Post-earthquake flood woes return in Christchurch


I do not remember though the Heathcote River or the Avon River overflowing as frequently as they seem to now. Significant overflow events have occurred in virtually every year since the earthquakes. Some of the were on relatively insignificant rainfall events such as one in early 2012 which occurred on the back of 90 minutes heavy rain. More serious (considerably more serious to be honest)events occurred in August 2012, June 2013 during a significant winter storm, significant storms in March and April 2014, and finally two storms just week apart in July 2017 (the last one being the one on Friday-Saturday just gone).

I visited both rivers on Saturday to see for myself the extent of the flooding. Both had road closures in place, or would shortly after I passed. The photos in this article are ones I took on a brief drive on Saturday morning to see how bad the flooding was. I deliberately avoided the lower part of both rivers and the estuary because significant road closures were in force at this point and I thought it would just hold up essential traffic.

Avon River (Wairarapa Terrace), R. Glennie

To be fair, it is worse in the lower reaches of both rivers near the estuary, where some parts have dropped by up to 50 centimetres. This means even a big king tide can now cause flooding, without any rain falling. So it was unfortunate then that 2.7m tide, which is the top of the tidal range around Christchurch, was due in Lyttelton at 1515 hours on

Avon River looking upstream, R. Glennie

Saturday.

But we have known about the flood hazard since the quakes. It existed before the 2010-11 earthquake sequence began, but was then largely understood to originate from the Waimakariri River, on whose flood plain the city is located. We have invested millions in reducing the flood risk around Flockton (Floodton)Basin, a low lying area east of the Palms Shopping Centre, where land subsided around the upper reaches of the tidal zone and does not drain easily as a result.

Heathcote River at Tennyson Street, R. Glennie

But it is not just the flooding that is the problem. Following the flooding is sometimes even more stressful as the insurance claims are sorted out, the houses cleaned out and the sanitation made safe again. Often carpet has to be ripped up and electronics replaced.

Heathcote River at Colombo Street, R. Glennie

Perhaps it is time to face an uncomfortable truth. The flood risk has significantly increased in these low lying areas. It is not going to decrease unless dwellings are either raised or moved. Yes it will cause a lot of stress for locals, but one only has to look at the number of times significant flood events have occurred since the earthquakes tells that perhaps these streets should not be occupied, or they can be occupied, but the insurance must be significantly more rigorous.  But one thing is for certain: the flood hazard has changed and we need to adjust accordingly.

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