Why don’t we dredge the river rather than build higher stopbanks?

We’ve often been asked the question – why build stopbanks when all we have to do is dredge the Hutt River to open up its channel and avoid flooding? The short answer is that the impact of gravel extraction on river flow is limited compared to raising stopbanks. Even dredging a deep channel in the shallow waters below Kennedy-Good Bridge wouldn’t allow enough water to drain into the harbour in a flood, whereas much greater volumes would flow through a wider river channel bounded by stopbanks.

What’s meant by a 100 year flood return period?

A return period is an estimate of periods between floods (when a flood of a certain size is likely to return) and the chance of one happening. So a 100 year return period flood is estimated to occur every 100 years (and therefore has a 1% chance of happening in any year).

The shorter the return period, the smaller the flood as smaller floods happen regularly (say 1 in 5 years). Bigger floods are rarer (say 1 in 100 years) and significant floods (say 1 in 440 years) rarer still.

But ultimately nature rules and like, buses, we expect floods to be regular but sometimes they all turn up at once. Our flood protection team adopts the view that we should expect the unexpected – which is why, especially in view of climate change, we want to improve flood protection in the Hutt CBD as soon as possible.

What is Riverlink?

RiverLink is designed to provide better flood protection, lifestyle and transport for Lower Hutt.

Three agencies - Greater Wellington Regional Council, Hutt City Council and the NZ Transport Agency - are working together through the project to deliver these benefits.

Better flood protection – we’re moving from a 1:65 return period flood protection standard to 1:440 to prepare for the higher rainfall and more volatile weather expected to be driven by climate change. Potentially at risk are lives and Lower Hutt’s $1 billion worth of assets.

Better lifestyle – we’re working together to rejuvenate Lower Hutt’s CBD through encouraging development to embrace the Hutt River and attract people to the area through providing recreational amenities along the riverbank, including pathways along the stopbanks and easy access to a restored river channel environment

Better transport – we’re working together to make Lower Hutt easier to access via SH2 and easier to get around