18 July 2016

South Dunedin natural hazards conversation urged

The time is right for a community conversation about how best to minimise the combined effects on South Dunedin of high groundwater levels, sea level rise, and land subsidence, ORC chief executive Peter Bodeker says.

For the past seven years ORC has been collecting information on South Dunedin’s natural hazards and making that information available to the public and the Dunedin City Council.

A technical report tying together all of this information will be presented to councillors on Wednesday (July 20).

South Dunedin is on soft, silty soils that in the 1800s were a tidal wetland, similar to Hoopers Inlet on the Otago Peninsula. This is a very different geological landscape to the rest of the city, which is largely built on a more solid, volcanic rock base. Today, about 2700 homes in South Dunedin lie less than 50cm above sea level.

ORC established three permanent groundwater bores in 2009 and a fourth in 2014 to monitor the relationship between sea level and groundwater levels.

Mr Bodeker said the key finding from this monitoring is the increased likelihood of surface flooding associated with rising sea levels. Because there is already a shallow water table beneath South Dunedin, an increase in groundwater levels will eventually result in occasional and possibly permanent surface ponding on parts of the area.

“Understandably, many people and groups want to know how these risks will be managed. To encourage a community conversation, and lay the foundation for further scientific work, we have produced a technical report on South Dunedin’s natural hazards and their effects, tying in all the research on the subject to date, including our own,’’ he said.

“While this further work is important, it must be the catalyst for bold decisions about risk management, rather than these reports just gathering dust on a shelf. The work also has to be co-ordinated between agencies and focus on the right issues, in the right sequence, at the right time,” Mr Bodeker said.

“We believe it is critical the community understands the current and future hazardscape of South Dunedin, and we will be presenting this information to the community over the next two months.”

“We are also firm in our belief that planning for South Dunedin’s future management is an immediate priority,” he said.

The South Dunedin plain is vulnerable to natural processes which occur reasonably frequently (such as coastal storms), and also events which occur rarely, but have significant consequences (such as major earthquakes on nearby faults).

It is densely populated, with approximately 10,000 permanent residents, and contains infrastructure and other assets which are important at a local, district and regional level.

The area’s physical characteristics mean it can be affected by water ponding on the surface (i.e. flooding) and it is this hazard which poses the greatest risk to community wellbeing, infrastructure, and other assets.

While the most recent example of major flooding was in June 2015, as a result of heavy rainfall, surface runoff, and a corresponding rise in groundwater, the ORC report shows that there are a number of naturally occurring physical processes and human activities which together, or separately, could affect flood hazard in South Dunedin.

Changes in mean sea level, climate and groundwater level are the processes most likely to exacerbate the effects of this hazard.

The report shows how these effects will vary across the area, with some areas likely to be affected sooner than, or to a greater extent, than others.

Mr Bodeker said ORC is working with the DCC to talk to the community about the report’s conclusions and the next steps required for planning for its future.

ends

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