Waimakariri and Kaiapoi Rivers sustain minimal extra earthquake damage

Preliminary estimates for additional repairs required to the Waimakariri and Kaiapoi Rivers after the 22 February 2011 earthquake amount to $2 million.

Environment Canterbury’s Principal River Engineer Ian Heslop says this figure is in addition to the projected repair costs of $3 million following the September earthquake.

“The additional damage to the Waimakariri and Kaiapoi Rivers was mainly limited to downstream of the Northern Motorway and the reconstruction team is working towards a completion goal of mid June 2011.

“The capacity offered by the Waimakariri and Kaiapoi stopbanks in its current state is estimated to be at least 2,500 cumecs,” says Mr Heslop.

Two goals were set after the September earthquake to restore design flow of the two rivers. The first goal of providing flood capacity was achieved at the end of 2010 with 2,500 cumecs successfully contained by the stopbanks.

“The second goal was to restore the flood capacity to the original 4,700 cumec design flow by the end of March. This was almost achieved when the 22 February earthquake struck.”

Mr Heslop says the February earthquake produced less ground shaking than the September quake at the Waimakariri and Kaiapoi Rivers which meant the repair work completed from the previous earthquake performed well.

The previously repaired Courtenay Stream, Jury’s Wetland, and Kaiapoi Borough School culverts and floodgates, as well as the Black Street, Bowlers Street, and Feldwick Drain pump station outfall culverts all appear to have sustained no damage.

“Overall stopbank damage was less with mainly minor to moderate damage. Where repaired damage did occur it was typically minor to moderate cracking of batter slopes and the occasional cracking of the stopbank crest.

“Liquefaction has not produced any additional stopbank foundation cracking however it is possible that there may be unidentified and untreated earthquake cracks in the stopbanks or their foundations. Liquefaction may have increased the risk of the ‘piping’ of sandy stopbank foundations, which could cause stopbank failure during a major flood,” says Mr Heslop.

A geotechnical investigation of the stopbanks which have suffered liquefaction damage will be carried out.

Mr Heslop says the analysis will determine what sort of retro-fitting will be required to minimise the risk when a flood occurs.

“A retro-fitting programme could take up to two years and this work would be outside of the earthquake repair cost estimates.

“Intensive condition monitoring over a number of years and ongoing monitoring during flood events or high tides, in order to identify and repair remaining problem areas will be important as well,” he says.

Environment Canterbury is continuing to monitor catchment rainfalls and river flows, and implement flood warning and emergency response procedures, as appropriate.