Sea level rise dooms Christchurch… but not for awhile yet

The writing would seem to be on the wall, large parts of Christchurch city are under a very real threat of deluge at some point in the future.

That was pretty much the ‘take home’ from last night’s lecture by Dr Deirdre Hart, senior lecturer at the University of Canterbury’s Geography department.

“What if ? Wednesdays” is a series of free public lectures that happen throughout the year. It is necessary to book online, but anybody may attend, you don’t have to be a student.

There was hardly a seat to spare this time around. Initially when I went to book, the website gave me an automated message that it was already full to capacity. I sent a personal email, pleading to the organisers, and had my name put on a reserve list. Later I got an automated email indicating I had a confirmed seat. This particular talk was held in the C3 lecture hall, with a capacity of about 350 I estimate.

Dr Hart’s presentation went for some 55 minutes and included many powerpoint slides, and some occasional demonstrations thanks to Christopher Gomez and his trolley of glass beakers and coloured water. This was followed by a Question and Answer session with the audience lasting about 45 minutes.

Words and terms were explained clearly at the outset. “Relative” sea level for example, as the height of the worlds various oceans and seas does vary. The total difference is only about two metres (the Pacific Ocean as higher than the Atlantic and Southern Ocean near Antarctica, for example).

Dr Hart explained how glacial ice sheets actually weighed down the land masses and that when that ice melted some thousands of years ago after the last ice age, land started to spring back up again. It takes time though and continues even after the ice sheets have finished melting. It was for this reason that she said Scotland was rising higher. Unfortunately this is tipping England somewhat like a see-saw and London is headed downwards. This is likely to exacerbate future risk of flooding in the London area.

New Zealand unfortunately is still sinking. This is a great concern because 75% of New Zealanders live within 10 km of the coast.

NZ has already experienced 170mm of sea level rise in the last 100 years, but in Christchurch we are 190mm worse off, because the land has sunk by 20mm.

There are various estimates for sea level rise by the year 2100. The figures range from 300mm to 1 metre around the world. In NZ it is expected to be somewhere in the range of 700mm to 1 metre.

Ocean behaviour though runs on many overlapping cycles. Some play out over 3 or 4 years and others play out over hundreds.

EL NINO and LA NINA climate cycles affect water levels in the Pacific Ocean near NZ by about 500mm. Keeping in mind that our usual tides are only about 2 metres in height difference, clearly effects can be cumulative.

One effect that is very noticable in NZ is APOGEE and PERIGEE. This is caused by occurrences of the sun and moon lining up occasionally, which can cause mega-sized Spring tides.

Atmospheric weather plays a big part in sea levels too. For every 1 hectopascal drop in air pressure, the sea level rises by 10mm. On the 4th of March 2014 the whole the Canterbury area was experiencing a severe storm (waves off of Banks Peninsula are said to have been 12 metres high). The air pressure was low enough that the sea near Christchurch was 400mm higher than on the 4th of April, when the weather was more normal.

In the famous storm of 1968 (which caused the sinking of the interislander ferry “Wahine” while inside Wellington Harbour, with the loss of many lives) the extreme low pressure of the air caused the sea level to rise by about one metre.

In October 2012 the superstorm known as “Tropical Cyclone Sandy” (which had already cut a path through the tropical areas of the Caribbean) and as it was moving north and fading it made a direct hit on the United States’ north east area and New Jersey got absolutely clobbered. The air pressure was so low, and the “superstorm” measured some 1,100 kms across, that the sea level in the area rose by some 4.2 metres. Waves of 10 metres were recorded near the Jersey shore.

Christchurch isn’t alone in being at risk, Tokyo is also a Holocene Progredation Plain. One picture slide clearly showed the various coastlines over the last few thousand years. Not all that long ago, Riccarton was a seaside suburb, and if you went anywhere further East than that, you were in the drink.

The “Bruun” rule was discussed. Going by this formula, one metre of sea level rise would likely lead to a retreat of the shoreline by anywhere between 30 and 100 metres. Most likely something in the 50 to 70 metre range.

However Dr Hart reckoned that the Bruun rule (intended for sandy beaches) has often been trotted out when it is not appropriate, and therefore conclusions drawn are likely to be wrong.

In Central Canterbury we have the Canterbury Bight just South of Banks Peninsula being blasted by the full force of wind and waves from Antarctica, whereas in Pegasus Bay, Christchurch and Northwards, the large promontory of Banks’ Peninsula forms an effective shield.

Our river systems, (and here the Waimakiriri stands head and shoulders above everything else put together) provide a constant diet of fresh soil, effectively pumping sand, silt and other sediments into the sea, right at Christchurch city’s Northern margin. For us here at the moment, that is actually a good thing.

(As an aside, I’ll explain some background for readers who are not locals. The “Waimak” as it is known is one of Canterbury’s famous “braided rivers”. The braided rivers are restricted to the Eastern South Island of New Zealand. Similar braided rivers are found in just a few places elsewhere, notably North America and the Himalayas. These wide scruffy looking river systems were initiated by ice age glaciers grinding away at tall mountains (Mount Cook in South Canterbury is still 12,000 feet high nowadays) During Autumn and Winter the water flow can be low, with a mostly dry riverbed of stones and small trees and shrubs that can grow quickly and survive some seasonal flooding, either NZ native species or introduced poplars and willows. However periodically these rivers can flood BIG TIME. Melting mountain snows in Spring and Summer can add to high rainfall events that may happen in the headwaters along the edge of the Southern Alps. Over several thousand years the Waimakiriri River has changed it’s course several times, usually during a time of major flood. During the human timescale that Christchurch has been settled by Europeans, it just so happens that the Waimak is at the Northernmost point it has been. Christchurch city’s northern suburbs stretch right upto the stop-banks that were constructed decades ago. Previously the river’s run would have been right through Christchurch’s Southern suburbs. Whether a really big flood event could cause this river to again shift further South, absolutely obliterating much of Christchurch City in one fell swoop, I do not know. But it would seem to be a possibility… perhaps the subject of a future public lecture ?).

For now though, Christchurch benefits from the material being brought down in the river and deposited into the sea around the river-mouth. Material like this, has helped build up the South Brighton and Southshore spit for example. One slide showed that even since the 1950’s considerable extra material had been deposited, effectively increasing the ‘dry land’ area of the spit.

Generally speaking we’re getting more than enough material and in areas North of Banks Peninsula (including Christchurch beaches) we are actually ‘accreting’ and NOT ‘eroding’.

Worldwide though, it’s a serious issue, with about 70% of coasts eroding. The annual rate of erosion runs somewhere between 500mm to 1 metre.

Dr Hart repeated this quote “Sea level rise doesn’t create new hazards, it exacerbates existing ones.”

At was at this point that Dr Hart spoke about “Multihazards”. She referred to the recent flooding in March 2014 of the Flockton Cluster (Flockton Basin ?). The area os Christchurch’s inner suburbs around the Saint Albans and Richmond Park areas.

This area may have been a little lower lying than some of the surrounds, but it didn’t used to have major flooding problems. Suddenly it does. The cause is the 2011 Earthquakes which resulted in the land area dropping by some 400 to 500mm in those suburbs.

Dr Hart went on to say something like ” ‘Earthquake land subsidence’ effectively gives us a ‘laboratory’ to study projected sea level rise.” One reason for more chance of flooding, she explained, was because we are closer to the water level of our rivers.

In 1868 South America experienced an earthquake of 8.5 magnitude. If a similar sized ‘quake today led to a tsunami, then damage to much of Eastern Christchurch would be made worse if the tsunami arrived during a time of High Tide.

Sea level rise makes (earthquake caused) liquifaction even worse.

One slide came from a Carter from 1989 and was a flow chart showing six rectangles. It laid out how when something like a flood occurs, the local population put pressure on local civic leaders etc to install defensive systems… dykes, stop-banks, drainage and pumping stations etc. However then when those protections are in place, there is a tendency to make use of the now flood-protected area and allow even more development (housing etc). Then when a major event later happens that overwhelms those defences, the losses of people and property are made very much worse.

In the question time that followed, a few points were covered that may not have been addressed in the initial talk. I won’t quote all the questions and answers here, you can listen to them yourself.

The presentation was videoed by some dude who’s name I don’t know but he had a pretty flash camera and had a wireless receiver so was capturing the sound directly from Dr Hart’s microphone. His recording will ‘soon’ appear on an official university Youtube channel. However they didn’t record the Q&A session. Luckily I did, and it can be accessed via the following links.

Q&A part 1

Q&A part 2

Beaker demonstration


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