MaximusDecember 4, 2008
Seaside cities: rising waters
Venice has been in the news lately, with the highest floods recorded for 20 years.
La Serenissima has always had a penchant for the wet and the melodramatic, but their recent experience is leaving them more than wet around the edges. While floods and Spring tides have always occurred, wetting the feet of the wealthy and raising the level in the tidal lagoon well above the comfortable; when it starts to get knee high and thigh high then even the most serene of Venetians must be getting their doublet and hose, if not their knickers, in a twist.
But how would we cope here in Wellington if a similar flood occurred?
Our city sits in a relatively well-restrained harbour, the great Eye of the Fish of Maui, with most of our central city streets sitting at about 2 to 2.5m above Mean Sea Level. Our tide in Wellington is typically just a little over 1m, meaning that at every high tide, every day, the sea level is perhaps only a metre or two below our feet. Open those manholes to the stormwater, and you may smell the salt, as some still drain to the sea. Luckily, we’ve separated out the sewerage lines to run to a giant holding tank under the Michael Fowler Centre’s carpark, so that if we get a storm surge, we can poo on assured that our harbour won’t get ‘soiled’.
That’s the way it used to be in Venice too: a metre above the waterline, except that they built on small islands in a muddy estuary, and then used timber piles to pile their brick and stone creations on. Over the years the timber has been decaying, and the city’s artesian wells have helped lower the city closer to the water – never mind global warming, Venice is in serious trouble regardless.
Venice has a similar tidal range to Wellington – slightly smaller. When they get tides over 1.1m, they start to get a bit damp around the edges, and so to change this, a project called MOSE has been introduced, and is due to be complete by 2011. Modulo Sperimentale Elettromeccanico is a project to install 79 inflatable metal barriers at the many entry points to the lagoon, which will be inflated to block water on exceptionally high tides. Venice’s highest tide ever was 1.94m in 1966: the tides this week must have been getting close to that record.
We get high tides here in Wellington too, but we’re built on rock (west of Lambton Quay) and reclaimed soil (but luckily, not timber piles). We’re unlikely to ever try to block off the entrance to the harbour with a giant inflatable bag – aside from the cost, size and the silliness factor, there’s bound to be a sharp rock or a taniwha that would puncture the balloon – and besides, water can just gurgle in over Kilburnie and Rongotai, which are so close to the high water mark that a mere couple of hundred years ago they were probably just tidal feeding grounds for Godwits. If a tsunami hits Wellington, as depicted in that tv doco a short while ago, K & R will be the first areas to succumb to the sea.
The most exciting place to see the big tides that we do sometimes get, is of course down on the waterfront, either in Athfield’s cutout wharf slot at Taranaki, or in the new Kumutoto stream mouth further north (where at a spring tide the water laps over the lowest platform). Rumour has it that the giant culvert exiting into the harbour here actually carries the remains of the Kumutoto Stream, which runs down from Woodward St. If you’re game enough, at high tide it is full to the top: you could swim all the way to Lambton Quay.