Dunedin currently safer from tides than first thought

South Dunedin was badly hit by flooding in June, 2015. Photo / Stephen Jaquiery, Otago Daily Times

Scientists studying groundwater in Dunedin have shared some surprise positives in a city well known for its vulnerability to sea level rise and flooding.

Researchers have been monitoring groundwater levels across the city, to get a clearer picture of its exposure to flooding.

Every 15 minutes, automated recorders collect data on how water beneath the city responds to rainfall, seasonal variation, hillslope runoff, pumping and the effect of tides.

Last year, Otago Regional Council scientists increased the scale and density of their urban Dunedin groundwater monitoring network from four sites to 23, covering the wider Harbourside-South Dunedin area.

"The first year of monitoring has yielded quite a few nice surprises," said Simon Cox, a principal scientist at GNS Science.

"Before we started, we heard widespread stories, such as people having to wait for low tide before digging to get a dry hole in their garden.

"We thought the monitoring would show groundwater is strongly tidal - but in reality, there are very few places where groundwater changes more than 20mm between low and high-tide.

"Chemical analyses also indicate saline-rich seawater barely infiltrates more than 800m inland from the sea or harbour."

The monitoring also indicates that some of the flat low-lying land beneath parts of the city is much less permeable than initially expected.

"Gentle pumping of wells is able to disturb the chemistry of groundwater long after water-levels have recovered, and many places have 'topsy-turvy' temperatures that are warmest in June and coldest in December," Cox said.

"This suggests water is not moving rapidly through the ground, and that is good news. Further monitoring and analysis needs to continue to improve our understanding of the relationship between present day tides and groundwater levels and the effects of future sea level rise."

South Dunedin Future, a collaborative project between Otago Regional Council and Dunedin City Council, was pleased to see these results.

"Technical staff from both councils will use the analyses presented in this report to help inform the next phase of scientific work, while consulting with the community on potential options to mitigate against natural hazards and climate change impacts through the South Dunedin Future project," the regional council's general manager operations, Dr Gavin Palmer said.

"Ongoing monitoring and further analyses of the likely effects of climate change on groundwater are still needed to confirm these initial observations based on one year of monitoring only."

Dunedin City Council has budgeted $35m to spend on flood reduction in the South Dunedin area over the next decade.

"This new report will form an important part of the technical basis for developing options for exactly how that money will be spent," the city council's infrastructure services general manager Simon Drew says.

Dunedin groundwater is monitored by Otago Regional Council and GNS Science, with chemical analyses completed by the University of Otago.

The city is seen as a test case for developing methods to monitor, map, model and help mitigate the effects of rising groundwater and sea-level rise.

South Dunedin - home to about 10,000 residents, 12 schools and six rest homes – is particularly exposed as it straddles a spread of low-lying flats between Otago Harbour and the Pacific Ocean.

In June 2015, when a monster deluge unloaded 142mm of rain in just 24 hours, hundreds of homes were swamped.

The emerging threat of sea level rise has caused particular concern – and experts have warned that just 20cm to 40cm of sea-level rise will mean South Dunedin's water table will rise, exposing it to flooding after heavy rain that could damage roads, pipes, cables, buildings and parks.