Prepare for more large deluges of logs being washed off hillsides in coming years - report

Tuesday, 20 October 2020

Forestry slash lines the riverbank in Ruatoria. (First published July 2020)

Huge tracts of pine forests on unstable land on the East Coast are soon to be harvested, meaning there's a high risk of more destruction and floods caused by forest debris.

That was one of the conclusions of a report into the source of the woody waste that littered beaches near the mouth of the Waiapu River, near Ruatoria after storms in June and July.

The report, by Gisborne District Council principal scientist Murry Cave, studied the material from the air and on ground.

By assessing a particular site the report found that about 46 per cent of the wood was forestry slash (pine logs, offcuts, waste and branches) of various ages. About 36 per cent was native wood, 14 per cent was willow or poplar and the rest was farm posts and battens.

**READ MORE:

Forestry debris washed up on Tikapa beach on the East Coast following storms in July, 2020.

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Map showing the pine forests in the Waiapu (brown), indigenous forest (green), and areas of recent harvest (yellow). Also shown are the positions of logs (either pine, indigenous or willow/poplar) distributed within the Waiapu riverbed.

* Prosecutions over flood-triggered logging debris on East Coast

* East Coast forest owners being prosecuted over flood-triggered logging debris

The wood differed from the huge amount deposited at Tolaga Bay after destructive flooding in 2018. That was nearly all pine.

While the forestry industry funded a clean-up of Tolaga Bay after the 2018 flood, it would be unlikely to do so for this year’s event given so much of the material was not from forestry, Cave said.

The report also used satellite imagery to establish the types of forestry in the catchment. While it was not possible to pinpoint exactly where the wood waste came from, it did reach several conclusions.

It found that the native wood mostly came from gullies formed following the clearing of land for pastoral farming, and that much of the pine came from forestry planted or stored near a river.

The willow and poplar was likely to be from erosion control plantings that had failed.

“Pine, while having short-term benefits as a result of its rapid growth, does not appear to be an effective long-term mitigation of erosion for large-scale gullies as the early benefits are then lost when the forest is harvested,” the report said.

The report ended by saying there was “a far larger area of pine forest now at or approaching maturity and thus the areas being harvested will increase markedly over the next five years”.

“More of this area of pine plantation is in areas of higher erosion risk and thus it is anticipated that the risk of Uawa [Tolaga Bay] – type events will increase over the next five years.”