New lake created by Cyclone Hale being closely watched

Sunday, 15 January 2023

A new lake has formed in the Waiorongomai valley, near Ruatoria.

A new lake created by a landslide in the hills near Ruatoria is being closely watched by the local council, particularly with more rain forecast this week and uncertainty around the stability of the dam created by the landslide.

The lake, thought to be about 1km long and about 15 metres deep, has been formed in the Waiorongomai River, a tributary of the Tapuaeroa River, which enters the Waiapu River near Ruatoria.

The lake was created by a massive landslide of earth and bush and is on privately-owned land.

A massive amount of earth and vegetation created the dam in the Waiorongomai valley.

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There was a concern about what impact more rain this week would do.

Gisborne District Council’s principal scientist, Murry Cave, said a much smaller lake had formed in the same location following a storm in July 2020, but that vanished when its dam failed a few weeks later.

Murry Cave, principal scientist at Gisborne District Council.

The new lake was much larger and while a catastrophic failure of its dam was unlikely to present any threat to properties downstream, it would pose a risk to anyone in its path, as well as a road bridge on Tapuaeroa Rd.

Cave said a satellite would measure the lake’s size and volume on Monday. He guessed it covered an area of about 10 hectares.

“The fact the lake has water flowing out of it is a good sign, but a lot will depend on what happens with the weather this next week. There’s a huge amount of material that could still come down and make it bigger,” Cave said.

Tolaga Bay resident Linda Gough's property is surrounded by a sea of logs in the wake of Cyclone Hale.

“If that occurred it would be a question then of whether there’s a catastrophic, rapid failure, or whether it gradually erodes away, or if it remains a lake,” he said.

Other lakes in the area created similarly in the past had lasted years before their dam walls blew out, he said.

“We’ll be monitoring it from now. We have a 150m span bridge down on the Tapuaeroa [River] and the concern would be what a rapid failure would do to the bridge support,” he said.

“You certainly wouldn’t want to be anywhere on those rivers if there was a rapid failure,” Cave said. Weather over the next two weeks would be key.

Tāirawhiti Civil Defence said they were keeping a close on the weather with staff monitoring the progress of a sub-tropical low forecast at this stage for Wednesday.

Fifty-three residents in Tauwhareparae, inland of Tolaga Bay, were still without power.

The East Coast’s main road, State Highway 35 was open daily from 7am to 7pm.

The Tolaga Bay wharf and Titirangi (Kaiti Hill) remained closed.

Contact with Gisborne’s waterways was not advised due to the amount of debris and wastewater in the rivers.

Temporary warning signs are in place along the rivers and beaches advising that no fishing, shellfish gathering, swimming or water recreation activities should occur until the signs were removed.