Sea Level Rise Assessment for South City and Harbourside

Dunedin (Friday, 18 July 2014) – There are likely to be ways to protect Dunedin’s harbourside and south city area from the impacts of rising sea levels.

An initial assessment of the potential options and costs will be discussed by Dunedin City Councillors next week.

DCC General Manager Infrastructure and Networks Tony Avery says the harbourside and south city area covers the most densely populated part of Dunedin, with a population of about 10,000 residents. It contains a lot of infrastructure, such as wastewater and stormwater assets and key roads. The area has an estimated asset value of $4.3 billion.

This low-lying area has been assessed as being the part of Dunedin most vulnerable to sea level rise in the medium to long term. Timeframes are estimates only and subject to change, but at this stage a 0.3m sea level rise above 1990 mean sea level, is predicted to occur by 2040. In the long term, a 0.8m to 1.6m sea level rise could occur by the end of the century.

The main threat, at least in the medium term, is from rising groundwater in the south city area, as groundwater levels are forced up by rising sea levels. Direct inundation from the sea becomes more of an issue around the Otago Harbour in the long term.

A report on the issues, prepared by consultancy firm Beca, will be presented to the DCC’s Planning and Regulatory Committee on 24 July.

The Beca report provides a high-level assessment of options. It indicates a solution to a 0.3m sea level rise in some areas could be to put in underground drains and pump the collected groundwater into existing stormwater systems. When sea levels of 0.8m are reached, predicted to be by the end of the century, wells could be put around the perimeter of the area to intercept incoming groundwater before it reaches the aquifer and pushes its level up. An aquifer is an underground layer of rock, sand or gravel that contains and transmits
groundwater.

Planning and Regulatory Chair Cr David Benson-Pope says, “The Beca report is a high-level and indicative assessment only and a significant amount of work will be needed before any decisions are expected. However, it is a useful starting point for further investigations.”

The effects of sea level rise are likely to be some way off, but the DCC needs to be responsible in terms of planning for these effects before they occur. There will be plenty of consultation with local property owners, as well as the wider community, as any decisions will have an impact on the city as a whole.

Beca’s Civil Engineering Technical Director Rob Crosbie says one of the key next steps is for the DCC and the Otago Regional Council (ORC) to find out more about the way the South Dunedin aquifer functions in order to better understand how the aquifer will respond to sea level rise.

Mr Avery says the DCC will continue to work collaboratively with the ORC in terms of monitoring and understanding the aquifer. The ORC has already installed an extra monitoring bore in the south city area this year and staff will report to their Council on 23 July.

The DCC is preparing a new District Plan, the second generation District Plan (2GP). In terms of the harbourside and south city area, the 2GP preferred option is to manage risks through the underlying zones of the area, rather than through the hazard overlay zones that are proposed for the rest of Dunedin. This is because the options to manage risks are still being explored and the District Plan will need to be easily adaptable to respond to any decisions made as a result.

In the interim, the 2GP preferred option for this area is to focus on maintaining the status quo in terms of development levels. With regard to residential development in residential zones, this means the proposed approach is to limit development to existing density levels, eg family flats would not be permitted in the South Dunedin residential zone.

For properties in the harbourside and south city area, the DCC will add a note on those Land Information Memorandums (LIMs) that refers people to the Beca report.

In her report to the Committee, Corporate Policy Team Leader Maria Ioannou notes that, as part of the development of the Long Term Plan 2015-25, staff will provide a proposed programme and funding estimates so more detailed investigative work can be carried out on options for the harbourside and south city area from 2015/16.

Information on the DCC’s climate change adaptation work programme, including the Beca report, can be found at www.dunedin.govt.nz/climatechange.

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