Most of the landslide debris from the Kaikōura earthquake is still stranded in the region's catchments — much of it perched high in the headwaters 10 years on — and a scientist says it will shape flood risks in the area for decades to come.
Tens of thousands of landslides thundered down hillsides when the 7.8 magnitude quake struck on November 14, 2016, cutting off towns and severing the main road and rail links along the coast.
University of Auckland geomorphologist Jon Tunnicliffe has spent years tracking the region's rivers from the air. He said early analysis of a recent LiDAR survey suggested around three-quarters of the material dislodged by the quake remained in the catchments.
Under the high climate model scenario, the cost of inland flood damage will see a 50% jump. (Source: 1News)
In July, Kaikōura had a reminder of what happens when it moves. About 450mm of rain fell over two days, breaching stopbanks and sending huge volumes of gravel down the rivers.
The Kōwhai River burst its banks, a state of emergency was declared, and the riverbed rose by 5m-7m in places.
Clarence Valley farmer Derrick Millton, whose land was lifted about 9m in the quake, said the Kōwhai was bearing the brunt of the problem.
“You can’t have land lifted 30 feet, as in our case we did, without something going on in the creeks and rivers. We’re always going to have issues with rivers and gravel and getting things back to normal, if it’s possible.”
'Jaw dropping' accumulation
Nowhere shows the scale of the quake's impact better than the Hāpuku River, north of Kaikōura township.
Tunnicliffe said one enormous landslide triggered by the quake sent about 26 million cubic metres of rock and rubble into the river, damming it and burying the valley floor.
The Hāpuku's riverbed rose by more than 30m, about the height of a nine-storey building. He said it is a figure he uses at conferences, where "people's jaws just drop".
"Nobody sees that kind of accumulation in a river system."
Scientists had once expected rivers to recover within five to 10 years of a major disturbance, but the Hāpuku showed it would take far longer, he said.
His team flew airborne LiDAR surveys over the river roughly every six months, and the Hāpuku has since become a reference case in international research, including in Nepal and China.
Now the latest survey suggests recovery has stalled, and has for now gone into reverse.
Preliminary analysis by Tunnicliffe's team of LiDAR flown on July 20, after the flood, shows the stretch of river measured in every survey holds about 5.4 million cubic metres more sediment than it did in a 2015 pre-quake baseline.
That is up from about 4.4 million in May 2025, when the river had appeared to be slowly shedding its load, and higher than at any earlier survey.
Tunnicliffe said more landslide material had come into the channel, but the bulk of the big deposit was still yet to work its way down the river.
Waves that take decades
Much of the debris left upstream had become "armoured", Tunnicliffe said, with finer material washed away and a crust of boulders proving difficult for rivers to move.
"In some cases, that might be more or less permanent, and that'll stick around until the next big disturbance or earthquake moves through."
He expects more large surges of sediment in future storms. He described the material as moving in "waves" that travel down the rivers over decades. A narrow canyon partway down the Hāpuku acts as a throttle, and wider sections of valley soak up debris "almost like a shock absorber".
The new survey fits that picture: scour in the upper valley, and thick deposits building in the reaches below.
The gravel dilemma
Tunnicliffe warned digging gravel out of rivers could backfire if it isn't carefully planned.
After the quake, gravel was removed downstream on the Hāpuku, bracing for a wave of debris that never arrived.
"They ended up extracting more gravel than the river was delivering, and so that resulted in the short-term lowering of the bed level," Tunnicliffe said.
Gravel extraction could work "in the right place and the right time", he said, but needed to be coordinated, monitored and planned for the long term, "rather than sort of knee-jerk reaction to the latest storm event".
ECan says gravel extraction is “a key part of the recovery effort”, but that it is demand driven, with commercial operators removing material for roads and other infrastructure.
The remaining landslides sit high in the headwater gorges and are “virtually inaccessible” for direct extraction, according to the regional council, which is still working to identify priority areas for future management.
Tunnicliffe said the scars of past disasters were still visible in the landscape.
"These rivers have memory, and it's really important to take that historical context on board," he said.
"When we're thinking about where infrastructure goes, or where the new highway corridor might go, it's important to take that history into account, and that may be going back several hundred years."
SHARE ME