From slash traps to helicopter removal, ways to mitigate the risk from forestry debris have been put to Tasman district councillors. PF Olsen, which manages the Tasman District Council forestry interests, was commissioned to evaluate the risk of woody debris from its forest estates. Source: Stuff NZ
PF Olsen regional manager Nelson Sam Nuske, who gave a presentation at a full council meeting, said of the council estates, the highest risk area was Kingsland, behind Richmond.
Kingsland had “relatively stable soils” but was on steep slopes, and its three main catchments flowed through urban areas.
However, all catchments contained “significant native riparian” areas which helped protect against slash deposits into waterways.
Around half of the Kingsland forestry area was harvested and retired into the Kingsland Forest Park in 2020. A further 20% is scheduled for harvest and retirement in 2023, with the remaining 30% planned for harvesting and retirement in 15 years’ time.
Borlase, Howard, and Sherry forests were identified as “moderate risk”, and Rabbit Island and Tunnicliff as “low risk”.
Discussed in the presentation were measures such as grass seeding, “not the be all and end all”, but an inexpensive measure which “slows the raindrops down a bit”.
Slash traps – gridlike structures in waterways that prevent the migration of slash downstream were “the ambulance at the bottom of the cliff”.
The report said woody debris traps required a resource consent, had ongoing maintenance costs, and if slash and water went over their limit could “release a torrent of water and woody debris far larger than what might otherwise have been observed”.
Helicopter slash grappling was a “practicable measure”, Nuske said, that was “actually not as expensive as it sounds”, particularly in areas where slash accumulated in gullies.
Nuske said one risk was that skid sites which held large amounts of slash would fail and end up in waterways.
Richmond-based wood energy company Azwood was “uplifting the slash as we log it” from the Kingsland site, and turning it into landscaping supplies.
However, the uplift of slash got more difficult further away from town. If slash couldn’t be taken away, it needed to be located in a stable area, he said.
Nuske said 95% of slash that washed up in storms came from hillsides – and not from skid sites.
A “fixed felling head” slowed the felling process down, by laying the trees down in a soft motion, which lead to less breakage and debris left on the hillside.
The report estimated six measures – 15m wide boundary plantings, alternative riparian plantings, a debris trap, helicopter slash grapple, slash removal or mulching, and grass seeding would cost NZ$585,000 to implement over five years.
In February, the Government announced a ministerial inquiry into forestry slash and land use on the East Coast.
Tasman Mayor Tim King said the first thing was “to wait and see what comes out of the inquiry and see what in the new rules and regulation framework might look like”.
“As a region, I do genuinely believe that a lot of our practices exceed the bottom line. Now that’s not exclusive and there are always areas or operators or owners who don’t do that, hence our role in regulation, and forest plan approval.”
The report did not discuss alternatives to clear felling but acknowledged that there was a “window of vulnerability” of six years post harvesting, and certain mitigation actions “may have a delayed effect” on the risk of slash deposits.
