A view of the Jones Creek rail bridge in northern Buller following a sudden rainstorm in early May 2022. Photo: Supplied / KiwiRail via LDR
West Coast Regional Council hired a helicopter to check the forest slash following a cloudburst in December.
The localised 'cloudburst' in the Grey Valley on 19 December sent a wall of logs down Callaghans Creek at Matai, washing out an approach to a rail bridge on the Stillwater-Westport rail link and closing the line for days.
Councillor Peter Ewen raised the matter at a council meeting this week, in the aftermath of Cyclone Gabrielle.
Acting consents and compliance manager Rachel Clark said that following their investigation into the Matai incident, the council decided that forestry slash "wasn't the total story".
The West Coast experience was that forestry slash was not a particular problem, the meeting heard.
"It is not to the scale here on the Coast that it is there. Certainly what's in the rules allows (foresters) to do what they do with the slash, but that's something going forward that everybody has to reach an agreement as to how the best way to handle it is."
Clark noted there was a requirement which meant "a certain proportion" of cutover material had to be left on the ground after felling to protect the slopes from eroding.
"That's why they do it. There's no doubt there needs to be better ways to (manage) it in the future, but they're not currently doing anything they're not supposed to do."
Following the meeting, Clark said at the time of the Matai incident, the council sent compliance staff up in a helicopter to check the creek catchment above where the damage to the rail corridor occurred.
While they observed "a small amount" of exotic plantation slash, the majority of the contributing problem at Matai was from fallen indigenous trees, following slips in the area as a result of the localised 'cloudburst'.
In her 18 years in the council compliance team, the issue of plantation forest slash had not been remarkable, however that was not to say it would be increasingly evident.
"It's not something that ever comes up as a major problem ... for us it hasn't been what it's like in the North Island. I haven't had too many inquiries."
At this point, the regional council did not have a specific rule addressing forest slash, but it could never rule that out.
Forestry operators also had to work to resource consent conditions which were monitored by the council.
The fact some West Coast stream catchments are at a critical erosion point, due to the loss of indigenous vegetation killed off by Cyclone Ita eight years ago, is on council's radar.
A case study by council engineer Paulette Birchfield on river and slope stability in the catchment of Lake Stream, a tributary of Mount Rochfort behind Westport, and on hillsides above Barrytown, was presented to the council nearly a year ago.
Her investigation showed the slopes in those locations, and others, were at "a threshold condition" and at a point of "mass movement", ranging from 'soil creep' to landslides.
The effects were being monitored at various sites and becoming critical.
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