Monday, February 26, 2024

Flood risk with riverbeds on the rise

Neal Wallace
Beds higher on almost a third of major Canterbury rivers.
Reading Time: 3 minutes

The height of some South Island riverbeds is rising, accentuating flooding events and prompting farming leaders to ask regional councils to take more action.

Environment Canterbury said data from 589 cross-sections on 11 major Canterbury rivers shows the beds on 30% are increasing, and a farming leader said in a recent flood, Southland’s Oreti River peaked well below previous events, but flooding was much greater due to river beds being raised by gravel deposits.

Mid Canterbury farmer Chris Allen estimates a 1km stretch of one branch of the Ashburton River has risen by a metre in recent years.

Allen and Southland Federated Farmers Junior vice-president Jason Herrick both know bridges where until recently vehicles could drive under them, but such has been the gravel buildup, people can now touch the underside of the bridge while standing in the riverbed.

Herrick said islands of gravel deposits have formed in many Southland rivers and are sprouting vegetation, which is exacerbating flooding.

He wants regional councils to encourage the removal of greater volumes of gravel for uses such as roading. Allan wants long-term management plans for Canterbury’s rivers, covering the mountains to the sea.

ECan surveys show the slopes of the Kowhai, Ashley, Waimakariri, Selwyn and the North Branch of Ashburton riverbeds have become flatter, trapping sediment.

This is attributed to changes in the slope of surrounding terrain and narrowing river channels, weeds in riverbeds slowing flows and changing weather patterns between periods of drought and heavy rainfall.

The council said these rivers have historically had the most gravel extracted and continue to need monitoring, management and gravel removal to maintain flood capacity.

Allen, who is also on the Ashburton River Liaison Committee, said Environment Canterbury is aware of the need to have plans for the region’s rivers.

In the interim, ECan is completing a riverbank tree-planting programme funded through the Infrastructure Reference Group’s shovel-ready projects, part of the government’s post-covid response.

Allen said the trees will assist with bank stability and keep the fastest flow in the middle of the river, but a plan needs to go further than that.

“It’s a very good start to stabilise the riverbanks, but they’re not doing anything with the gravel.

“They’re only doing maintenance, they’re not beefing up flood protection, they’re not future-proofing it looking 20-30 years into the future at what are they trying to achieve.”

He said a plan is needed, to determine what is to be achieved, how to address weak points, how to manage a “once in 100 years” flood, where flood waters can be channelled before returning to the bed and how to pay for it.

This has to be achieved by working as much as possible with nature.

Allen said one option is to encourage contractors to extract gravel where it needs removal rather than where it is economic for them to do so.

Herrick said a rain event on the scale of 1978 or 1984 would cause devastating flooding today for Southland communities and farms alongside rivers given the increased gravel deposits.

Roading contractors and farmers have been sourcing gravel from farm quarries instead of riverbeds, which he attributes to a consenting process that prioritises environmental and cultural values over practical river management.

Environment Southland said a lack of large flooding events in the past 20 years has allowed a build-up of gravel.

Smaller floods tend to disperse sediment across gravel bars without moving it.

A statement from the Ministry for the Environment says that the Resource Management Act devolves responsibility for local management of rivers to regional and territorial authorities to allow localised community issues and decision making.

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