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A Rapid Assessment Of The Extreme Rainfall Event In Canterbury, New Zealand And Environs

There have been numerous media reports of the 1 in 100-year flood event in the Canterbury region over the last several days. The preliminary data is readily available for hourly, daily and multiday rainfall totals across the region. The primarily daily data used in this brief assessment is still to be verified but checking across a handful of NIWA stations from the coast to more inland sites does reveal some interesting variations.

For example, one of the heaviest rainfall sites over three days, and not all sites in the region have been checked, nor are all sites reported as yet, is the Mayfield At Ruapuna Forecast site with 282.6 mm over three days. When checked against the very accessible HIRDS 4.0 dataset developed by NIWA that is very close to a 1 in 250-year event. Note, that is for a three-day accumulated rainfall.

However, it gets more interesting. Over the first day of the event at the Mayfield site between 9:00 am on Friday and 9:00 am on Saturday only 30 mm fell. The distribution of that rainfall over the 24-hour period is important as it was relatively evenly spread through the day which would have started the process of filling the water filled areas or zone of saturation. The second day of the event from 9:00 am Saturday to 9:00 am Sunday was critical when 110.2 mm was recorded. For a one-day event at that site based on HIRDS 4.0 historical data that was only a roughly 1 in 10-year event. The final day of the event running from Sunday at 9:00 am to Monday at 9:00 saw a further 142.2 mm. For a one-day event that accumulation represented about a 1 in 50-year event.

However, it is the accumulation of rainfall over multiple days that is critical to understanding this type of flood event. Short duration high intensity rainfall can cause flash flooding and such events tend to be more localised. Multiday events such the recent Canterbury floods are a different story. As seen, the damage can be much more widespread and severe. When multiple days are examined the accumulated two-day event from Saturday at 9:00 am to Monday at 9:00 of 252.4 mm exceeded the 1 in 250-year HIRDS 4.0 value of 247 mm. The three-day total at 282.2 mm also just exceeded the 279 mm 1 in 250-year return period found in as the HIRDS 4.0 value for such an event.

Other sites across the region such as Methven and Lismore also, on preliminary examination, experienced three day accumulated rainfalls in the 1 in 250-year event range. While Christchurch airport over three days received 101.2 mm which is only a 1 in 5-year event.

It is important to note that all the data for this geographically expansive event has yet to be thoroughly checked and validated. Yet this preliminary data undoubtedly points to an extreme multiday event. Such events also point to the need to fully understand the implications of a wide range of rainfall event types. Short duration, minute by minute downpours to events that potentially can last for even longer than three days can all potentially exceed the 1 in 250-year return period and threaten life, livestock and property. As climates continue to change New Zealand will not be immune from the full range of potential rainfall event types.

About CLIMsystems Ltd: CLIMsystems has assembled an excellent team of climate change adaptation and risk assessment experts, with a combined experience of over 200 years with projects in over 50 countries. Our expertise ranges across climate model development, Task Force on Climate-related Financial DIsclosures, adaptation pathways and economic analysis of climate risk mitigation projects.

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