Tuesday, March 5, 2024

Environment Southland taking holistic route to river management

Neal Wallace
Fewer large floods in Southland allows build-up of gravel and sediment.
Smaller floods tend to disperse sediment across gravel bars without moving the gravel.
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The fact that there are fewer large flood events in Southland has allowed a build-up of gravel in the region’s riverbeds, according to Environment Southland.

The council’s manager of integrated catchment management, Paul Hulse, said in the past 20 years the province has experienced few of the large flooding events that traditionally move gravel.

“Smaller floods tend to disperse sediment across gravel bars without moving the gravel,” Hulse said.

“This can lead to the gravel bars being suitable for weed and vegetation growth, and we see more root mats and stems forming, making the gravel more resistant to river flows.”

Silt deposits can also reduce the quality of gravel.

The council relies on market demand for the mechanical removal of gravel but fewer consents have been granted so the council is considering funding gravel removal in its next long-term plan.

Data provided by the council shows a halving in consents for the removal of gravel, from 203 in 2011-12 to 293 in 2012-13 – and only 100 in 2022-23.

Hulse said consent was recently granted for gravel management trials in three rivers that involves vegetation removal and beach skimming.

Less gravel removal improves the natural character of the river, he said, and can also reduce bank erosion.

Hulse said the council is adopting a holistic approach to river management and is developing a strategic gravel management policy.

This includes improving data and science to support consents for minor gravel works such as beach skimming, mitigating riverbank erosion, sediment loss and managing gravel build-up under bridges.

He said this will extend the council’s ability to manage the impacts of gravel more effectively.

Larger extraction projects will still go through the normal consent process.

The council is undertaking a resource assessment using bathymetric LiDAR to survey the Mataura and Waikaia rivers. The survey is the first of its kind in New Zealand, and will eventually be extended to all of Southland’s major rivers.

Hulse said data collection has been completed and processing is underway.

It will provide elevation mapping and show gravel build-up, landscape changes, catchment hydrological processes such as stream flow estimation and catchment size. 

Environment Southland is also working on a climate adaptation project aimed at identifying ways of better protecting high flood-risk areas in Southland.

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