River protection
We're using two methods to protect the edges of Te Awa Kairangi / Hutt River against flooding.
Hard edge protection
In the first half of 2025 we'll be placing over 22,000 tonnes of rock along the river between Kennedy Good and Melling Link bridges, in places where the river runs close to roads, substations or other important structure, such as the new stopbank at Mills Street.
We’re using rocks that are 0.4m to 1.4m in diameter as they need to be large enough to withstand the river’s force when it’s in flood. They are a dense crystalline marble with trace quartz and woolstonite veins.
Other common methods, such as big wire baskets full of smaller stones wouldn’t work here. They'd get smashed up by the debris that comes downstream with the force of the water.
Soft edge protection
In other areas, we’re creating soft edge protection, to slow down the velocity of the water and create a natural habitat for river life. We’re laying horizontal bundles of willows that we harvested to make way for the rock lines.
They're anchored by large concrete blocks buried under the beach and tied in place with biodegradable manila rope.
The uprooted trees stay in contact with the water and sprout quickly. They eventually grow into a natural river berm on the pebbly beach, known as a groyne.
As well as willow bundles, we're also planting willow poles, which do double duty as stabilisers. They slow floodwater over the berms and are angled to help reroute flood water to the river.
Check out this video showing how our soft edge protection is done.
Transition to natives
Willows have been used in New Zealand since the 1840s to manage rivers. In future, we intend to transition to natives between Kennedy Good and Melling Link bridges.
We’ve been running a trial to see which natives might thrive, and testing various planting supports, such as wooden root pegs, wool bags, biodegradable plant guards and wool mats.
In the test area, 18 species have been planted out among young willows, which provide shade as well as slowing down the floodwater. The natives have survived four floods since they were planted in August 2024.