Upper Motueka Flood Recovery

Client: Tasman District Council

In May 2023 flooding caused significant damage across the Tasman District Council’s maintained river network.

One site in the Upper Motueka River suffered significant damage when part of the flow jumped out of the active channel and began flowing through a landowner’s paddocks before returning into the river downstream.

Key features

• Hard engineering
• Bio engineering tools
• Trenched and weighted willows
• Willow planting
• Debris fences/traps
• Sacrificial gravel bunds

When the flood waters receded, there was a large volume of gravel deposited on the site, which meant that there was virtually no difference in height between the riverbed and the landowner’s paddocks. This meant that rainfall events as small as 19 cumecs would result in water flowing back through the paddocks. 

“Within the conditions of the global resource consent held by the client, there was no option to build a bank to provide a protection for this site in future events,” says Taylors’ Rivers Department Manager, Ben Burbidge.

After a series of planning discussions and site visits, it became clear that it would not be possible to prevent floodwaters from flowing through the site in high flows. However, the desired solution was to prevent flood debris from being transported into the landowner’s paddocks during these events. It was suggested that the best approach would be to turn the site into a trial for using a mixture of some hard engineering and bio-engineering tools to encourage the natural construction of a bank over time. 

Hard engineering

It was decided that the installation of a rock retard at the upstream end of the site was going to be the best way to incorporate rock into this site.  The function of this is to reduce the velocity of the floodwaters when they overtop the structure and to encourage the build-up of silts behind.

Bio-engineering tools

Across the site, the planned works incorporated four different bio-engineering tools – trenched and weighted willows, willow plantings, debris fences/traps, and sacrificial gravel bunds. 

Trenched and weighted willows

Five packets of willows salvaged across the river were placed into a trench, weighted down using large rocks as an anchor, and tied together with rope.

Willow planting

A total of 765 locally sourced willow poles (donated by Goldpine) were planted in rows between the packets of trenched willows. These plantings were later supplemented with an additional 223 in 2024.

Debris fences/traps

Two types of debris fences/traps were installed on the site. One type was made from salvaged pine trees trenched in to form a cross, while the other used untreated wooden posts and ropes.  Both methods are designed to catch any debris in the flood waters and encourage the natural build-up of silts. 

Sacrificial gravel bunds

On the downstream side of the packets of willows there was a gravel bund created to serve two purposes: one was to provide some protection for the willow poles that were planted from flood debris and the other is to work in a similar way to the rock retard to reduce the velocity of floodwaters to encourage the build-up of silts.

Like any new idea, there was some initial scepticism, says Ben, but the system has already proven its worth.

“This is a solution that had never been implemented on this scale before,” says Ben. “However, the results achieved have exceeded the client’s, landowner’s, and our own expectations,” says Ben. “There have been subsequent events of up to 50 cumecs that have not resulted in water flow through this site and even in larger events there has not been any flood debris flow into the landowner’s paddock.

Ben credits Taylors’ Rivers Supervisor, Ian Hogg, with working to gain buy-in from the Tasman District Council and the local landowner to trial this solution. Based on the success of this project, Ben thinks that other councils will use the same approach.

“It’s a solution that works in the right spot,” says Ben.