Another bout of heavy rainfall is likely to “remobilise” vast masses of flood waste left carpeting low-lying areas of the East Coast, a geologist says.
The fresh downpours – potentially bringing another 50mm to 100mm to cyclone-hit areas this afternoon and evening, on top of 50mm recorded since 10am yesterday - has prompted a new round of warnings to Hawke’s Bay and Gisborne residents still recovering from Gabrielle.
Authorities have now ordered the evacuation of the severely-hit Esk Valley area.
Martin Brook, an associate professor of geology at the University of Auckland, said it remained to be seen what impact this rainfall had on some hundreds of thousands of cubic metres of waste that Gabrielle had left blanketing floodplain areas.
Those impacts would depend on what type of material the rain was hitting, he said, but another deluge was likely to “rework” what was on the ground – while also adding to local landsliding problems.
While the sludge coating people’s properties had been widely described as silt, Brook said it actually contained a wide range of different minerals and grains, from sand and gravel measuring in the millimetres, down to fine soils, like silt, as small as 0.002mm.
The specific make-up of that material mattered, he explained, because it determined how it would drain in big rainfall events like this one.
“Different grain sizes have got different permeabilities - so they drain and behave differently – but there is a possibility for a lot of that material to be re-worked, particularly if it’s been deposited on a gradient,” he said.
“And the finer the material, the easier it is to be transported as suspended sediment, and the further it can be transported by water,” he said.
“Coarser particles, however, are heavier and won’t be transported as easily.”
Given much of the material causing problems was likely to be finer, more soluable “dispersive soils”, Brook expected the heavy rain would send large amounts into waterways and out to sea.
Satellite imagery captured in Gabrielle’s wake showed enormous plumes of sediment washing out into the ocean, with likely damaging impacts on local marine ecosystems.
Earlier this month, one marine scientist warned how such climate-related flooding events would push estuary ecosystems to the brink of collapse, with some likely to never recover.
In other cases, Brook said the rainfall could make the waste situation worse for clean-up efforts.
“It’s very site-specific, but it might create a bit more of a mess for people who’ve been cleaning up their yards.”
As for landslide risk, Brook said more rain risked bringing down more slips across the East Coast’s already heavily scarred landscapes.
“Some of the soils which will have already failed will now be at a residual strength – and not at their peak strength – and these are the ones that’ll be more susceptible to failing again as shallow-earth flows, or mud flows.”
Large sediment plumes entering Hawke's Bay from the Mohaka & Wairoa Rivers following the devastation wrought by Cyclone Gabrielle. Could better land management practices have mitigated this?@CopernicusEU #Sentinel2 2023-02-19
— Francis J. Burdon (@frank_burdon) February 20, 2023
HT @i_ameztoy
Full Size: https://t.co/uAR2pi0piA pic.twitter.com/0rSBZ0uhmO
Niwa meteorologist Ben Noll said soil moisture right across the north and east of the North Island was far in excess of the limits of Niwa’s daily anomaly maps.
“With the consistent rainfall we’ve seen, I’d say we’re looking at some of the highest soil moisture levels we’ve ever seen for this time of year,” he said.
“There will be many places that are just completely saturated: they’re reached field capacity, they’re at water surplus, and the grounds are unable to take any more additional moisture.”
Rain run-off was just one reason why the East Coast faced fresh risk from flooding in this storm.
Looking further out, Brook said Gabrielle’s flood deposits could have long-lasting consequences for property owners.
“Again, as we’re talking about a range of grain and particle sizes, that has a massive control on the engineering properties of material and how easily it’s eroded or remobilised in future,” he said.
“Ultimately, people might decide to build on these fresh flood deposits, but they might find the bearing capacity is now different.”
New sedimentary soils also came with higher liquefaction potential during future earthquakes, which affected the eastern North Island.
“As we saw in Christchurch, soil liquefaction potential can be very site-specific and grain-size is one of the many factors that is important,” he said.
“So, the new ‘silts’ may be too problematic to be built on.”
In the worst-affected areas of the East Coast, another geotechnical expert this week described the volumes of silt and sand left by Gabrielle as even worse than those created by the Christchurch quakes, which measured in the hundreds of thousands of tonnes, and took months to clean up.
Assessing precisely how the cyclone had reshaped the region’s geological hazard environment wouldn’t be straight-forward.
“Recent work by Landcare, Massey and Auckland universities has shown the ability to geochemically ‘fingerprint’ river deposits to determine the approximate area they were sourced from, but it’s very broad scale,” Brook said.
“Perhaps in our AI-driven utopian future, this geochemical fingerprinting could be sharpened up – poor land management that leads to a hillside being eroded and transported downstream onto someone’s property, could then yield a fine for the hillslope’s landowner.”
Victoria University ecologist Associate Professor Stephen Hartley said Gabrielle had forced society to look more closely at the consequences of poor land use choices.
“Put bluntly, denuding steep hillsides of complex, multi-tiered forests has allowed millions of cubic metres of topsoil to slip off the face of our land, down raging, flashy rivers, only to be deposited across flat land, estuaries and ocean beds, smothering everything beneath it,” he said.
“Topsoil and subsoil we now call silt. Draining floodplain wetlands, so that water runs out of them quickly, has removed buffers that can soak up water during a downpour and then release it slowly downstream in the weeks and months that follow.”
“Of course, we need agriculture and forestry for good reason, but these land-uses can and should be embedded in a matrix of diverse ecosystems that between them can regulate extremes.
“In a volatile and precarious system, too much of one thing is never a good thing.”
Emergency services were called to the scene about 8.30pm.