Reduced tidal flow after quakes

FLOW REDUCTION: The flow of the Avon-Heathcote Estuary has been reduced due to earthquake-related lifting.
FLOW REDUCTION: The flow of the Avon-Heathcote Estuary has been reduced due to earthquake-related lifting.

A million cubic metres of water – enough to fill 400 Olympic-sized swimming pools – is being kept from flowing in and out of the Avon-Heathcote Estuary because of earthquake-related lifting.

The reduction in flow is expected to spark changes at the Estuary's tidal inlet and the surrounding beaches.

The Port Hills Fault, responsible for the February 22 quake, runs across the Estuary and pushed up the southern section of the Estuary bed by up to half a metre, while parts of the northern section sank by up to half a metre.

However, the elevation changes did not even out.

Click here to view a graphic of the estuary flow changes.

A report by the National Institute of Water and Atmospheric Research (Niwa) said that overall the bed had risen by 14 centimetres, shrinking the area of the Estuary covered by water by about 18 per cent.

About 50 hectares more – an area greater than twice the size of the Christchurch Botanic Gardens – of the bed is now exposed at mid-tide.

Niwa river and coastal expert Murray Hicks said the estimated loss to the "tidal prism" – the volume of water exchanged between the Estuary and the ocean with every tide – was about one million cubic metres, or about one-sixth of its original flow.

He said the Estuary's higher sections now stayed underwater for a shorter time at high tide. "Indeed, some of them aren't even underwater at high tide any more."

Hicks said that because of the diminished water flow, the width of the tidal inlet at Shag Rock was expected to shrink by 15-20 per cent.

The changes would also reduce the size of offshore and inner-Estuary sand mounds, known as tidal deltas, which could eventually enlarge beaches at Southshore and Sumner.

Niwa estimated up to 40 per cent of the Estuary bed was covered by liquefaction mounds after February 22.

Professor David Schiel, who heads Canterbury University's marine ecology research group, said many little "critters" and cockles living in the Estuary's sediments were smothered when the mounds emerged like volcanoes. He said many of those animals were sensitive to nutrient flows and other "gunk" from raw sewage pumped into the rivers and the Estuary. Contaminated shellfish found in the Estuary prompted a public health warning this month, because of "extremely high levels of norovirus".

Hicks' report pulled together three previous surveys of the Estuary, which used aircraft-mounted laser-scanning equipment as well as ground and boat surveys using GPS. These were compared with surveys taken after the February quake.

Hicks said difficulties in comparing surveys meant it was not possible to reliably calculate elevation changes on 11 per cent of the Estuary.

The city's rivers, which snake through the city and flow into the Estuary, also took a battering in the quakes, with banks collapsing and slumping in places, as well as elevation changes in the channels.

The Press