Rebuilding on a rising tide

by Gareth on June 15, 2011

It’s been a shaky week in Christchurch and Canterbury. Another M6.3 shock hit the city on Monday afternoon — renewing the misery for many in the city’s eastern and seaside suburbs, but thankfully not adding to the death toll. Attention has now turned — with some force — to the question of which suburbs should be rebuilt, and an excellent feature by David Williams in last Saturday’s Press on sea level rise and its implications for the rebuilding of Christchurch should cause some pause for thought. Williams interviewed James Hansen during his visit to the city last month (shortly before I did, in fact), and uses Hansen’s views on sea level rise to kick off his discussion:

Hansen says a multi-metre sea level rise is possible this century if greenhouse gas emissions, caused by things such as coal-fired power plants, vehicle engines and agriculture, are not reduced.

Williams goes on to put that into the context of a city where the baseline has shifted:

But the sea-level implications of his predictions are particularly significant for low-lying, quake-hit Christchurch.

The city has two rivers snaking through it and much of it is drained swamp land. As it is, the city’s main surveying marker — a stone in the foyer of the city’s broken Anglican cathedral, 8.5 kilometres from the beach — is barely five metres above the high tide line.

Since the run of earthquaked began last September, the city’s eastern and riverside suburbs are living with a new normal:

Tidal flooding from the Lower Styx River has swamped some Brooklands properties twice a day since the February earthquake.

Last month high-tide flooding hit Christchurch’s river suburbs and residents are anxiously waiting to see if it’s a wet winter.

GNS Science geophysicist John Beavan, of Wellington, has been surveying post-quake Christchurch. He confirms that isolated areas, rather than whole suburbs, have dropped by up to a metre.

Modelling done after the quake – quite a bit of which has been verified by surveying – showed that the Avon- Heathcote Estuary and part of the Port Hills has risen by “several tens of centimetres”. Meanwhile, land to the north of the Estuary, such as Bexley, has gone down by maybe 10cm. Subsidence because of liquefaction is on top of that.

It remains to be seen if Monday’s shock has added to those figures, but it would be unwise to presume that things haven’t got worse. Williams also digs out the views of another Williams who is taking a precautionary view of where sea level will eventually end up:

Former Christchurch man Nigel Williams, a traffic planner who now lives in Auckland, follows Hansen’s work closely and is developing a retirement property, with solar panels, well out of harm’s way.

Williams, 64, has a blog called The 100 Metre Line, which urges people to move to higher ground while they still can. He says 10 metres above mean sea level passes through St Andrew’s College, in Papanui, and parts of Halswell.

“If by 2050 there has been half a metre of sea level rise, we’ll be up to our arses by 2100,” he says. “I’d move to Geraldine.”

Nice place, Geraldine. Any city rebuilding plan which fails to take account of projected sea level rise over the next 150 years will not be worth the paper it’s written on, but given that the process is being supervised by coal-loving former energy minister Gerry Brownlee, I don’t hold out much hope for common sense and forward thinking being properly applied. [I should note that chez Hot Topic is 170m above sea level — and a rather large earthquake-prone fault is expressed as a cliff about 30 metres from where I write this. I’m not sure which will get me first.]

{ 9 comments… read them below or add one }

Bob Bingham June 16, 2011 at 9:24 am

If you set the flood map http://flood.firetree.net/ to 1 meter and look at Christchurch it is clear that the water is coming up the rivers. I wrote to Christchurch pointing out the problem and I am sure that more authorative people that me have done the same. It will be interesting to see if it gets a mention when they declare the reasons for abandoning some areas of the city

Thomas June 16, 2011 at 10:05 am

The fact that the tide may rise fast is underlined by an article in the latest Scientific American:
http://www.scientificamerican.com/article.cfm?id=the-last-great-global-warming

“Surprising new evidence suggests the pace of the earth’s most abrupt prehistoric warm-up paled in comparison to what we face today. The episode has lessons for our future”

The article compares pre-historic episodes of warming and the speed they took to our current one and concludes that past warming episodes saw temperatures rise much more gradually allowing oceans to better balance acidity, species to adapt and migrate etc. while we are currently on an rise slope not seen before.

Graphs in the print edition (or for subscribers the online edition) are stark.

Kiwiiano June 16, 2011 at 5:29 pm

With the best will, I have reservations about firetree. With a 1m setting there are some parts of Hagley Park underwater and with a 4m setting, Cathedral Sq is well high and dry even though according to Google Earth’s numbers it should be rather damp.
Note that the Avon is tidal up to the rapids between the Barbados and Kilmore St bridges, just upstream of The Bricks monument.

_R2D2 June 17, 2011 at 9:22 am

“Any city rebuilding plan which fails to take account of projected sea level rise over the next 150 years will not be worth the paper it’s written on”

Any ‘projection’ going out 150 years is not worth the paper its written on.

Gareth June 17, 2011 at 10:32 am

On human timescales, sea level rise is effectively a one way process – and there’s only one way it’s going to go. To fail to take that into account, or to allow an inadequate margin for rise would be a gross dereliction of civic duty. We’re going to rebuild a city that should expect to be around for hundreds of years. I suggest you look up “equilibrium response” to climate forcings.

R2D2 June 18, 2011 at 3:31 pm

You must obviously have a bad grasp of human history. Following the Tobo eruption sea levels decreased. This was an important part of human history, which is believed to have decreased human populations down to perhaps less than 10,000 individuals about 70,000 years ago. These remaining individuals would populate the world through the land bridges made possible (even Australia, however not New Zealand until maritime travel was possible!)

Thomas June 17, 2011 at 7:53 pm

_R2D2: Been to Europe ever? Wandered around towns that stood there for 1000 years plus? Visited a grand European Cathedral that took 20 Generations @ 25 years each to complete from plans to finish?
Do you have any idea about the value of long term thinking?

I give you another example: In the German Harz mountains they were mining for precious metals as early as the 1200ds.. They dough vertical shafts down the top of the mountain range following ore. Water intrusion was their biggest issue. So they decided to connect the many independently owned vertical shafts with a horizontal one that would come out the side of the mountains many Km further, a 200 year project. They shook hands and set to work. Many generations later the project was completed and the miners reaped the benefits.
Thinking long term is sort of unfashionable these days and look what is happening to the prospects of future generations!

R2D2 June 18, 2011 at 3:19 pm

You both elude to the current inter-glacial period that has resulted in rising sea levels over the last 12000 years.

Lets be thankful for this period of warm temperatures that have allowed the human population to flourish and human culture and technology with it.

I guess this warm climate wont last forever and we should really be preparing for an eventual decrease in temperatures.

Kiwiiano June 17, 2011 at 10:52 pm

Just off the top of my head…. they failed to plan ahead in Venice and New Orleans and look where that is getting them.
To be fair no-one up until the last decade or three would have envisaged the problems we are aware of now, but pretending they aren’t happening is, as noted, an appalling dereliction of duty.

{ 1 trackback }

Previous post:

Next post: