Cyclone slash already in rivers

FORESTRY SLASH: Woody debris at Wigan Bridge on Tauwhareparae Road following Cyclone Cook in April. The debris found in the Mangatokerau catchment was linked to forestry activities and the company removed it from the stream and carted larger wood away from the downstream property. Tairawhiti Roads completed the clean-up at Wigan Bridge because the origin of that wood was not known. Picture supplied

A FORESTRY slash investigation in the Tolaga Bay area following Cyclone Cook has revealed the majority of woody debris was pine and had already been in the river system.

The impacts of Cyclone Cook in April were felt across the region, with debris avalanche events occurring in a number of catchments, particularly inland from Tolaga Bay.

Gisborne District Council staff found the worst damage in Mangatokerau Valley, inland from Tolaga Bay, with landslides on harvested slopes, woody debris in waterways and a build-up of material in the Mangaheia River at Wigan Bridge.

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

“The objective of the exercise was to gain a better understanding of what happens with woody debris in our rivers and on the beaches following major storm events,” said principal scientist Dr Murry Cave.

“For our environmental and science services team, this led to an investigation into the sources of woody debris in this event.

“Was it forestry slash or was it streamside vegetation such as willow?”

Slash at Wigan Bridge and on Tolaga Beach was 70 percent pine, with the remainder poplar and willow.

60 percent pine

About 60 percent of the material overall was pine logs that were weathered and abraded, indicating they had been in the river system for one or two years.

The assumption before the event was that slash in the waterways and on the beaches was driven by debris flows, but the investigation found only one contributing debris flow, while the rest were sourced from slash stored on the flood plain.

“This suggests a lot of logs get caught up on the banks of the rivers as the flood waters drop, and these are then caught up and moved downstream in the next event,” Dr Cave said.

About 6 percent were freshly-cut logs, which indicated they were likely stored on a landing in preparation for loading but were caught out by the storm.

While the majority of the slash was found to be from forestry operations, the investigation showed slash was a wider community issue.

“Wood stowed on flood plains and failed landings are a major contributor, but so are willow/polar.”

The study culminated with a project with Tolaga Bay Area School this week, looking at the impact of woody debris on the beach environment.

The objectives were to allow for an empirical evaluation of the woody material on the beach, and to initiate a citizen science project as a pilot for further school-based science projects.

When the final report of the investigation is complete, it will be shared with the forestry industry for comment and be peer-reviewed by an independent expert from the School of Forestry at the University of Canterbury.

“It is anticipated that the data will be used to help identify the areas that are most at risk of slash mobilisation, so that mitigation measures can be put into place to reduce future risk,” Dr Cave said.

A FORESTRY slash investigation in the Tolaga Bay area following Cyclone Cook has revealed the majority of woody debris was pine and had already been in the river system.

The impacts of Cyclone Cook in April were felt across the region, with debris avalanche events occurring in a number of catchments, particularly inland from Tolaga Bay.

Gisborne District Council staff found the worst damage in Mangatokerau Valley, inland from Tolaga Bay, with landslides on harvested slopes, woody debris in waterways and a build-up of material in the Mangaheia River at Wigan Bridge.

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

“The objective of the exercise was to gain a better understanding of what happens with woody debris in our rivers and on the beaches following major storm events,” said principal scientist Dr Murry Cave.

“For our environmental and science services team, this led to an investigation into the sources of woody debris in this event.

“Was it forestry slash or was it streamside vegetation such as willow?”

Slash at Wigan Bridge and on Tolaga Beach was 70 percent pine, with the remainder poplar and willow.

60 percent pine

About 60 percent of the material overall was pine logs that were weathered and abraded, indicating they had been in the river system for one or two years.

The assumption before the event was that slash in the waterways and on the beaches was driven by debris flows, but the investigation found only one contributing debris flow, while the rest were sourced from slash stored on the flood plain.

“This suggests a lot of logs get caught up on the banks of the rivers as the flood waters drop, and these are then caught up and moved downstream in the next event,” Dr Cave said.

About 6 percent were freshly-cut logs, which indicated they were likely stored on a landing in preparation for loading but were caught out by the storm.

While the majority of the slash was found to be from forestry operations, the investigation showed slash was a wider community issue.

“Wood stowed on flood plains and failed landings are a major contributor, but so are willow/polar.”

The study culminated with a project with Tolaga Bay Area School this week, looking at the impact of woody debris on the beach environment.

The objectives were to allow for an empirical evaluation of the woody material on the beach, and to initiate a citizen science project as a pilot for further school-based science projects.

When the final report of the investigation is complete, it will be shared with the forestry industry for comment and be peer-reviewed by an independent expert from the School of Forestry at the University of Canterbury.

“It is anticipated that the data will be used to help identify the areas that are most at risk of slash mobilisation, so that mitigation measures can be put into place to reduce future risk,” Dr Cave said.

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