Thursday, December 7, 2023

Flood and sediment lessons to learn across catchments

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Scientists team up to study effects of silt on highly valuable land, and how to recover from it.
Scientists sampling silt deposits in Hawke’s Bay post Gabrielle.
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The devastating flood events that bowled through Northland, Te Tairāwhiti and Hawke’s Bay offer no upsides for the regions’ primary producers but may yet provide some valuable insights on how best to manage future events’ impacts upon highly productive land and production.

AgResearch scientist Dr Alec Mackay is working alongside scientist Dan Bloomer and Alex Dickson of LandWISE. 

The project they are co-ordinating aims to provide growers with advice about stabilising and restoring impacted sites and, secondly, establish baselines to support a longitudinal study of site recovery to build soils back better to increase land resilience to mitigate future events.

“In the week following the cyclone, Dan reached out to AgResearch, Massey University and Plant and Food Research looking for  information on past flooding events to put up on the LandWISE’s website for landowners to consider what was best to do with eroded surfaces or sediment,” Mackay said. 

Mackay and his colleagues were able to contribute information based on the experiences of growers following the 2004 Manawatū flood event, and lessons from the 1950 Gisborne floods.

“A decision tree developed from the experiences from the 2004 Manawatū flood impact on pastures gave growers some outlines on how to manage  sediment deposits less than 5cm deep, 5-20cm deep or  over 20cm.”

But he said in Hawke’s Bay most of the major impact of the sediment was on very high-value arable and vegetable  land, and  in perennial crops like apples, grapes, kiwifruit, citrus and cherries.

“The wake of Gabrielle has left sediment in excess of 10-20cm across many vegetable and arable farms, orchards and vineyards, and in places over 1m of sediment.”

“Here you really want to get back down to the original surface, but there is very little knowledge out there on how sediment should be managed in such crops and of the options available, what impact that will have to the future productivity of the soils and their resilience to future events.”

Backed by some core Crown research funding and working with Plant & Food Research and Massey University, the science team has been assisting LandWISE, supported through HortNZ with funding from the Ministry for Primary Industries for nutrient and contaminant testing.

They have developed a programme across Hawke’s Bay, Northland and Te Tairāwhiti catchments to collect relevant data to determine soil biology and physical condition and the origin of sediment deposits, contaminants, depth of deposits, composition and mineralogy.

Data from this initial survey will provide important baseline information for extending growers’ decision support tools and management strategies to all land use types, building on lessons from 1948 Gisborne floods and the 2004 southern North Island storm.

The plan is to  take up to 200 samples across the major catchments, covering all major land-use types but with an emphasis upon the highly productive land in arable, vegetables and perennial horticultural crops. 

In Hawke’s Bay and Te Tairāwhiti the flooding came with large amounts of sediment. 

In Northland there was little or no sediment with the flooding, but prolonged wetness. 

“The sampling also involves talking with landowners about what they propose to do, and the plan is to return in six months, one year and later to examine what did and did not work.” 

These sites will become living laboratories to follow-up on post-flood management.

Mackay can see a longitudinal study will be invaluable in shaping future event recovery responses.

“We will be able to add to the 2004 decision tree with a greater range of land use, more information on sediment types and remediation options.” 

For the deeper 20cm-plus sediment deposits on pastures, the 2004 recommendations were to sow something as quickly as possible and get something growing that starts the soil development process, with the plant roots stimulating biological activity on the largely inert sediment body.

“But site remediation for arable and vegetable, and especially for perennial horticultural crops, requires different thinking. 

“With perennial crops you have a huge investment. By the time you get new trees established, it could be at least five years until you are back to full production, so how do you decide how much to invest per hectare removing sediment?”

Reports from Hawke’s Bay have included one kiwifruit operator spending almost $100,000 a hectare to remove silt over a metre deep from rows using Bobcats.

“For these highly productive soils the challenge is, can we build soils back better, to mitigate future challenges?”

While the scientists have been focusing on the characteristics of sediment and flood damage, Mackay said it is impossible not to go past the impact of the flooding on people, and at the same time the amazingly positive attitude of those so deeply affected.

“The resilience of the people and the strength of the communities they are in is remarkable. They are just getting on with recovery.  And also, their openness to talk about their experiences and [the fact that they are] prepared to share with others on their learnings and thinking since the flood – there is an enormous level of good will there.”

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