Mapping the riverbed

Aorangi Pilings

Mapping to better understand the amount of gravel and sediment moving through the rivers at Waiapu and East Cape catchments will take place this month.

The Gisborne District Council is working with local contractors Landpro to map the riverbeds using a small aircraft called LiDAR (Light detection and ranging system).

Council environmental scientist Olivia Steven said the mapping will be used to produce a 3D map of the riverbeds.

“The data collected by the LiDAR survey will be used to create a digital elevation model and allow us to understand the profile of the river,” she said.

The information will then be used to develop a gravel extraction management plan in partnership with the community.

Gravel riverbeds provide important habitat for a range of native birds such as banded dotterel, invertebrates, plants and fish, Ms Steven said.

Landpro director Mike Borthwick said the company would be operating a VQ880 LiDAR, which is the only model that can penetrate the water surface and map the riverbed.

“The laser sends out a light signal, which, like a laser beam, shoots down to the ground. The aircraft system then measures the time it takes for the signal to return. This is how it measures the differentiating height of the ground,” he said.

The mapping will be done 600 metres from the ground and the aircraft will fly at a speed of 300kmh and cover 200km of catchment, one area at a time.

The mapping process is non-invasive and poses no risk to people, animals or vegetation, Ms Steven said.

“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,” she said.

An excess of sediment and gravel, increasing the riverbed height, is known as “aggrading” the riverbed.

The headwaters of the Tapuaeroa River, west of Ruatōrea, are an example of the phenomenon, and they have been aggrading steadily (30 metres in some parts) since monitoring began 25 years ago.

Riverbeds degrade when too much gravel is extracted, or the flow of water is not strong enough to move the supply of gravel.

Gravel is often extracted to build new roads and to maintain forestry and local roads.

“It is not about whether the phenomena are good or bad . . . it is kind of like a sand dune, it naturally rises and falls. Allowing the cycles of aggradation and degradation is important,” Ms Steven said.

“Ongoing extreme degradation from human influence can shrink the river size and deepen the channel, reducing the available habitat.”

The reasons for the high aggradation rates are that the geology is relatively young and prone to erosion, the catchment has steep gullies, and removal of native trees which causes erosion erosion, she said.

The Waiapu River has some of the highest suspended sediment loads in the world for the size of its catchment.

“On average, the Waiapu transports 35 million tonnes a year, with erosion of its gullies producing large amounts of sediment,” she said.

However, not all braided river systems — systems with many branches — produce the same amounts of gravel.

“Bed-level data gathered through surveying and LiDAR is essential for deciding what is a sustainable amount of gravel to extract from the gravel riverbed systems.”