Ūawa River seen in full flood on February 13, 2023, during Cyclone Gabrielle. Photo / Uawa Civil Defence
The leader of a team that conducted a study on riverbank erosion caused by Cyclone Gabrielle in the Gisborne-Tairāwhiti district was shocked by a level of erosion, sediment and land loss equating to “about 270 tennis courts”.
Dr Hamish Biggs, an ecohydraulics and remote sensing scientist with Earth Sciences NZ, said he was surprised at what research by Earth Sciences NZ and the University of Canterbury revealed.
“During Cyclone Gabrielle response work and helicopter Lidar [light detection and ranging] surveys we saw massive bank erosion,” he said. “However, the volume of bank erosion and extent of land loss revealed by the data is still astounding.”
The study showed “220,000cu m of sediment was eroded from just 9.1km of Ūawa/Hikuwai riverbanks in Gisborne (Tairāwhiti)”, a press release from Earth Sciences NZ said.
“This corresponds to 24cu m of sediment for each metre of river channel, or around 1500 dump truck loads for each 1km. The researchers ... also observed major land losses, with riverbank erosion claiming 72,300cu m of surrounding land in the study area – equivalent to around 270 tennis courts.”
The study was done shortly after Cyclone Gabrielle to support response activities, and results were published in the journal Earth Surface Processes and Landforms (ESPL).
The on-site study leveraged helicopter Lidar data collected by the University of Canterbury and Niwa (which recently merged with GNS Science to form Earth Sciences NZ) team and regional Lidar and aerial imagery data collected by Gisborne District Council and Hawke’s Bay Regional Council.
“Similar findings were observed in the Wairoa River in Hawke’s Bay, with more than 140,000cu m of sediment eroded from riverbanks and 35,000cu m of land lost from an 8.3km study area,” Earth Sciences NZ said.
“This corresponds to 16 tennis courts of land lost and 6.7 Olympic swimming pools filled with sediment for each 1km of river.”
Biggs said the team’s work was ongoing.
“We are still actively involved in research projects focused on understanding the geohazard impacts of Cyclone Gabrielle.”
The work carried out was significant for the future, he said.
“The volume of sediment eroded from channel banks and land loss is much larger than those typically observed in rivers internationally, or in other parts of New Zealand. This is likely due to the fine sediment-dominated channel banks and extreme size of the floods.
“In the Ūawa/Hikuwai River, there was more erosion of inner channel banks than outer banks. This is completely different than [what] occurs in most meandering single-thread rivers, where it is more common to see larger erosion of outer banks.
“We hypothesise that this is due to the severe out-of-channel flow across floodplains during Cyclone Gabrielle, which basically ‘shortcut’ the inside bank of meander bends, flowing across floodplains, then back into the main river channel.
“At these locations there is much higher bank erosion. This is an interesting result and phenomena internationally,” he said.
The researchers also found that more work was needed on vegetation to help mitigate future floods.
“Further work is needed to determine which species of plants/trees were eroded and which were retained. Erosion of riparian vegetation contributes woody debris to rivers, which, together with forestry slash, can accumulate as debris dams at bridges and cause significant damage to infrastructure,” said Biggs.
“It is important to determine which species to plant (i.e. willows or specific native species) as planting the wrong species (depending on rooting structure) may not help to reduce bank erosion, but only increase woody debris.
“It is also possible that very tall trees may exacerbate bank erosion due to the weight of their trunks and biomass on saturated channel banks. Further research is needed into this (notably for tall pine trees and poplars on riverbanks).”
A range of factors came into play in affected rivers, he said.
“There are many issues in these rivers, including fine sediment supply from upstream (e.g. due to land use, landslides and bank erosion), and significant issues with woody debris. Major work is needed to reduce fine sediment and woody debris inputs to these rivers.”
The study could also be expanded to other New Zealand rivers, he said.
“This work was a small internally funded project. However, it could be upscaled to cover more rivers, longer river reaches and to include classification of riparian vegetation to species level. Hopefully we can obtain support to do this work in the future.”