Rebuilding on a rising tide

By Gareth Renowden 15/06/2011

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.]