Quake-built spit best defence
Christchurch's best protection from a highly damaging tsunami may be the next major Alpine Fault earthquake.
Scientists researching the Christchurch area's palaeo-tsunamis - those before the written human record - say the Brighton spit is the best safeguard for the city against tsunamis.
But in one of those strange twists of nature, they say the spit will start shrinking unless it is soon replenished by millions of tonnes of sediment released by a major quake on the Alpine Fault.
A diminishing spit will expose Christchurch more directly to any tsunamis generated by great quakes of more than magnitude 8 along the western coast of South America.
In a paper in the latest edition of Quaternary Science Reviews, James Goff and Catherine Chague-Goff, of the University of New South Wales' Australia-Pacific Tsunami Research Centre, discuss seven tsunami events that affected the Canterbury coast to varying extents over about the last 6500 years.
The most recent, in 1604, was generated by a magnitude 8.5 to 9.0 quake in northern Chile.
Goff said the relative exposure to these tsunamis of the land on which Christchurch was later built had fluctuated over time.
"This is most notable over the past 2000 years or so since the Brighton spit extended south to close off the bay, then retreated and formed again, causing the Avon-Heathcote Estuary to change from open to closed bay and then from open bay to estuary.
"The relative extent of the tsunami hazard is therefore controlled to a certain extent by the presence or absence of the spit, which is in turn governed by interactions between seismic activity and fluvial processes."
A magnitude 8 quake on the Alpine Fault would cause massive amounts of rock and fine sediments to fall into the upper catchment of the Waimakariri River, the paper said.
"The fine sediment, sand, is rapidly transported to the coast, with longshore drift adding to the length, width and height of the. . . spit.
"It is now nearly 300 years since the last large Alpine Fault earthquake (1717) and as such the relative supply of fine material is starting to slow, which will in turn cause the spit to diminish . . . thus exposing the shores of the Estuary to larger tsunamis.
"In order to maintain the sand volume of the spit and protect the city from future tsunamis, another large Alpine Fault earthquake is required."
Goff told The Press the protection from a bolstered spit was a "potential silver lining on the disaster" of such a major quake.
Christchurch did not appear to have a "huge problem" with tsunamis, but Lyttelton Harbour and Banks Peninsula were more of a concern.
People tended to focus on the quakes, landslides or tsunamis as distinct events, but there were knock-on effects that could arise years later.
"When the next big one happens, and it could be in an hour's time or in years, it is going to be huge and everyone will be associated with the hell of that, the immediate after-effects, and nobody is going to think about where that sediment is going.
"We'll be halfway through clearing up the mess and there'll be a lot of sand coming down the spit and going in through the front door.
"We wondered why palaeo-tsunami weren't getting through into the most exposed parts of the city. [But] when that extra lump of sand comes down to the coast and is delivered to the spit, it will get longer and wider and higher."
The spit could grow long enough to close off the Estuary and turn it into a lagoon, he said.
QUAKE MAY FORCE WAIMAK INTO CITY
Sand from an Alpine Fault earthquake may boost the Brighton spit and protect Christchurch from tsunamis, but a glut of alpine boulders could eventually send the Waimakariri River flowing through the city.
In the Quarterly Science Reviews paper, co-author James Goff says a large Alpine Fault quake could have grim repercussions for Christchurch.
Massive rocks and other coarse sediment sliding into the upper Waimakariri catchment would ultimately cause the river to avulse, or change its channel.
"The Waimakariri currently flows out to the sea at the northern extent of its alluvial fan, which extends from Kaiapoi in the north to Lake Ellesmere in the south," Goff said in the paper.
"Avulsion will divert the river south either through Christchurch city, as it did around 600 years before present, or further south into Lake Ellesmere, or both.
"This north-south-north migration across its alluvial fan is a normal process and can be charted over several millennia and has fundamental implications for the city beyond a variable exposure to tsunami inundation."
Goff told The Press there was archaeological evidence that when Maori wanted to go inland from the coast they went to Lake Ellesmere and travelled up the Waimakariri from there.
"In say 50 to 200 years after the quake, all hell could start breaking loose on the [Canterbury] plains and along the coast. When those coarser gravels come down out of the Southern Alps and get on to the plains, there'll be so much sediment coming out. I doubt we'll have the capability of moving enough gravel away fast enough," Goff said.
"The Waimak is on the northern part of its fan, so it has to go south.
"It doesn't take a rocket scientist to figure out what's south of where it is now. There's a slightly naive sense that we can control mother nature," he said.