Study highlights improvements to slash debris

File picture

IMPROVEMENTS are needed in the forestry industry to reduce slash getting into waterways, especially from contractors new to the district, a Gisborne District Council investigation has concluded.

Councillors heard the recommendations during a presentation of the Cyclone Cook woody debris study, at a meeting of the environmental planning and regulations committee.

Cyclone Cook, which occurred in April, was not a particularly large storm, a one in 4.5 year event, but had significant impacts in certain areas including inland from Tolaga Bay.

Impacts of woody debris flows from storms include damaging infrastructure and degrading the visual appeal of rivers and beaches, and impacts on water quality and ecology from the increased sediment load lost from the land.

The storm gave the council a chance to use a new monitoring system, co-developed with Landcare Research with the assistance of the forestry sector last year, which includes tools for reporting and evaluating the risks of forestry slash and woody debris from landslides.

The council’s environmental and science manager Lois Easton said the results challenged some previously-held beliefs, including where the wood originated and what caused it to enter waterways.

Analysis of woody debris piles at Wigan Bridge found the majority (70 percent) was from pine plantation forests, with 60 percent already in or close to the rivers, and six percent freshly cut logs. Thirty percent was willow and poplar.

Previous argument from sector

The forestry sector had previously argued willow and poplar was the main source of woody debris on beaches after storms.

The study found while willow and poplar were contributors, the majority of the material was generated from forestry.

Before the study, Landcare Research had predicted the main driver of such woody debris was from storm-induced debris flows, but in this case only one debris flow contributed.

“Management practices in the forests were the key contributor,” Ms Easton said.

She said the wood piles at Wigan Bridge included newly-cut timber waiting to be transported to the port that was left on a flood plain, and was subsequently caught up in the event.

“It raises consent issues about those landing areas.”

Some of the pine was very old, indicating it had been deposited in the river in previous storms.

They found slash-catchers, structures placed across rivers and streams to catch woody debris, were not as effective in heavy rain and high river flow as previously thought.

“There is only a certain amount they can catch before they are by-passed,” Ms Easton said.

“It is partly about placement, but also about how we stop slash getting into the rivers in the first place.”

The primary source of material was woody debris already within the flood plain, deposited there by previous storms.
An equivalent amount of material remains in the catchment after Cyclone Cook, and will mobilise during the next high intensity storm.

“This creates a vicious cycle,” Ms Easton said.

“Woody debris gets into river systems, then is mobilised again in the next event.”

Storms every two years

On average there are storms large enough to mobilise woody debris every two years.

The investigation was a key part of work being done with the forestry sector to better manage the environmental impacts of forestry harvest.

“These issues are going to be ongoing,” Ms Easton said.

“130,000ha of the 160,000ha in forestry was planted as protection forest.

“When you harvest protection forest, it is land just waiting to go, and it will.”

A major concern was a lot of new contractors coming into the district, some who have not been involved in forestry before, and many not familiar with the particular challenges in this region.

“There is a world of difference between forestry harvest in pumice country on gentle slopes in the Bay of Plenty, and on hill slopes like this, which are just waiting to slip,” Ms Easton said.

“We need to step up our monitoring of compliance, particularly with new contractors.”

The volume of wood being harvested meant mistakes could have large environmental impacts.

End-of-life willow and poplar in the district’s river margins was also identified as an issue and options for how to deal with it would be investigated.

The investigation findings have been reported to the Ministry for Primary Industries to the National Standard for Plantation Forestry, which comes into effect in May 2018.


Slash at Wigan Road bridge

  • Pine: 70 percent, including 60 percent abraded logs, six percent cut, and four percent wind-thrown
  • Willow/poplar: 30 percent

Key drivers of the Cyclone Cook woody debris mobilisation

  • Landing failure and debris flow (Mangatokerau)
  • Loss of newly cut timber stored on flood plains
  • Slash catcher failure (Mangatokerau, Mangaheia and Whakaou)
  • Landings poorly located in vulnerable positions on flood plains
  • Widespread slash from forestry harvest left in vulnerable positions in gullies and on flood plains
  • The poisoning and consequent failure of willow plantings upstream of Wigan Bridge
  • <

IMPROVEMENTS are needed in the forestry industry to reduce slash getting into waterways, especially from contractors new to the district, a Gisborne District Council investigation has concluded.

Councillors heard the recommendations during a presentation of the Cyclone Cook woody debris study, at a meeting of the environmental planning and regulations committee.

Cyclone Cook, which occurred in April, was not a particularly large storm, a one in 4.5 year event, but had significant impacts in certain areas including inland from Tolaga Bay.

Impacts of woody debris flows from storms include damaging infrastructure and degrading the visual appeal of rivers and beaches, and impacts on water quality and ecology from the increased sediment load lost from the land.

The storm gave the council a chance to use a new monitoring system, co-developed with Landcare Research with the assistance of the forestry sector last year, which includes tools for reporting and evaluating the risks of forestry slash and woody debris from landslides.

The council’s environmental and science manager Lois Easton said the results challenged some previously-held beliefs, including where the wood originated and what caused it to enter waterways.

Analysis of woody debris piles at Wigan Bridge found the majority (70 percent) was from pine plantation forests, with 60 percent already in or close to the rivers, and six percent freshly cut logs. Thirty percent was willow and poplar.

Previous argument from sector

The forestry sector had previously argued willow and poplar was the main source of woody debris on beaches after storms.

The study found while willow and poplar were contributors, the majority of the material was generated from forestry.

Before the study, Landcare Research had predicted the main driver of such woody debris was from storm-induced debris flows, but in this case only one debris flow contributed.

“Management practices in the forests were the key contributor,” Ms Easton said.

She said the wood piles at Wigan Bridge included newly-cut timber waiting to be transported to the port that was left on a flood plain, and was subsequently caught up in the event.

“It raises consent issues about those landing areas.”

Some of the pine was very old, indicating it had been deposited in the river in previous storms.

They found slash-catchers, structures placed across rivers and streams to catch woody debris, were not as effective in heavy rain and high river flow as previously thought.

“There is only a certain amount they can catch before they are by-passed,” Ms Easton said.

“It is partly about placement, but also about how we stop slash getting into the rivers in the first place.”

The primary source of material was woody debris already within the flood plain, deposited there by previous storms.
An equivalent amount of material remains in the catchment after Cyclone Cook, and will mobilise during the next high intensity storm.

“This creates a vicious cycle,” Ms Easton said.

“Woody debris gets into river systems, then is mobilised again in the next event.”

Storms every two years

On average there are storms large enough to mobilise woody debris every two years.

The investigation was a key part of work being done with the forestry sector to better manage the environmental impacts of forestry harvest.

“These issues are going to be ongoing,” Ms Easton said.

“130,000ha of the 160,000ha in forestry was planted as protection forest.

“When you harvest protection forest, it is land just waiting to go, and it will.”

A major concern was a lot of new contractors coming into the district, some who have not been involved in forestry before, and many not familiar with the particular challenges in this region.

“There is a world of difference between forestry harvest in pumice country on gentle slopes in the Bay of Plenty, and on hill slopes like this, which are just waiting to slip,” Ms Easton said.

“We need to step up our monitoring of compliance, particularly with new contractors.”

The volume of wood being harvested meant mistakes could have large environmental impacts.

End-of-life willow and poplar in the district’s river margins was also identified as an issue and options for how to deal with it would be investigated.

The investigation findings have been reported to the Ministry for Primary Industries to the National Standard for Plantation Forestry, which comes into effect in May 2018.


Slash at Wigan Road bridge

  • Pine: 70 percent, including 60 percent abraded logs, six percent cut, and four percent wind-thrown
  • Willow/poplar: 30 percent

Key drivers of the Cyclone Cook woody debris mobilisation

  • Landing failure and debris flow (Mangatokerau)
  • Loss of newly cut timber stored on flood plains
  • Slash catcher failure (Mangatokerau, Mangaheia and Whakaou)
  • Landings poorly located in vulnerable positions on flood plains
  • Widespread slash from forestry harvest left in vulnerable positions in gullies and on flood plains
  • The poisoning and consequent failure of willow plantings upstream of Wigan Bridge
  • <
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