Safeguarding Canterbury from a major flood – The Waimakariri Flood Protection Project

September 13, 2019 posted by

It’s a beautiful space, it’s a beautiful river The Waimakariri is particularly steep by New Zealand
standards, it carries a lot of gravel, it’s very high energy. In the upper reaches it’s quite braided. The Waimakariri is a particularly aggressive river. In the early days of European settlement, in
the 1850s, it was recognised by the settlers that they couldn’t in fact develop sustainable communities
without managing the river and managing that flood risk. When you look at what the Waimakariri River
has historically done it is a major threat. There’s photos of the floods in the late 1800s, where there’s
2.5 to 3 metres of water flowing through Cathedral Square. So, that shows you just the power of the river
and the damage it could do to Christchurch. We’re standing on the primary stopbank at Englebrechts.
This is ground zero in the river engineering sense. The river broke out through the stopbank in 1957 and
caused flooding in Lower Coutts Island and through the Belfast area, right out through to Spencerville. The project’s been subject to quite
rigorous project management. A huge amount of planning, of working out
where water could flow in a given event and making sure that the primary stopbanks
that were in place were up to scratch would perform, how they would perform and
then providing a secondary flow path and secondary stopbanks in behind them to
really provide a belt and braces approach. Working out from hydraulic modelling how high
flood levels get and where floodwaters go and working out how we contain those floodwaters. It’s a very clever piece of engineering; very clever piece of
thinking about how to manage a massive beautiful braided river. In terms of the primary stopbank
we had to upgrade 35km. We had to construct 25km of secondary stopbank. In terms of rock armouring we constructed
rock riverbank protection and that involved 260,000 tonnes of quarried rock. The primary system is designed for a
flood event of 5,500 cumecs of water; that’s a lot of water. The secondary system has been
designed for 6,500 cumecs of water. 1 cubic metre is of course a tonne of water so that’s
equivalent to the weight of a Honda Civic. So 5,500 cubic metres per second, is equivalent
to 5,500 Honda Civics going past. We have confidence that what
we’ve designed is going to work. The budget for the project was $40 million, it was to be
rolled out over ten years, kicking off in 2010. We’re on target in terms of budget and it’s
a very pleasing situation for us to be in. I also have to pay credit not only to the
project managers but also to the contractors who’ve been involved all the way through. Without a flood protection system in place there’s at
least $8 billion of asset at risk on the flood plain. It’s about looking after our backyard; turning
a risk in our backyard to an asset, And now we’ve seen through this project we’ve been
able to accelerate the development of those areas and create quite unique recreational assets, and biodiversity
assets for the people of greater of Christchurch. It’s the wildness of the river, that makes it so
attractive to the local community. On a nice day I love it.
It’ll terrify me when it’s in flood. There are always likely to be challenges in the future there might be the superflood, or there might be
what climate change brings us in decades to come, but what we know is that we’ve created an asset here that is a legacy, a foundation legacy for the future.

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