The slash traps debate – solution or even more damage?

5 April 2023
In New Zealand, could slash traps help curb the flow of woody debris that has battered and cloaked the east coast of the North Island during recent storm events? Forest managers say yes but Gisborne District Council (GDC) says the traps could make the devastating effects of the debris even worse.

Some forest managers say research and trials they have conducted show debris or slash traps are a viable solution. Eastland Wood Council also recommends them as one of six sound practices in its Good Practice Guideline for Catchment Management.

But GDC says the force of slash mobilising off the East Coast’s steep slopes is so significant it would take massively engineered structures to contain it. The force would be accelerated and potentially do more damage downstream if it suddenly burst out of a slash trap that could not hold it.

Aratu Forests Limited is one of five companies successfully prosecuted by GDC for contributing to slash waste that devastated parts of the region during storm events in 2018. The company says it has since spent four years and NZ$100,000 investigating the viability of slash traps for its forests that were involved.

It believes several strategically placed slash traps made from modern, engineered, high-tensile steel wire nets could reduce the impact of woody debris movement during future severe weather events. But it was unable to convince GDC the proposed traps would not impact fish passage or be at risk of failure, so it withdrew its application to focus its resources on other slash management options.

GDC said it had previously granted consent for some small slash traps but was concerned the traps proposed by Aratu could risk overtopping, formation of a debris dam, or trap failure during a storm event. Due to those concerns, GDC wanted to publicly notify the application.

Debris traps are widely used in the US and Europe. They can be large concrete and rock structures involving complex engineering or rudimentary designs simply constructed from iron fence posts and thick wire rope. They either trap debris that comes down a waterway or direct it onto a “safe zone”, where it can be stored or transported away.

They have been used in New Zealand forests for some time, albeit the technology is constantly evolving. In 2020, the district council commissioned a report into international best practice for debris traps.

GDC chief of strategy and science Joanna Noble said while slash traps sounded good on paper, the council’s view was that they were really just a safety net at the bottom of a cliff. GDC considered prevention — including measures such as significantly reduced harvesting areas, removal of all debris from forests during harvesting, cessation of cut-to-waste practices — was better than the suggested cure.

Ms Noble said that until recently, clear-felling had occurred over large areas (hundreds or thousands of hectares) simultaneously in the Gisborne region. This practice was high risk and these risks were exacerbated by the region’s steep topography, highly erodible soils and vulnerability to regular severe storms.

While debris traps were used in Europe, large-scale clear-felling practices were normally avoided there, Ms Noble said. The traps Aratu wanted to instal are similar to one being trialled by Rayonier Matariki Forests in an 850ha catchment area in part of its Willowflat forest, about an hour and a half south of Wairoa.

A ‘valuable tool’ if used strategically

Rayonier Matariki national environmental manager Andy Fleming said the company installed debris traps over water courses in several of its forests nationally but the Willowflat trial debris barrier was more highly engineered than others.

The slopes in Willowflat were similarly steep to those on the East Coast, albeit not as long and a different geology. The Willowflat trap cost the company about NZ$150,000 and was designed to hold 3000 cubic metres of debris in a catchment area considered one of the forest’s most vulnerable.

Made from interwoven steel mesh, the trap was modelled on the rockfall barriers used to safeguard roadside cliffs in Kaikoura after landslides associated with the 2016 earthquake, Mr Fleming said. The mesh is supported top and bottom by wire rope and held secure at each side with fixings mechanically drilled eight metres into the stream banks.

Mr Fleming believed slash traps were a valuable tool for managing forestry waste. If deployed strategically they would have helped curb the amount of slash in debris that had inundated properties and beaches on the East Coast in recent years.

But he noted slash traps were not designed to mitigate poor forestry practice. They were installed to help mitigate the effect of land failures and were deployed along with other land management tools. The traps needed to be regularly cleared. As a condition of the consent for the Willowflat one, Hawke’s Bay Regional Council requires the company to inspect it after rain events involving more than 50 millimetres in 24 hours.

After Cyclone Gabrielle, Rayonier Matariki had not been able to access Willowflat forest by road. Mr Fleming said he had flown over it but had not been able to assess from that height how the slash trap had performed.

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Source: gisborneherald