Tuesday, March 5, 2024

Flatter slopes cause sediment build-up

Neal Wallace
Sustained flow that historically carried sediment and gravel out to sea has reduced.
Reading Time: 2 minutes

The slope of several Canterbury rivers has become flatter, allowing sediment to be trapped, according to Environment Canterbury.

The regional council’s senior river engineer, Matthew Surman, said it has long-term information on 589 cross-sections on 11 major Canterbury rivers, with 70% showing a drop in bed levels but 30% an increase.

The data shows slopes on the Kowhai, Ashley, Waimakariri, Selwyn and the North Branch of Ashburton rivers have flattened in recent years.

“These rivers have tended to have the most gravel extraction historically and continue to need monitoring, management and gravel removal to maintain flood capacity,” he said.

Braided rivers are especially dynamic and changeable. Where gravel deposits are extensive, this can cause rivers to change course and lead to increased flooding.

The council uses weed control, gravel extraction and land use management to prevent gravel build-up and to ensure sufficient room when there is flooding.

Surman said gravel demand from industry exceeds the natural build-up on some rivers, including from the North Branch of the Ashburton and the Selwyn rivers.

On the Kowhai, Ashley and Waimakariri rivers, demand exceeds supply.

Surman said gravel extraction has increased in recent years but shifted from areas where bed levels have dropped and to those where there is a surplus.

“A secondary trend has been for extraction to move away from some traditional river sources, towards some smaller rivers and streams further from towns.”

Extraction from Canterbury rivers increased from an average of 740,000 cubic meters per year in the 1990s to 1 million cubic metres per year in the early 2000s, and to an average of 1.3m cubic metres per year from 2006 to 2016.

The natural process of flooding has historically flushed sediment out of river systems.

“However, as the flow is reduced, particularly in larger braided river systems and on rivers where water takes have reduced flows, the sustained flow that historically carried sediment and gravel out to sea, even when not in flood, has reduced.” 

This means some riverbeds are lifted, with the council encouraging gravel extraction for use in roading, concrete and bulk fill.

Surman said historical imagery shows an increase in weeds in some of the region’s rivers, which requires more intervention to manage flood risk as they tend to slow flood flows, raise water levels and encourage the build-up of sediment.

Surman said in some cases land use intensification has narrowed river berms, which changes the way water, sediment and gravel move through the river.

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