Monday, 11 June 2012

The Christchurch Estuary-bed rises ~ earthquake aftermath

The Christchurch estuary is a large body of water.  It's very familiar to me as the home I grew up in overlooked it and its presence and contours were part of my life from my earliest years.  I'm familiar with its tides and moods as well as the shifting shapes of its sandbars, so the question of how it has been affected by the earthquakes has been an immediate concern.

These two photographs show it as I remember it - ever-changing!



The equation of earthquake disturbance is not as simple as a paddling pool, the water of which slops over the side if jolted: many creatures have their homes there, and a whole range of vegetation and tiny organisms complete the estuarine niche in which all are inter-dependent, so what affects one affects the whole habitat.  Substantial changes to the bed of the estuary and the burden of effluent dumped there have had far-reaching impacts.

Some very interesting articles have been published and I include links to these at the foot of this article.

Briefly, the earthquakes have caused the bed of the estuary to rise -  by an average of 14 centimetres overall, but diverse changes in level have resulted: southern parts have risen by up to half a metre whereas northern parts have dropped by as much as half a metre.  The net reduction of depth has resulted in a greatly reduced water capacity, as well as considerable loss of water coverage: the equivalent of about 50 hectares.

Liquefaction silt has smothered large areas of the bed resulting in the death of many creatures who lived there.  Liquefaction silt is not simply mud or sand but a silt which is both heavy and hard to shift and which sets very hard when exposed to the air, as much of the estuary is at low tide.  It doesn't simply melt or wash away.  The photograph linked to here shows this dramatically:
The other immediate effect of the earthquakes was the loss of the city's usual waste water and sewage infrastructure.  This resulted in a great mass of untreated effluent being poured directly into the city's waterways and estuary for an extended period of time as it had nowhere else to go.

Birds also flocked to the area - not a good combination.  The water passage between the estuary and sea is fast-flowing but narrow, meaning that the exchange of estuarine and fresh seawater is limited, so build-ups of waste water take time to clear.  Given the amount of stagnant water left standing it isn't surprising that in the early part of the year, which is summertime in this part of the world, disease set in and thousands of birds died, probably of avian botulism.  The overall loss is estimated to have been about 10%, with some species more badly affected than others.  Fortunately, the resident bird population is high and expected to recover its numbers over the next few years. 

The view from Sumner beach near Shag Rock across to New Brighton spit

I'm particularly interested in the sandbars, and hunted through old photographs to see what I had from years gone by.  Here is a series of three taken twenty years ago.  The panorama is from above Redcliffs and extends from left to right starting with the one at the top:




By way of comparison here are two taken from a nearby spot in March 2012.  The sandbars form an almost continuous mass right in to the edge of New Brighton, whereas the earlier photographs show a network of channels.  In one of the articles linked to below a resident remarked that the tide no longer comes in as close as it used to, which certainly looks to be the case from these images.
 


Earthquakes and disasters notwithstanding, the Estuary is always interesting and often beautiful.  I took these photos in March when travelling back to town one evening.  This one was taken from the foot of Balmoral Hill, looking across McCormack's Bay towards Mt Pleasant:


I stopped at the foot of Mt Pleasant to watch the birds flying about in clouds before roosting for the night:


The striking image below was also taken from the foot of Mt Pleasant within minutes of the one above, which shows how swiftly the mood of the sky and water can change!  Locals will recognise the nor' west sky!


The outlook for the recovery of the estuary is good but expected to take some years.  Those interested in reading further will find plenty to browse through in the articles linked to below.    

Links for further information:

A complete list of my earthquake articles can be found on the following page:

You can find my other articles about exploring the beach and its rock pools via the link below: 

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