NZ
The origin of the vast amount of sediment moved around by Cyclone Gabrielle, and what happens to it
Michael Daly
Feb 28 2023
Mountains of silt and slash blanketing Esk Valley should stay there, resident says
Steve Wheeler has up to 2 metres of silt blanketing his property in Esk Valley. The smart move would be to leave it there, he says.
Among the most shocking sights to emerge from recent extreme weather events were the vast amounts of sediment washed down by floodwaters onto the plains of Hawke’s Bay and Tairāwhiti.
In some of the worst cases, the sediment left behind after the water had gone completely buried houses up to their roofs.
Professor Troy Baisden, from the University of Auckland, studies the flow of carbon, nitrogen and water through ecosystems. He said the North Island’s east coast was among the world’s leading hotspots for the uplift erosion and deposition of sediment.
In areas such as the Waipaoa catchment above Gisborne, sediments from the 20th century were more than 5m deep.
What is in the sediment?
While the sediment is often generically called silt, technically silt is particles of sediment that range in size from 0.002mm (or 2 microns) to 0.063mm.
READ MORE:
* Cyclone Gabrielle: Mountain of silt engulfs Esk Valley property, no way to move it
* Cracking the code of catastrophic floods in New Zealand
Larger particles up to 2mm are sand. Sediment particles smaller than silt – under 0.002mm or 2 microns – are referred to as clay or mud.
Baisden said much of the silt and sediment was repeating a cycle, having been deposited some millions of years ago in thick marine sediments like those washed down rivers two weeks ago.
CHRIS SKELTON/STUFF
Sediment brought down by floodwaters during Cyclone Gabrielle buries a building up to its roof in the Esk Valley, north of Napier.
It was then uplifted but only poorly consolidated, leaving the steep, soft-rock hill country of the east coast.
Where does it come from?
Three soft-rock terrains were the main sources of the deep deposits of sediment left after the flooding, Baisden said.
Of those, steep mudstone hill country covered the largest area. It was good for fertile pasture, until roughly the upper metre of soil and weathered rock became a slip. In events like Gabrielle, slips could cover roughly a quarter of the area.
To the north, in Tairāwhiti, there were terrains where gullies expanded at the top of stream channels in steep catchments, Baisden said.
Thirdly, towards the tip of East Cape, most notably in the Waiapu catchment near Ruatoria, there were massive slopes composed of harder but highly fractured rocks that generated earthflows.
“These areas have amongst the highest sediment generation anywhere in the world.”
Associate Producer Peter Almond, head of the Department of Soil and Physical Sciences at Lincoln University, said the sediment ending up on the floodplains was coming from material eroded from hill slopes, or from river channels and riverbanks.
In places, flooding rivers had also scoured sediment from floodplains, moving it to somewhere else on the floodplain, or out to sea.
"In the big picture, the fertile plains of Hawke’s Bay and Tairāwhiti owe their origin to the kinds of events we have witnessed with Cyclone Gabrielle,” Almond said.
“However, the frequency of the large sedimentation events building the plains has increased as a result of changes to land use, primarily the clearance of native forest from the hills.”
Climate change, bringing more frequent and more intense storms, would speed up the frequency and rate of sedimentation on the plains.
“These events pose a greater hazard as populations grow, and more people are put in harm’s way."
A section of road known as ‘Devil’s Elbow’ on SH2 between Napier and Wairoa crosses what is normally a small stream. During Cyclone Gabrielle slips blocked the road, and torrents of floodwater poured down the hillside.
What happens to the eroded hills?
On hill slopes where soils had been eroded, usually the topsoil and subsoil down to the bedrock were removed by the processes of land sliding, Almond said.
Other kinds of erosion involving flowing water, including rilling and gullying, also removed all pre-existing soil.
“This stripping and reforming of soils is a common phenomenon in the rapidly eroding, steep soft-rock hill country of eastern North Island. It is common too in the Rangitikei, Manawatū and Whanganui regions.”
How is the sediment distributed?
The way different sediment types were distributed on the floodplains depended on the energy of water flows.
Close to rivers and in channelised zones, only the larger-sized material (sand) could settle because of the strong current, Almond said.
In areas where flows were lower, dominantly silt and fine sand could settle. In ponded areas clay (mud) could settle.
CHRIS SKELTON/STUFF
Sediment left in the Esk Valley after Cyclone Gabrielle.
What about the sediment that gets to the sea?
University of Waikato Professor of Marine Science Conrad Pilditch said the volume of sediment entering the coastal environment from this year’s weather events was a “big issue”.
Although the water in estuaries and coastal zones may become clear after a short period, the silt would still be around in those environments for a long time.
For some estuaries, a lot of silt was already accumulating. Material landing on the seafloor or on rocky reefs could smother animals and plants.
As that dispersed, there could be a background increase in turbidity, “which creates a longer term effect of silt being resuspended and dispersed around and generally browning those coastal waters”, Pilditch said.
“All of these ecosystems are dependent on plant growth – microscopic phytoplankton or large kelps and seaweeds.
“When you put these suspended sediments in the water column, it stops some light from hitting the seafloor.
“This cuts down the amount of food at the base of the food web, and then all the animals that feed on that, it has devastating consequences for them as well."
MARK TAYLOR/STUFF
What can be done where land is covered in sediment?
Almond said the sediment was usually low in organic carbon and nitrogen, phosphorus and available potassium, unless it was eroded from a nearby fertile soil and redeposited quickly.
Where sediment was thin – less than 5cm thick – many existing plants would readily grow through it. The sediment could be readily incorporated into pre-existing topsoil or direct-drilled to re-establish pasture.
For sediment up to 25cm thick, deeper cultivation could be used to achieve the same effect, although the greater concentration of fresh sediment would mean lower fertility.
With thicker sediment, where it could not be removed, a new soil would have to form in the sediment. Depending on management approaches, the new soil could be growing pasture at somewhere more than 70% of its original productivity within 18 months, Almond said.
Fertiliser additions were essential, particularly when the sediment was thick because fertility of the sediment was normally low.
”Experience from the 2004 Manawatū floods showed that despite a relatively quick recovery to productive pasture, land affected by sedimentation tended to be affected by pugging and weed problems for a sustained period after sediment deposition.”
DAVID WHITE/STUFF
Silt damage to a farm inland from Wairoa.
What could be done to reduce the amount of sediment washed down?
Satellite mapping had shown that closed-canopy indigenous and exotic forests were effective at preventing slips because roots hold the soil together, Baisden said.
Two areas to look at were whether increasingly accurate elevation and slope data could show more clearly what land was at risk. And second, how much risk were vulnerable terrains planted with exotic forests at in the years after clearcutting?
”Studies of mudstone hill country around Lake Tutira suggest that indigenous reforestation can be effective, and mānuka honey can complement carbon credits as an income source from this land.”
Mountains of silt and slash blanketing Esk Valley should stay there, resident says
Steve Wheeler has up to 2 metres of silt blanketing his property in Esk Valley. The smart move would be to leave it there, he says.
Among the most shocking sights to emerge from recent extreme weather events were the vast amounts of sediment washed down by floodwaters onto the plains of Hawke’s Bay and Tairāwhiti.
In some of the worst cases, the sediment left behind after the water had gone completely buried houses up to their roofs.
Professor Troy Baisden, from the University of Auckland, studies the flow of carbon, nitrogen and water through ecosystems. He said the North Island’s east coast was among the world’s leading hotspots for the uplift erosion and deposition of sediment.
In areas such as the Waipaoa catchment above Gisborne, sediments from the 20th century were more than 5m deep.
What is in the sediment?
While the sediment is often generically called silt, technically silt is particles of sediment that range in size from 0.002mm (or 2 microns) to 0.063mm.
READ MORE:
* Cyclone Gabrielle: Mountain of silt engulfs Esk Valley property, no way to move it
* Cracking the code of catastrophic floods in New Zealand
Larger particles up to 2mm are sand. Sediment particles smaller than silt – under 0.002mm or 2 microns – are referred to as clay or mud.
Baisden said much of the silt and sediment was repeating a cycle, having been deposited some millions of years ago in thick marine sediments like those washed down rivers two weeks ago.
CHRIS SKELTON/STUFF
Sediment brought down by floodwaters during Cyclone Gabrielle buries a building up to its roof in the Esk Valley, north of Napier.
It was then uplifted but only poorly consolidated, leaving the steep, soft-rock hill country of the east coast.
Where does it come from?
Three soft-rock terrains were the main sources of the deep deposits of sediment left after the flooding, Baisden said.
Of those, steep mudstone hill country covered the largest area. It was good for fertile pasture, until roughly the upper metre of soil and weathered rock became a slip. In events like Gabrielle, slips could cover roughly a quarter of the area.
To the north, in Tairāwhiti, there were terrains where gullies expanded at the top of stream channels in steep catchments, Baisden said.
Thirdly, towards the tip of East Cape, most notably in the Waiapu catchment near Ruatoria, there were massive slopes composed of harder but highly fractured rocks that generated earthflows.
“These areas have amongst the highest sediment generation anywhere in the world.”
Associate Producer Peter Almond, head of the Department of Soil and Physical Sciences at Lincoln University, said the sediment ending up on the floodplains was coming from material eroded from hill slopes, or from river channels and riverbanks.
In places, flooding rivers had also scoured sediment from floodplains, moving it to somewhere else on the floodplain, or out to sea.
"In the big picture, the fertile plains of Hawke’s Bay and Tairāwhiti owe their origin to the kinds of events we have witnessed with Cyclone Gabrielle,” Almond said.
“However, the frequency of the large sedimentation events building the plains has increased as a result of changes to land use, primarily the clearance of native forest from the hills.”
Climate change, bringing more frequent and more intense storms, would speed up the frequency and rate of sedimentation on the plains.
“These events pose a greater hazard as populations grow, and more people are put in harm’s way."
A section of road known as ‘Devil’s Elbow’ on SH2 between Napier and Wairoa crosses what is normally a small stream. During Cyclone Gabrielle slips blocked the road, and torrents of floodwater poured down the hillside.
What happens to the eroded hills?
On hill slopes where soils had been eroded, usually the topsoil and subsoil down to the bedrock were removed by the processes of land sliding, Almond said.
Other kinds of erosion involving flowing water, including rilling and gullying, also removed all pre-existing soil.
“This stripping and reforming of soils is a common phenomenon in the rapidly eroding, steep soft-rock hill country of eastern North Island. It is common too in the Rangitikei, Manawatū and Whanganui regions.”
How is the sediment distributed?
The way different sediment types were distributed on the floodplains depended on the energy of water flows.
Close to rivers and in channelised zones, only the larger-sized material (sand) could settle because of the strong current, Almond said.
In areas where flows were lower, dominantly silt and fine sand could settle. In ponded areas clay (mud) could settle.
CHRIS SKELTON/STUFF
Sediment left in the Esk Valley after Cyclone Gabrielle.
What about the sediment that gets to the sea?
University of Waikato Professor of Marine Science Conrad Pilditch said the volume of sediment entering the coastal environment from this year’s weather events was a “big issue”.
Although the water in estuaries and coastal zones may become clear after a short period, the silt would still be around in those environments for a long time.
For some estuaries, a lot of silt was already accumulating. Material landing on the seafloor or on rocky reefs could smother animals and plants.
As that dispersed, there could be a background increase in turbidity, “which creates a longer term effect of silt being resuspended and dispersed around and generally browning those coastal waters”, Pilditch said.
“All of these ecosystems are dependent on plant growth – microscopic phytoplankton or large kelps and seaweeds.
“When you put these suspended sediments in the water column, it stops some light from hitting the seafloor.
“This cuts down the amount of food at the base of the food web, and then all the animals that feed on that, it has devastating consequences for them as well."
MARK TAYLOR/STUFF
A plume of sediment in the sea off the Hawke’s Bay coast after Cyclone Gabrielle.
What can be done where land is covered in sediment?
Almond said the sediment was usually low in organic carbon and nitrogen, phosphorus and available potassium, unless it was eroded from a nearby fertile soil and redeposited quickly.
Where sediment was thin – less than 5cm thick – many existing plants would readily grow through it. The sediment could be readily incorporated into pre-existing topsoil or direct-drilled to re-establish pasture.
For sediment up to 25cm thick, deeper cultivation could be used to achieve the same effect, although the greater concentration of fresh sediment would mean lower fertility.
With thicker sediment, where it could not be removed, a new soil would have to form in the sediment. Depending on management approaches, the new soil could be growing pasture at somewhere more than 70% of its original productivity within 18 months, Almond said.
Fertiliser additions were essential, particularly when the sediment was thick because fertility of the sediment was normally low.
”Experience from the 2004 Manawatū floods showed that despite a relatively quick recovery to productive pasture, land affected by sedimentation tended to be affected by pugging and weed problems for a sustained period after sediment deposition.”
DAVID WHITE/STUFF
Silt damage to a farm inland from Wairoa.
What could be done to reduce the amount of sediment washed down?
Satellite mapping had shown that closed-canopy indigenous and exotic forests were effective at preventing slips because roots hold the soil together, Baisden said.
Two areas to look at were whether increasingly accurate elevation and slope data could show more clearly what land was at risk. And second, how much risk were vulnerable terrains planted with exotic forests at in the years after clearcutting?
”Studies of mudstone hill country around Lake Tutira suggest that indigenous reforestation can be effective, and mānuka honey can complement carbon credits as an income source from this land.”
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