A big piece of infrastructure preventing large parts of land near Foxton from flooding is getting a $2.5 million upgrade to mechanics that are now more than 60 years old.

The Moutoa floodgates on the Foxton-Shannon Road was originally built in response to the ravaging floods of 1941, and was fully operating by 1962.

Horizons Regional Council catchment manager Jon Roygard says a routine asset inspection recently revealed some of the electrical, mechanical and structural components have exceeded their design life. They are being replaced with components more in line with modern engineering that will enable the gates to be operated off-site. 

Project manager Barry Hofmeyr says gearboxes have come from Italy, along with more winch systems.

In heavy rainfall the floodgates divert water from the Manawatū River into a specially designed 10km floodway. The diversion of floodwater protects about 280 square kilometres of land in the lower Manawatū Plain from flood damage.

At full capacity, 2500m³ of floodwaters rush through every second – the exact size of an Olympic swimming pool.

The floodwaters take a shortcut through a channel that is 600m wide, with stopbanks 5.5m high, allowing it to bypass 30km of meandering river before meeting up with itself again further west at Whirokino on its way out to sea.

The floodgate network has a book value of $50 million and is by far the largest in the Manawatū catchment.

The curving reinforced concrete structure contains nine steel radial gates, each 15m wide by 4.5m high and weighing 15 tonnes. The gates are raised by a pulley system attached to the concrete piers and are operated by a series of electric motors.

Each gate can be operated independently, although in extreme weather events all the gates can be opened. Jon says how often the gates are opened can vary from year to year. During the flood of 2004, the gates were opened seven times.

Three-quarters of the project was being funded by Kānoa, the government’s Regional Economic Development & Investment Unit, with other funding coming from a cyclone protection fund.

The Manawatū River originates north of Norsewood, west of the Ruahine Range, and drains 6000sq km on its way through the Manawatū Gorge to the sea at Foxton.

Along the way it’s fed by several fast-flowing tributaries, including the Mangatainoka, Tiraumea and Mangahao rivers above the Manawatū Gorge, and the Ōroua, Pohangina and Tokomaru rivers below it.

Jon says weather and river levels are monitored intently. It can take 10 hours for floodwater to arrive at the Moutoa floodgates after an upstream alert at Palmerston North, allowing time for landowners in the flood plain to be notified.

The upgrades have allowed a switch-over to a new electrical system, with new Italian-made and Australian-designed winches, while new overhead powerlines have also been installed.

The project is due to be completed some time next month.