Risk of ‘permanent' flooding by 2030

South Dunedin. File photo
South Dunedin. Photo: ODT files
Rising groundwater could cause permanent surface flooding in parts of South Dunedin as early as 2030.

It would be limited to just a few areas by the end of this decade, but over time the "areas of permanent groundwater inundation" would become more widespread and better defined, a new South Dunedin Future report says.

The "Stage 1: Risk Identification Report" is part of a suite of new reports both Dunedin city and Otago regional councillors will consider this week as climate change adaptation plans for the low-lying Dunedin suburb progress.

South Dunedin was home to 6000 households, 12 schools, six early childhood centres and six rest-homes as well as churches, community halls, medical centres, parks, recreational grounds, heritage structures and social housing, the risk assessment said.

It was subject to severe flooding in 1923, 1929, 1960, 2015 and 2018.

The first stage of the three-stage risk assessment under way in the joint-council South Dunedin Future work programme remained "high level", but it did canvas the range of natural hazards the area faces, including rising groundwater.

"The groundwater table is usually very shallow within South Dunedin," the report said.

"This can cause significant drainage and water ponding issues in some places, as well as dampness in buildings and structural instability of foundations and roads.

"Groundwater infiltration into the stormwater and wastewater networks is a widespread issue across South Dunedin."

The report said groundwater levels were expected to rise with climate change and if emissions reduction strategies were kept at status quo levels, rising groundwater would permanently flood some areas of the suburb by 2030.

Maps included in the report showed groundwater flooding becoming increasingly widespread from 2050 to 2100.

Monitoring of groundwater chemistry had shown there was increased salinity in groundwater closer to the coast, but the amount of salt water in groundwater was not as far inland from the coast as previously thought, it said.

However, there was groundwater contamination in some places because of the former Dunedin Gasworks.

The report also highlighted how prone to rainfall-induced flooding the suburb was.

South Dunedin had no major waterways flowing through it or a natural connection to the coast — it was ‘‘generally flat, low lying, and is bounded by relatively elevated land, resulting in a basin’’.

Roads in the suburb were ‘‘highly exposed to flooding’’, which would worsen with the increasing intensity of extreme rain brought about by climate change.

Yet, while flooded roads presented access issues, roads also provided ‘‘important flood storage volume to minimise the flooding of private properties and buildings’’.

The risk assessment went on to say although South Dunedin was bounded by both Otago Harbour and the Pacific Ocean, the St Kilda to St Clair sand dune system and the elevated reclaimed land along the Otago Harbour shore provided some protection against tsunami and storm surge.

A staff report to councillors said a technical peer review process had identified issues to be resolved in the next stage of risk assessment work, which was due to be completed by the middle of next year.

At separate meetings tomorrow and Wednesday city and regional councillors will be asked to note the report, endorse a ‘‘generic longlist’’ of ways to respond to climate change impacts in South Dunedin and approve community engagement on the approaches outlined in the longlist.

Creating stormwater detention basins, wetlands to sponge up excess water, modifying drainage systems, raising the ground level above the floodplain, relocating or abandoning land and buildings before significant damage occurs and coastal protection are all options up for discussion.