Some Thoughts About Drainage

Thursday’s storm is now behind us although unfortunately some will still be mopping up and repairing damage.  It served as yet another proof of how bad weather coming from the south-east can be very bad: it causes water levels to rise in the Ashley and in the tributaries of the Kaiapoi.  As someone who witnessed, from Waikuku Beach, the 1953 flood in the Ashley, I know how high the Ashley can get. (I was five at the time – pretty exciting stuff for a small boy!)

Preliminary assessment suggests that the worst six hours of the storm was about the level of one in 125 years’ frequency.

In preparing for this kind of eventality, we have to ask ourselves: what do we want to pay for and what level of risk are we prepared to stand?  We can protect ourselves with stop-banks, building requirements, etc from various levels of flooding, but how far do we want to go?  The more we protect ourselves, the more it can cost.

On the other hand, if experience tells us that problems are occurring frequently, something obviously needs to be done.


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