Groundwater study boosts South Dunedin confidence

GNS Science principal scientist Simon Cox discusses a detailed study of South Dunedin and...
GNS Science principal scientist Simon Cox discusses a detailed study of South Dunedin and Harbourside groundwater. The images supplied (right) show how sea-level rise could lead to increasing amounts of groundwater breaching the land surface. Darker blue signifies surface water now permanent, on average, and lighter shades represent water that might be added from infrequent storms or floods. PHOTO: GREGOR RICHARDSON
An in-depth study of groundwater has boosted confidence South Dunedin can manage the impact of sea-level rise for decades.

However, challenges will mount as water beneath the low-lying land is forced to the surface.

They could include inundation of stormwater and wastewater systems and damage to buildings, and eventually lead to groundwater emerging at the surface as springs or causing localised flooding.

Direct inundation from Otago Harbour was unlikely before 2100, as sea-level rise would first need to get to 60cm-70cm, the Otago Regional Council said.

The study, published by GNS Science, was of the South Dunedin and Harbourside areas, and one point to emerge was seawater penetration of the land was not as high as had been feared.

Nonetheless, rising groundwater is expected to be at the heart of issues in South Dunedin.

The research findings showed a lesser impact in the Harbourside area.

As groundwater rises, the ability for rainfall to be absorbed into the ground like a sponge decreases.

The study is the first in New Zealand to model the contribution of groundwater to several sources of flooding hazard.

GNS Science principal scientist Simon Cox said measuring what was happening now enabled projections to be made.

"The research shows that Dunedin needs to plan for water coming from above, below and sideways from the harbour and ocean," Dr Cox said.

"There are some tidal effects near the coast but most of South Dunedin is sitting on relatively old, thick mud which groundwater moves slowly through, and this may mean there are more options for the future."

Dr Cox said monitoring showed the ground was "less prone to tidal flow and entry of seawater than initially feared, which provides a little more time for the city to plan".

The study was based on four years of data.

Water temperature and pressure were measured by automated sensors every 15 minutes.

Dr Cox said the flat-lying coastal land was vulnerable.

"Our study shows that in some areas, as the sea level rises, groundwater will contribute to flood problems in two ways — through loss of the ground’s capacity to absorb rain, and then flooding from below — and this can be expected before any inundation directly from the sea."

Otago Regional Council natural hazards manager Jean-Luc Payan said the study provided a detailed picture of where and when issues would arise, mapped out against 10cm increases in sea-level rise.

"This helps us to understand how hazards will evolve and enable planning for the future," Dr Payan said.

The study indicated groundwater moving to the surface would move from periodic occurrences to being a permanent feature when sea-level rise got to about 40cm.

That could be about 2080.