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Moutoa Floodgates

The Moutoa Floodgates, located between Foxton and Shannon on the Manawatū River, provide vital flood protection for land in the lower reaches of the Manawatū River.

The curving reinforced concrete structure contains nine steel radial gates, each 15 m wide by 4.5 m 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 and standby power is available in case of electricity failure.

The gates were built in response to a 1941 flood, and were completed in 1962. When operated at full capacity, they redirect approximately 2,500m³/s of floodwater through a specially designed, 10km long floodway channel. This diversion of floodwaters protects approximately 280km2 of land in the lower Manawatū Plain from flood damage.

After more than 60 years of operation, the gates are being upgraded to ensure they provide flood protection for decades to come. Read more about these upgrades further down this page.

Watch the video below to learn more about the Moutoa Floodgates.

Upgrade works

After over six decades of service with minimal upgrades, the latest investigation into the condition of the gates revealed that some electrical, mechanical and structural upgrades are required to ensure effective and successful operation of the floodgates.

Upgrades began with the successful installation and switch-over to a new electrical system in October 2024. The remaining upgrades are being undertaken in stages to maintain service levels as close to regular operation as possible. Flood modelling has also been undertaken to assess potential impacts of more severe flooding and determine necessary flood contingency plans should adverse weather hit the area while upgrades are underway.

The contract for construction was recently awarded for $2.5 million, with 75% of that from Kānoa, central government’s Regional Economic Development & Investment Unit.

Please note: Motorists travelling through the area should be advised that traffic management measures may be in place between 7am-5pm on weekdays at any time during the next 12 months depending on work being undertaken.
 

One of the new H-structure poles.

Above: One of the new H-structure poles.

Electrical

The overhead lines previously spanning across the gate piers, have been relocated downstream of the gates. New H-structure poles have been installed by Electra on both sides of the floodway to support the approximately 200m span of conductors. This allows easier access to the piers for routine maintenance and for the installation of the upgraded motor and winch sets scheduled to start in 2025. 
 

Electrical System Upgrade

The motor control system has been upgraded to allow for a smoother and more resilient gate control. This upgrade also included the installation of an automatic switchover between mains and generator in the event of a power failure. 
 

Mechanical upgrade

The current mechanical components are more than 60 years old and have exceeded their design life. The motors, winch sets and sheaves will be upgraded. This contract, which includes detailed design, was awarded in August 2024, shortly after which design commenced. Installation of the new winch sets and motors are expected to commence late Feb/early March 2025 and is expected to run through to mid-September 2025. The new components will have a 50 year design life, providing many more decades of flood resilience to the area.
 

Sheave component on floodgate.

Above: Close up of a sheave on one of the gates, which forms part of the winch set that lifts and lowers each gate.

Close up of winch drum, motor and gear box.

Above: Close up of a winch drum, motor and gear box that forms part of the winch set.

The Manawatū River

The Manawatū River originates north of Norsewood in Tararua District, to the west of the Ruahine Range. It drains 6000 square km on its way through the Manawatū Gorge to the sea at Foxton. On this circuitous route it is joined by several major, fast flowing tributaries, both above and below the Gorge, including the Mangatainoka, Tiraumea and Mangahao Rivers and the streams of the South East Ruahines above the Gorge, together with the Ōroua, Pohangina and Tokomaru below the Gorge. 

On reaching the wide Manawatū Plain the river adjusts to a shallower gradient and meanders over the last 48 km of its journey, with a fall of just 18 cm per km. The Manawatū Plain was formed from silt carried downstream by the river. Much of this flat expanse of land is at sea level or below and consequently the area can flood easily.

Horizons continues to further climate resilience throughout the region through construction of new flood protection infrastructure and upgrades to existing infrastructure.
 

Moutoa floodway

Located on Foxton-Shannon Road, south of Ōpiki and just above Moutoa, the gates divert water from the main river into a specially designed 10km floodway that rejoins the river at Whirokino, just before Foxton Loop.

The floodway bypasses the 30 km of slow-flowing, meandering channel that can easily flood and pour water over many hectares of valuable land. It is 600m wide and is bounded on both sides by 5.5m high stopbanks.

Several features of the design prevent the floodway from being damaged by flood flows:
  • energy dissipation blocks immediately adjacent to the gates reduce the speed of the water as it comes under the gates;
  • the curved shape of the structure enables the water to fan out and spread evenly across the floodway; and
  • the floodway rises slightly away from the gates so the water has room to pool and does not gouge out a channel.