River mapping with LiDAR
How LiDAR is making waves in river resource management and planning in Ruatoria.
“The Waiapu River provides the lifeblood of Ngāti Porou. It is of great cultural and spiritual significance to us. We reference it whenever reciting tribal pepeha, we compose and sing waiata in its honour, we tell stories about its history. But the river is suffering; choking under the weight of devastating erosion in its surrounding catchment.”
In 2015, local iwi Te Runanganui o Ngāti Porou adopted a joint management agreement with Gisborne District Council to ensure that natural resources are managed in line with freshwater policy and iwi rights are recognised, enabling Ngāti Porou to have a greater voice in resource decision making.
One example of ongoing decisions that are being made in consultation with Ngāti Porou involve the provision of gravel extraction consents for the Waiapu river. In recent years, the amount of gravel being extracted commercially from the Waiapu river has been increasing. This resource represents a substantial economic opportunity but is also an activity that is in conflict with community aspirations to restore and rehabilitate the river.
Extraction works being carried out in the Waiapu catchment.
Here, LiDAR has been a very useful tool in helping both council and iwi sustainably manage the environmental impact of gravel extraction to protect the mauri of the river. The Waiapu river is a complex system and there are places in the river identifiable with LiDAR where gravel is aggradating (accumulating). It would be more sustainable to extract material from these areas to mitigate aggradation issues as opposed to overextracting in areas where gravel is leaving the river. Overextraction of gravel from an area where there is no influx would cause the river to narrow and contract, triggering erosion and impacting the balance of the river system and surrounding ecology.
These images show where aggradation (a build up of sediment) has occurred over time, changing the river morphology which poses a flooding hazard amongst other issues.
With the support of council through the Envirolink fund, Jon Tunnicliffe , a river scientist from Auckland University has been assisting this project by using council’s present and past regional LiDAR datasets to compare and observe changes in the river over time. Unlike other techniques like river cross section surveys, LiDAR’s ability to quickly capture and deliver high quality information has made a huge difference in terms of planning for this resource.
LiDAR and photogrammetry have been used to observe changes in a 6km stretch of the Waiapu river over a one year period (Feb 2019 - Feb 2020). Material leaving the river via erosion is marked in blue and material accumulating is marked in red. With this information the balance between erosion and deposition can be accurately assessed, allowing for the more sustainable management of extraction operations.
In the future, annual LiDAR surveys of the river are planned for ongoing monitoring purposes. There is also an opportunity to use LiDAR to generate 2D hydrodynamic models capable of simulating the behaviour of the river over time. Upcoming data from the LINZ National Elevation Programme will be very valuable for this purpose.
As part of this project, Gisborne District Council has plans to present results of this work back to the community and to run a workshop to show people how the data was used. From a community perspective, LiDAR has been a useful tool in helping build greater appreciation of the changes happening with the landscape. Iwi and hapū in Ruatoria are also keen to do more with the data, not just for environmental purposes but also economic and cultural ones.
Read the full report here:
If you have a LiDAR story you'd like to share, we'd love to hear from you. Get in touch: elevation@linz.govt.nz
