Innovations to stop slash and logs entering waterways
LAST YEAR’S DISASTROUS TOLAGA BAY floods prompted forestry researchers to seek ways to prevent harvesting residues ending up in waterways during extreme weather events.
This year, the results from two of those projects were unveiled at the 2019 Forest Growers Research conference in Wellington last month by Harvesting Programme Leader, Keith Raymond.
One that will be familiar to NZ Logger readers is the fixed felling head being used by Mechanised Cable Loggers in Nelson, which featured in our March issue.
FGR conducted field trials to quantify the success of these heads in reducing tree breakage compared to a dangle head, which would mean less slash and offcuts being left on the hills where it could be washed away. The study showed that fixed heads produced stems that are on average 40% longer, as more whole trees were brought to the landing by a yarder.
The second project was to improve extraction of residues by using a slash grapple slung under a helicopter to clear locations that are hard or undesirable to reach with machines.
The idea originally came from PF Olsen’s Hamish McPherson and was taken up by FGR, which worked with Helihawk of Taupo to develop a prototype multi-tine slash grapple that was built by Colchester Engineering of Matamata.
The first trial earlier this year flown with Wairarapa Helicopters proved the theory behind the slash grapple, with net extraction of 18.5 tonnes per productive flying hour when recovering slash only – larger logs slow down the process and it’s recommended these are extracted first.
Mr Raymond says that while helicopters are expensive to operate, the cost of slash extraction averaged NZ$135 per tonne of slash removed, or approximately NZ$112 per metre of stream length. When the cost of cleaning the stream using the helicopter slash grapple was averaged over the volume of wood extracted from the harvest area, the net cost was only NZ$1.05 per cubic metre of wood produced.
It would be preferable to the dangerous and difficult job of clearing waterways using people on the ground, or with machines, which may result in soil disturbance and sedimentation of waterways.
A demonstration using an updated design of the slash grapple that holds onto residues more effectively was due to take place in Gisborne at the time NZ Logger went to press. Mr Raymond believes the device is almost ready to commercialise and says that plans for a slash grapple that can be fitted to a yarder or hauler are also under way.
NZL