Turning Forbury Park racecourse into a lake, building a wall
around it or raising the whole area have been rejected as
options for defending Dunedin's harbourside and south city
areas from sea-level rise.
But a series of pumps at low-lying points and wells around
its coastal fringes could be viable ways of saving one of the
city's most valuable areas - home to $4.3 billion worth of
infrastructure and property assets and 10,000 people - from
going under water, an initial assessment of the options says.
The report, prepared for the Dunedin City Council by Robert
Crosbie, from consultancy Beca, says the main threat to the
harbourside and south city area from sea-level rise in the
next 100 years will be from rising groundwater, forced up by
the rising sea level.
The groundwater in the area generally sits about 40cm below
Inundation was not expected to be a serious issue this
Although time frames were estimates only, at this stage the
sea level was predicted to rise 30cm by 2040 and 80cm to 1.6m
by 2090. It seemed the lower end of that range was more
likely, council corporate policy team leader Maria Ioannou
said. That meant there was time before decisions had to be
made about how to protect the south of the city.
After doing a high-level assessment of the options, Mr
Crosbie said a solution to a 30cm sea-level rise could be to
install a system of underground drains in lower-lying areas
and pump the collected groundwater into existing stormwater
When sea levels 80cm higher were reached, 70m-deep wells
could be dug around the perimeter of the area to intercept
incoming groundwater before it reached the aquifer and pushed
the groundwater levels up.
How many wells depended on how high the sea rose.
Options such as building up the whole area, building dykes,
installing a surface drainage system, digging out Forbury
Park racecourse and making a lake, building an impermeable
barrier 70m deep along coastal fringes and putting a tidal
barrier in at Andersons Bay inlet were also considered, but
dismissed because they largely still required pumping water
He estimated the pump system could cost about $10 million to
install, noting costs would vary greatly over the long period
being considered and included assumptions about things like
the dune system remaining in place, and the wells about $65
Before any plans could be made, extensive underground
investigative work needed to be done to understand exactly
how the groundwater would react to the rising sea level. He
noted that had already begun, with the Otago Regional Council
recently installing an extra monitoring bore.
DCC planning and regulatory committee chairman Cr David
Benson-Pope said the Beca assessment was indicative only, but
was a useful starting point for the work that was needed
before the council made any decisions about what to do.
There would be plenty of consultation with local property
owners and the wider community before any decisions were
made, he said.
He expected people in the harbourside and south city areas
would welcome the report, because it showed the area could be
''I think it will be quite reassuring for people to look at
this and see we have a series of options to work through that
are not necessarily earth-shatteringly expensive and are a
very long way off.''
ORC engineering, hazards and science director Dr Gavin Palmer
said the report gave a useful indication of the scale and
nature of interventions potentially required, and was a good
place for the councils to start as they teased out the
feasibility of the options.
The Beca report would be added to land information
memorandums, but Cr Benson-Pope did not think that should
affect house values because most people were already aware it
was a future issue in the area.
Ms Ioannou said that because the options for protecting the
area from sea-level rise were still being explored, the
council was recommending the next iteration of the district
plan stick with the status quo, which was to limit
development to existing density levels in the residential
Plans to communicate it to the south city community were
being developed and there would be plenty of opportunities
for people to have their say on the matter as protection
plans were developed over the next decade or so.
The next step was for council staff to work when, and how
much it would cost, to do the more detailed investigative
work needed on the options for protecting the area. That work
would begin in 2015-16.
Councillors will consider the Beca report next week.