occurred was due to slips rather than flooding, and the most costly damage occurred on roads rather than buildings.
In contrast, during the 2013 storm the sheer volume of water exceeded the design capacity of the culverts and drains, and resulted in widespread flooding. Damaging flows originated not only from rivers and creeks, but also from sheet flow running overland off the Richmond and Stoke foothills. Stormwater systems had little hope of draining this water away, and it continued downhill in pathways previously unused to water flow. It accumulated where culverts and grates were blocked or overwhelmed, and backed up in areas behind streets, highways, landscaping, or natural contours. The principal source of damage was water flowing through buildings and homes.
Outwash fans – the accident waiting to happen
The suburbs of Stoke and Richmond near Nelson were vulnerable to this type of flooding because, like many places in New Zealand, they are built on outwash fans. Outwash fans are prone to flooding especially when they are steep, as they typically do not have well-defined permanent flow channels. The fans’ very existence stems from many floods over thousands of years depositing sediment haphazardly across the surface.
During the 2013 storm, once free of the usual flow paths, water travelled downhill across the fans in a number of directions, often in areas difficult to predict. The image below displays the contours along the Richmond foothills, and is a good illustration of the lack of defined waterways for each outwash fan. It is easy to see how water displaced from the usual watercourses, or water collected locally by heavy rain, can flow in unpredictable directions.
How extreme was the April 2013 storm?
We know that localised extreme rainfall events occur reasonably often across New Zealand. However it is uncommon for these to occur exactly where a raingauge is located. Because the 2013 storm occurred in an urban area, the extreme rain was measured by not one but six raingauges, with the maximum 1-hour total of 101 mm. Based on all previous data collected across our district, the chance of this occurring in any given year is 0.2%, or “500 to 1” in betting terms. The common way of expressing this is to say this is a 500-year event, but this incorrectly implies that the storm will happen only once every 500 years.
The most extreme rainfall over 1 hour measured in New Zealand, a total of 134 mm, occurred high in the Southern Alps in the Cropp Valley, in the Hokitika River catchment. Many of the greatest rainfall totals measured in New Zealand have occurred in this location, including 682mm over 24 hours, 2927 mm over 1 month, and 18442 mm over a 12-month period.
The most intense rainfalls measured in the world are seen in the warmer climates. Spare a thought for the town of Holt in Missouri, USA, where 305 mm was dumped in the space of 42 minutes in 1947.
[Editor’s note: Here is some TV news coverage with good footage of the flooding.]
Martin Doyle is a hydrologist at the Tasman District Council.