Council is working to better understand the amount of gravel and sediment moving through Tairāwhiti rivers.

Light Detection and Ranging (LiDAR) technology is being used to map the Waiapu and East Cape River catchments from the air. The LiDAR system is flown low over the river and uses pulses of light to map the riverbed. LiDAR is completely non-invasive has no risk to people, animals, or vegetation within the survey area.

Council Environmental Scientist Olivia Steven says the mapping will improve the understanding of sediment and gravel levels carried in rivers.

“The last time LiDAR was flown over these riverbeds was in 2014, so we can now compare the previous levels with recent mapping to understand which sections are aggrading or degrading.”

An aggrading riverbed is when there is an excess of sediment and gravel, and the riverbed height is increasing. Ms Steven says the headwaters of the Tapuaeroa River, west of Ruatorea, is an example of an aggrading riverbed, and has been aggrading steadily since monitoring begun 25 years ago.

Degrading riverbeds can be created when too much gravel is extracted, or when there haven’t been many erosion events upstream.

“The Aorongiwai Steam, upstream of the Matahaia Bridge (inland from Ruatorea), exhibits degradation, which is shown clearly by the lack of gravel around the bridge abutments.”

Ms Steven says the data gathered using LiDAR technology is essential when deciding what a sustainable amount of gravel extraction is.

“Gravel riverbeds provide important habitats for a range of native birds, fish, animals and plants, and the LiDAR information is invaluable when monitoring levels.”

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