Flood protection project to handle flows 30% higher

The latest works are the next piece of the puzzle and will involve a combination of stopbanks and floodwalls designed to manage peak flows of around 380 cubic metres per second (the equivalent of an olympic swimming pool every 6 seconds!) — almost 30 percent higher than the flow recorded during the 2022 event.

Detailed flood modelling for the work has been carried out by Tonkin + Taylor.

“They assess rainfall events under different climate change scenarios to ensure the infrastructure can withstand floods of that magnitude,” says Tonkin + Taylor Principal Planner Sara McMillan.

“Typically, these projects are designed for a one-in-100-year flood — meaning a flood that has a one percent chance of occurring in any given year. There are two main types of flood improvement works. The first gives the river more space by increasing the capacity of the channel, making it wider or deeper. That can be difficult to achieve in developed areas.”

The Maitai Flood Mitigation Project will focus on the second option: forming a barrier along the riverbank.

“A stopbank is an earth embankment built along the riverbank, essentially creating a barrier to hold back the river,” says Sara. “A floodwall is built on top of an existing stopbank using very strong materials and deep piles that anchor it to the ground, allowing it to withstand the immense forces of a river in flood.”

The area behind Hanby Park was first developed in the late 1940s and early 1950s and has experienced large floods in the past.“In 1954, a major flood inundated around a dozen homes in the area,” says Toby.

Other significant floods occurred in 1970, 1986, and a smaller event in 1995.

Following the 1954 flood, Council installed a stopbank behind Hanby Park, Mill Street and down to Clouston Bridge. This stopbank has provided protection during most flood events since then.