A better understanding of how braided rivers such as the Wairau interact with groundwater systems is the focus of the Gravel Bed Rivers (GBR) National Research Project, now in its fourth year.
Findings from this ongoing work were presented to Council’s Environment and Planning Committee recently by Research Hydrogeologist Scott Wilson from Lincoln Agritech in Canterbury, and Council’s Environmental Scientist Groundwater Quantity and Quality Peter Davidson.
The study, funded by the MBIE Endeavour Fund with contributions from Marlborough District Council, Hawke’s Bay Regional Council and Environment Canterbury, is looking at three rivers - the Wairau, the Ngaruroro in Hawke’s Bay and the Selwyn/Waikirikiri in mid-Canterbury.
“A key finding from the research is that an unintended consequence of providing the community with flood protection from the Wairau River has been a reduction in natural aquifer recharge rates. This has likely been compounded by gravel extraction and a depleted natural gravel supply since the 1960s,” Peter Davidson said.
“It’s important to note that this research is also being done with our rivers and policy teams here at Council as any changes in the way we manage the mighty Wairau will need a coordinated approach. There is a review of the Wairau River Scheme coming up next year and this work will feed into that,” he said.
“In some cases, the processes are natural so Council cannot regulate them. However, we can manage gravel extraction and developing a gravel management strategy is a likely output from Council’s point of view,” Mr Davidson said.
This research has coincided with Government policies as part of the NPS for freshwater management and in particular the principle of te mana o te wai or the mana of the water.
Research Hydrogeologist Scott Wilson said there was a “pretty clear” relationship between the form and dropping of the average riverbed level and long-term groundwater levels, all of which have been declining since the mid to late 1990s.
Sonic core drilling has revealed loose clean gravel near the surface and a silt and clay rich gravel underneath. “The contact between these two levels is created and affected by flooding and river levels. If you drop the bed of the river, it affects the aquifer. The scouring of the river is also an issue as it drops the water table in the riverbed,” Mr Wilson said.
The research has identified there is a clay rich layer buried beneath the Wairau River which has formed from erosion of the Northbank hills. “This layer is not very permeable and therefore does not transmit water very well. We have dubbed it the peanut butter gravel,” he said.
“In some places along the Wairau the riverbed is now sitting within this layer due to bed levels dropping over time and this affects the ability of the aquifer to recharge. It is likely this is an unintended consequence of flood protection works, gravel extraction and a depleted natural supply. We could see there was a long-term decline in groundwater recharge rates and now we believe we have identified the causes,” he said.
With one year of the research project to go, the team will now focus on modelling across the three rivers to look at the impact of changing the way the rivers are managed and the economics of this. The findings will be presented to all three Councils in the middle of next year.
