As severe storms and significant weather events continue to increase in New Zealand, so does the amount of erosion.
According to the Ministry for the Environment’s Our land 2018 report, almost half (44 per cent) of this loss is from
pastoral land.
A new collaborative five-year MBIE funded programme ‘Smarter Targeting of Erosion Control’ (STEC) led by Manaaki Whenua
Landcare Research is now underway, with the task of exploring cost-effective ways of targeting land-based erosion
control in the hope of slowing the damage and improving water quality.
“This research will improve our understanding of where erosion occurs, sediment volume, what type of sediment is
produced, and by which processes,” says Manaaki Whenua Landcare Research Geomorphologist Dr Hugh Smith co-leads the
project.
As significant weather events continue to increase, so does the amount of erosion - triggering the urgent need for
research to understand how to best target erosion control to slow the damage and improve water quality.
Fine sediments are the most common culprit behind widespread contaminants in New Zealand’s rivers and estuaries. They
muddy rivers and reduce water quality, but also carry other pollutants such as heavy metals, nutrients and microbes.
“Year to year there is large variability in storm events,” says Dr Smith.
“We have severe storm events like the one in 2004 in the Manawatu River where huge quantities of sediment are generated
and flow down the river, and there are massive impacts from landsliding and flooding.”
“Then other years we have extended low rainfall periods with no major storms and less sediment, but the water quality is
still quite troubling,” he adds.
Current erosion modelling tools model annual average sediment loads over entire catchments but through STEC new data
will be collected and used in building new models.
“Our ambition is to move from steady-state annual models where things are currently treated on a long term annual
average using historical rates of change, to an event scale modelling approach that better links erosion sources to the
fine sediment affecting water quality in streams and rivers,” Dr Smith explains.
The models will indicate how sediment, and in particular fine sediment, moves through catchments and how erosion and
sediment transport can be targeted and mitigated cost-effectively to help strategically minimise erosion effects
“The smarter targeting of the whole STEC programme is saying ‘well if we had better data, and better models that allowed
us to actually link erosion on the land to instream related water quality impacts could we then better inform managers
when it comes to deciding where to target their investment in erosion control to maximise the benefits downstream?” he
says.
STEC is starting sampling in four river catchments across New Zealand to collect this new data. The Manawatu - for new
and continuing data collection; the Whanganui - to look into impacts from the 2018 storm and legacy sediments; and the
Oreti in Southland and Wairoa near Auckland - to research bank erosion and sediment fingerprinting methods in
collaboration with NIWA’s Managing Mud programme.
Erosion mitigation research will also be conducted on farms to learn more about how effective erosion techniques are,
and build on the base of research for new models.
“Once we have an operating model, there is the opportunity to run scenarios and look at questions relating to the
targeting of erosion control and the benefits at a catchment scale,” says Dr Smith.
“We will then link in with the economic analysis and include that as part of the equation so we can get a fuller view of
the wider benefits of an erosion control strategy and be able to predict outcomes for different catchments.”
Sampling is already underway in the Oreti and Manawatu in the upper Tiraumea catchment - where sensors are being
installed into earth flows to provide data on the rate of movements. A weather station will also be installed to look at
climate conditions and how they vary and relate to movement rates.
Further erosion sampling will take place over the coming months and continue for three years.
STEC is led by Dr Chris Phillips and Dr Hugh Smith from Manaaki Whenua Landcare Research in collaboration with NIWA,
Massey University, Waikato University and an international research network.
The programme partners with Whanganui iwi (Tamaūpoko Community group) and
Rangitāne o Manawatū.