The odds of a 1 in 100 year flood is probably greater than winning Lotto. But people buy a Lotto ticket and win, yet ignore the chance of an extreme weather event.
All of our flooding probability work is based on historical information. However, if we look forward, which we have to do with climate change, there is still some uncertainty on what to expect. What is likely is that we will experience lower lows and higher highs, which poses some issues around our management of flood protection schemes.
Our assets minimise risk only to a certain level of service, such as a 1 in 100 year flood event, but it’s not fool proof. We manage our flood and drainage schemes to minimise risk as much as possible, however there is still residual risk and, despite our best efforts, things can fail. If something goes wrong then there will no doubt be impacts.
We interpret forecasts from MetService to provide relevance for our region. What does this mean for us? Part of this is understanding our region’s rivers and schemes, and how susceptible certain areas are to rainfall and coastal events. If we expect any impacts, we tell our stakeholders, the likes of Civil Defence, and the New Zealand Transport Agency, and we let our council staff know, those who look after our flood assets and drainage.
The weather is a dynamic beast and things do change from what was forecast. So it’s a bit of a fine line between getting people prepared and that underlying uncertainty. Everything we do know, we put on Flood Room Live. It’s about giving people relevant information so they can make informed decisions.
The March and April rainfall events of 2017 caused widespread impacts, mainly in the Coromandel, Hauraki and Lower Waikato River area. There were three events quite close together. A lot of the Hauraki Plains was underwater. Some spillways overtopped as they’re designed to do into ponding areas. People had to be evacuated.
We had the Lower Waikato very high. We had to work with Mercury Energy and Genesis Energy to manage the water coming out of Lake Karāpiro and Taupō to minimise the impacts across the catchment. We have all this water and we need to make sure it comes out over a longer period of time rather than all at once.
If it ever gets to a stage where people, property and animals are at risk, Civil Defence take over the management of the event, and we provide information on potential impacts. During every flood event, our staff are out in the field. They have procedures for doing manual checks to make sure pump stations and floodgates are working properly, and keeping drainage clear.
Of course, if it’s a big storm event or a series of events then there is only so much we can do. We haven’t reached that stage where our schemes are overwhelmed, luckily. But it could happen.
During wet weather events, the regional council works with Mercury Energy and Genesis Energy to manage the amount of water coming out of the dams on the Waikato River, to minimise the impacts across the catchment.
"We haven’t reached that stage where our schemes are overwhelmed, luckily. But it could happen."
The Piako River spills into a ponding zone, to relieve pressure on the flood scheme.