Opinion: Lies, dam(ned) lies and flood statistics

Dr Fuller

By Dr Ian Fuller

So, another “500-year” flood event in the Bay of Plenty. The last was in 2005 at Matata. Of course, this doesn’t mean that in either Edgecumbe or Matata we need to wait another 500 years before seeing a repeat of such a big event. As ably put by NIWA hydrologist Roddy Henderson in Laura McQuillan’s recent article on Stuff it’s about probabilities: a 500-year flood has a 0.2 per cent likelihood of happening in any one year.

These are rare events. No doubt the exact statistic assigned to Edgecumbe’s flood will be debated. What matters most is that it exceeded stopbanks designed to protect Edgecumbe, and did so by 30 per cent, according to the Bay of Plenty Regional Council.

As noted by Mr Henderson, the problem with flood statistics is that they are derived from short records; just 65 years in the Rangitaiki River. We do not have data to reliably determine how often such large floods recur. The likelihood of the largest floods occurring during short periods of data measurement is inevitably small. Further complicating the issue is climate, which changes. If the climate shifts to a more extreme phase, frequent larger floods can be expected. Models cannot take this into account, because they only use existing data.

We need to better understand the nature of the problem. How does last week’s flood compare with floods that occurred earlier than 65 years ago? What documentary evidence do we have for floods in our rivers that occurred prior to measuring flows? Beyond that, what insight can tangata whenua provide, using oral histories spanning 800 years or so? At Massey University, we are now looking at deposits left behind by floods in the sediments that make up floodplains in order to extend flood records. In the Manawatu River we have a flood history spanning at least 2000 years, and we are currently working on sediment extracted from 5m and 8m cores in the Whanganui valley. We need  sedimentary archives linked with gauged, documentary and oral history information to produce a robust understanding of floods in our catchments. A multi-layered approach will help understand how large past floods have been and link these episodes of flooding to past climate, or other significant events affecting our rivers. This knowledge is fundamental if we are to protect our communities.

Building higher stopbanks is not the answer, I believe. Higher stopbanks actually increase the risk of devastating failures such as we saw in Edgecumbe. When they fail, the power of the water at the breach is far greater than it would be in a naturally rising flood spreading across the floodplain. There will always be an upper limit to flood defences, determined largely by how much society is willing to pay. Even if defences could be built that would withstand the 500-year flood, there remains the risk posed by the 1000-year flood. Inevitably flood defences fail and the higher the stopbank the greater the devastation. In New Zealand many flood protection schemes simply have not been designed to accommodate very large floods, nor, practically, can they be.

We need to learn to live with floods by allowing more (although not necessarily all) floodwaters to spill onto floodplains. We saw this in Edgecumbe last week, when breaches were made in stopbanks upstream to relieve pressure downstream. Having to respond in such a way does suggest that we haven’t quite got it right. Nevertheless, the principle is sound: by permitting flooding in areas where people don’t live, areas where people do live can be better protected. Inevitably in intensively farmed floodplains, this principle means losses in the agricultural sector, but compared with devastating loss sustained by communities like Edgecumbe, this may be a price society at large should consider paying.

Floodplain development should be restricted. Encroachment of housing, industry and infrastructure on floodplains puts people in harm’s way. Past mistakes should not be perpetuated. Controlled flooding, flood-proof housing and floodways re-routing floodwater are all part of the answer. In some cases, managed retreat may be the best option.

It’s been suggested that the Matahina Dam was responsible for Edgecumbe’s flooding. In reality dams are effective in moderating impacts of small floods, but have less capacity to mitigate very large floods. It’s easy to apportion blame after the fact, and community anger and frustration are entirely understandable. What is now required is a response that learns from the past,  so that we can better prepare for the future and build a more resilient response to coming floods, which are 100 per cent likely to recur.

Dr Ian Fuller is an Associate Professor in Physical Geography in the College of Sciences’ Institute of Agriculture and Environment, working with Professor Mark Macklin (University of Lincoln, UK) on New Zealand flood histories.

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