isi

Over 220,000 m³of sediment was eroded from just 9.1km of ūawa/Hikuwai riverbanks in Gisborne (Tairāwhiti).

This corresponds to 24 m³ of sediment for each metre of river channel, or around 1,500 dump truck loads for each 1 km.

The researchers from Earth Sciences New Zealand and the University of Canterbury also observed major land losses, with riverbank erosion claiming 72,300 m² of surrounding land in the study area, equivalent to around 270 tennis courts.

Lead author Dr Hamish Biggs was shocked by what the data showed.

“During Cyclone Gabrielle response work and helicopter lidar surveys we saw massive bank erosion; however, the volume of bank erosion and extent of land loss revealed by the data is still astounding.”

Similar findings were observed in the Wairoa River in Hawkes Bay (Te Matau-a-Māui), with more than 140,000 m³ of sediment eroded from riverbanks and 35,000 m² of land lost from an 8.3 km study area.

An example section of the Wairoa River showing aerial imagery before Cyclone Gabrielle (left), after Cyclone Gabrielle (centre) and elevation change (right).

This corresponds to 16 tennis courts of land lost and 6.7 Olympic swimming pools filled with sediment for each 1 km of river.

In their study, the authors also investigated the role of riverside vegetation and river morphology on bank erosion.

Dr Arman Haddadchi, a sediment transport expert at Earth Sciences NZ, said they found differences in erosion depending on these factors.

“We saw that inner banks were more eroded than outer banks in the ūawa/Hikuwai River, and there was far less bank erosion where vegetation was retained, compared to where it was removed, notably in the Wairoa River.”

The researchers stress that further work must be done on this to fully understand the relationship between vegetation and erosion.

“The next steps will be to see which species are most effective at reducing bank erosion, as we did see differences between the ūawa/Hikuwai and Wairoa Rivers that may have been due to the type of vegetation present and the weight of tall trees on channel banks. Shrubs and small trees were more likely to be eroded than tall trees; however, where tall trees were removed there was typically much larger bank erosion or collapse,” said Dr Biggs.

This work follows recent Earth Sciences New Zealand research that showed Cyclone Gabrielle was one of the most extreme landslide-triggering events globally.

The study has been published in the journal Earth Surface Processes and Landforms (ESPL) and is available here: https://doi.org/10.1002/esp.70136.

Helicopter lidar surveys of the Uawa River post-Cyclone Gabrielle, showing coloured point-cloud (top) and DoD (bottom). This example illustrates out of channel flow shortcutting across the inside of tight meander bends, resulting in deposition of fine sediment on floodplains and erosion of channel banks. Bank collapse and removal of riparian vegetation are also visible where flood waters re-enter the main channel, for both inner and outer bends. The green line shows the top edge of the riverbank post-Cyclone Gabrielle; this is the boundary inside which bank erosion, deposition and net change are calculated.

(Source: NIWA, https://niwa.co.nz/)