The misleading ‘one in 100 flood’

This week I had a chat with Joshua Sargent with the Environmental Research Institute at the University of Waikato. He is an environmental spatial scientist, which is someone who combines the elements of environment, people, and place, with a strong interest in human-flooding relationships.

Janine Krippner

This is a good place for it, floods are our number one hazard in terms of declared Civil Defence Emergency Management emergencies, how often they occur, and losses. During our larger events, terms like “100-year flood” pop up, which that some say is the most misunderstood term in disaster preparedness.

The term came about in the 1960s through a national flood insurance programme and was not intended to be used for specific locations. It can be helpful to have a scenario like this for understanding how things that may be impacted, but these can become part of our everyday terminology where the context is lost and incorrect meanings take on a life of their own.

While a 1-in100-year-flood sounds like a flood of a given size happening once every 100 years, that’s not what it means. It means that there is a 1 in 100 (1%) chance of floodwaters exceeding a given level in any single year. More than one can occur in a year too. This sounds like a small chance but ignoring it can lead to much greater impacts if we as individuals and communities don’t take steps to be prepared.

The impacts – what is damaged and how, vary greatly. As with every hazard, there are groups and individuals more vulnerable than others. On the broader scale we have our farmland versus our towns and preparedness can look different.

For example, farmers can work with their lands to redirect water and have raised areas for livestock to avoid harmful contact with standing water. For households, having an emergency plan is a great idea, and not just for flooding.

If something hasn’t happened recently we tend to forget that it does happen. If we aren’t prepared, it’s a bigger problem when it does happen. We can make small improvements and undertake maintenance that can reduce the impacts and speed up our recovery.

Since flooding is frequent in our region with our 20 large rivers and about 1400 smaller river systems there is a lot more information on the Waikato Regional Council website, Civil Defence has advice for what we can do to before, during, and after a flood on their website, and our district council websites have information for us too. NIWA also has a quick and easy “Township Flood Challenge” game online.

If we know what to do, we can act faster, and taking quick action is important with rising water. Remembering that we should stay out of flood water, not drive through it or play in it, can prevent harm to us and our loved ones.

It is important for us to know that flooding can happen any time (even when the sun is shining), they could happen more frequently than we think, what the impacts could be for us, where to go for relevant information, and what we can do to help ourselves.

The decisions we make today will influence future flooding scenarios. Once again, we are not powerless.

 

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