Wood at the Esk River mouth, north of Napier, following Cyclone Gabrielle. Photo / Warren Buckland
Wood at the Esk River mouth, north of Napier, following Cyclone Gabrielle. Photo / Warren Buckland
A new study looking into the origins of large amounts of pine that washed down rivers during the Hawke’s Bay floods could soon be commissioned - providing a clearer picture of where that debris came from.
Hawke’s Bay Regional Council released a 45-page report last week assessing thetypes of woody debris at 17 locations where bridges were damaged, or debris had accumulated on land and beaches, following Cyclone Gabrielle on February 14.
That included six beach and 11 river sites.
A surprising finding was that slash (waste product from commercial forestry) made up only a small percentage of the woody debris.
“At all but one of the surveyed sites, there was little evidence of slash ... the site with the most cut pine was Mangaone at Rissington, where 9 per cent of the timber had cut marks,” a regional council statement read.
However, the study did find that - on average - more than half of the woody debris at each site was pine (around 56 per cent). The other main tree species identified were willow and poplar.
Debris up against the Tutaekuri River Bridge near Napier in the days after the cyclone. Photo / Mark Mitchell
Hawke’s Bay Regional Council senior freshwater scientist Dan Fake, who was involved in the study, said the study’s scope did not include where the woody debris originated from further up each catchment.
As such, it remains unclear exactly where the large amounts of pine came from - such as specific pine forests or other locations.
Fake said that would fall under another study which could be given the green light in the future.
“It was outside the scope of the current study, but could be a next step to try and figure out,” he said.
“So looking at where it’s come from, what is the slope of that land, what is the geology of that land, are there things that we could put into regional plans which could prevent that from happening in the future, or is it just a case of it was a one-in-500-year event in some places and erosion is a natural process and it is relatively unavoidable.
“These are the questions we want to find out.”
He said there were no timeframes around when that new study could get under way, but it would likely include analysis of satellite imagery and flyovers.
He said the regional council was having to prioritise a lot of projects at present, following the cyclone, and may look at partnering with another organisation to help with such a study - such as GNS.
Fake said if they were to partner with another organisation “we would like to work with them to tailor the research in a way that can answer some of the questions for ourselves”.
Fake said it was not particularly surprising that large amounts of pine were found at the locations assessed, as pine forests were a significant land use further up many of the catchments.
