Thursday, April 25, 2024

Woody debris report shows mixed results for Hawke’s Bay

Hawke’s Bay Regional Council today released its report on the make-up of the woody debris washed up against bridges, on beaches and in flooded areas in the wake of Cyclone Gabrielle.

The Council’s survey of deposited woody debris was undertaken at 17 sites. The detailed assessments followed guidelines created and used in the Gisborne/Tairawhiti region.

Regional Council’s Group Manager – Asset Management, Chris Dolley says the report aimed to understand the likely origins of large woody debris that washed down the waterways during Cyclone Gabrielle to help inform future land use planning.

“The woody debris assessed in Hawke’s Bay consisted of a mixture pine, willow, poplar and ‘other’ – native timber and debris that could not be identified,” he said.

“The woody debris composition differed from catchment to catchment and largely was the result of whatever tree species was predominant in the catchment upstream.”

He said Council was particularly interested to see whether the pine debris included harvested logs, “slash”, or were whole or parts of trees sourced from erosion of hillsides and streambanks.

“At all but one of the surveyed sites, there was little evidence of slash, indicating that the majority of pine came from erosion of hillsides and streambanks. The site with the most cut pine was Mangaone at Rissington, where  9 percent of the timber had cut marks,” said Mr Dolley.

The results are summarised below:

Catchment / AreaMix of debris
CHB*Willow
NgaruroroLargely clear of debris
Upper Tutaekuri and Mangaone– Dartmoor and RissingtonPredominately pine with the remaining being willow
Lower Tutaekuri – Puketapu – AwatotoPredominately willow / poplar with the remaining being pine
Esk River and BeachPredominantly pine
Te NgarueMix of pine and willow
Aropaonui (beach)Predominantly pine
Waikare (beach)**Predominantly pine
Mohaka (beach)Predominantly pine with some ‘other’
Wairoa (rivermouth)**Predominantly pine
Mahia (beach)Predominantly pine with some ‘other’

*Sites in CHB weren’t formally surveyed as it was evident from preliminary assessments that willow / poplar was the dominant timber source.
** Waikare beach and Wairoa River mouth sites were assessed by air as access to a suitable site was not found.


Much of the pine that was found at the sites tested constituted wind thrown or previously dislodged pine, pine remaining from community operations or clearance, or general pine pieces that lacked evidential origins but were in the vicinity of flood waters and our river systems, the Council found. 

Willow proximity to rivers combined with the volume and strength of the flood waters from Cyclone Gabrielle contributed to the willow found within the sites tested. Areas of riverbank willow plantings have remained in place despite the flood event, proving they remain one of the best options for erosion control, it was found.

Council says it is working with the other councils in the region, and Waka Kotahi to ensure that flood control and bridges are considered together so that correct decisions can be made around bridge deck heights and spans as it rebuild for the future.

The full post-cyclone large woody debris assessment can be found here:  https://hbrc.info/woodydebris2023.

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