Close watch on 'highly mobile' Franz Josef river

Major shifts within the Waiho River bed are increasingly triggered by minor rain and the situation is "highly mobile", says the West Coast Regional Council.

Regional council chief executive Darryl Lew said on February 27 officials have now "formed a view" around some stop bank maintenance on the river's southern bank, to be formalised by the joint rating district for the area and council.

"We're still digesting the geotechnical advice and drawing up a bit of a plan… but the main channel is moving around a lot. The river is highly mobile," Lew said on Tuesday.

That mobility reflected the significant trend north of the main channel shifting north after January 20 and then splitting again on February 15.

"Things are moving very quickly down there… we have to be very confident and considered about what we do," he said.

He expected repairs to the southern stopbanks to be confirmed by next week.

Of concern was the Waiho northern bank avulsion into the neighbouring Tatare Stream, below the Franz Josef sewerage ponds.

Lew said the river at that point had als now begun to "back cut" by about 50m toward the sewerage pond area.

Intensive surveys of the avulsion at that point had begun.

Lew said the risk of the river cutting back had been "well predicted" including in geotechnical reports going back to the 1990s on the natural hazard risk to Franz Josef, and by the Waiho Technical Advisory Group (TAG).

Loose, unpacked sediment rapidly building up in the bed was the main cause.

"The unconsolidated sediments on that side give a huge opportunity for back cutting — this was well predicted.

"What the TAG said was in two to five years. What we're seeing in terms of the avulsion could actually happen sooner. It's happening quicker."

The significant avulsion in the river bed on the northern side in mid-2023 forced council to abandon bunds to keep the Waiho River and Tatare Stream beds separate.

The avulsion trend north became more permanent after the January 18-20 red alert weather event when 95% of the Waiho's main flow began to run via the Tatare Stream.

That left the usual river bed past Waiho Flat on the southern bank virtually dry.

At the same time, it heightened the potential threat to the Franz Josef sewerage ponds.

Since then a fairly moderate event overnight on February 15 saw about 50% the river channel split again away from the north side.

Lew said the worrying trend is that the river is increasingly vulnerable to sudden shifts following relatively low rainfall events.

"The river is highly, highly, active to even small rainfall events, to the course it chooses to run."

The heightened risk of the river changing even in fairly moderate rains meant council was keeping a closer watch, Lew said.

"We're continuing to monitor the situation in the river and we've significantly increased our staff presence down there.

"We're not even talking orange and red (weather) alerts here given how highly mobile the river is," Lew said.

Westland Mayor Helen Lash said she was surprised the river was now cutting back towards the sewerage ponds but the district council was well aware the river is highly mobile.

"It's changing almost daily… it's certainly a beast and a half."

Pending future decisions to move the ponds, the district council meantime has begun its build of about 200m of new protection for the Franz Josef sewerage ponds, off the end of the existing northern Havill Wall stopbank.

"Our work's going to plan."

Lash said it was proceeding on the basis of buying enough time for council to finalise a new wastewater treatment site, expected to take 18-24 months in planning and consents.

But she hoped new fast track legislation by the Government for infrastructure would benefit council on this.

"What we're really wanting is some space."

Local Democracy Reporting is local body journalism co-funded by RNZ and NZ On Air