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Aerial views monitor environmental impacts of storm

10 May 2017

Aerial views monitor environmental impacts of storm

Council’s land and soil resources team carried out a visual assessment by helicopter on 21 April to look for evidence of landslides and woody debris in waterways following Cyclone Cook.

In a presentation to the Environment Planning and Regulations Committee, councillors were shown aerial images of landslides on harvested slopes, woody debris in waterways and a build-up of material in the Mangaheia River at Wigan Bridge.

It was through the post-storm survey staff found the worst damage in the Mangatokerau Valley. A local resident said their property was flooded with woody debris but they did not report it to Council.

“The issue of woody debris migrating out of forests, causing damage to downstream property and landing on beaches is a significant concern for the affected communities,” said senior land management officer Nicki Davies.

“It’s a complex problem and we want to understand where and why the debris migration occurs so we can manage the effects of forestry as a land use, and the longer-term effects on the environment.”

Ms Davies said this was the first weather event the new system for post-storm monitoring had been employed since it was co-developed with Landcare Research last year.

“A risk matrix is used to identify debris flow susceptibility, likelihood of occurrence and potential consequences of an event.”

The post-storm monitoring will also capture observations from the public and forestry owners, volume of debris deposited on flood plains and beaches, and the value of damage.

“Forestry owners are required to operate within the conditions of their consent, which include mitigating the effects of woody debris migration, like employing the use of slash catchers,” says Ms Davies.

“In waterways where the origin of wood can be identified, the owner is responsible for clean up and removal.”

Acting environmental and regulatory group manager Lois Easton said there were over 1000 active forestry consents that contain a wide range of conditions issued at varying times over the past 10 years.

The monitoring information will be used to inform actions that would reduce the occurrence of woody debris and its impact on downstream environments.

“Harvesting areas we identify as causing issues could lead to a review of their consent conditions, as well as a change to conditions on new consents going forward,” said Ms Easton.

“Through this event we identified gradual migration of wood left on slopes and land slippage had led to debris ending up in a number of waterways.”

“Woody debris stored on flood plains and left in or next to waterways from previous events was also an issue.”

“Several valuable refinements to installation and maintenance of slash catchers arose from this event.” 

“We are sharing this information with forestry owners to inform better compliance with consent conditions and improve their practices.”

Council are also working on tools like mobile apps and a web based spatial tool to capture quantitative data and expect to refine the risk matrix over the next six months.

If you see or experience any damage from a storm event please contact us 06 8672049 or email landslide.report@gdc.govt.nz

Woody debris left in the Mangatokerau Catchment

Woody debris left in the Mangatokerau Catchment 

Woody debris at Wigan bridge

Woody debris at Wigan Bridge, Tauwhareparae Road

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