Close watch kept on integrity of stopbanks

DAVID WILLIAMS

Last updated 05:00 31/03/2011

 

Earthquake-weakened floodbanks on the Waimakariri and Kaiapoi rivers might fail during a major flood, engineers fear.

Environment Canterbury (ECan) has commissioned a geotechnical investigation on stopbanks in Kaiapoi and near the northeastern Christchurch suburbs of Brooklands and Spencerville, to determine the risk of failure.

In a report be considered today by Government-appointed ECan commissioners, regional engineer Ross Vessey said "intensive" monitoring of floodbanks would be required "over a number of years".

"There may be unidentified and untreated earthquake cracks in the stopbanks or their foundations [after February's quake], or earthquake liquefaction may have increased the risk of `piping' of sandy stopbank foundations, which may cause stopbank failure during [a] major flood."

There should be ongoing monitoring during floods, to identify and repair problem areas, including checking stopbank foundations, the report said.

Vessey told The Press residents should not be unduly alarmed but when it came to floods "it's all a game of chance".

Big floods usually happened in summer, so the risk heading into winter was decreasing, he said.

ECan had brought in Riley Consultants to determine the risk of stopbank failure after the two quakes.

Its geotechnical report will take months to complete and the resulting retrofit programme could take two years to implement.

Retrofitting costs had not been factored into the latest quake repair estimates.

Vessey's report said damage to stopbanks from February's quake was less severe than the 7.1 magnitude September shake, and was generally restricted to previously affected areas. Repairs after the February quake are estimated to cost ECan a further $2 million, on top of the $3m already spent.

The report said Kaiapoi's walkway had "disguised" significant cracking. The weakened banks could withstand a 15-year flood, at a flow of 2500 cubic metres of water per second (cumecs).

When repair work was finished in June, the flood capacity would return to the original 4700 cumecs design standard.

Waimakariri District Mayor David Ayers said ECan was prudent to check the structural integrity of the stopbanks and residents should be reassured the organisation was taking the matter seriously.

"Their comment that the same areas were affected as last time fits in with our observations on our streets and people's properties."

Vessey's report said stopbank repairs would be delayed in some areas so work can be co-ordinated with dams being built to prevent liquefaction and lateral spreading.