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Cyclones affecting New Zealand’s Highways

  • Wednesday April 19, 2023

By Dilip Datta CEng FIHE
IHE Fellow & Senior Engineer – Transportation, Blue Barn Consulting Engineers

It has been a rough start to 2023 for New Zealand. Two cyclones and a major rain event have left the roads in a mess. We are mopping up (quite literally in some cases) after the latest one to hit, Cyclone Gabrielle.

I hadn’t seen a cyclone before and the first one was not that bad (Cyclone Hale). The area that got the worst rain and wind was Hawkes Bay, but Auckland and the Northland (all areas north of Auckland) had hundreds of millimetres of rain during the cyclones. They say we Aucklanders have received six months’ worth of rain in two months.

For some it’s good as work will pick up, but for farmers and people who have lost a house, it’s really terrible. A lot of the less affluent were not even insured. A lot of farmers can’t even obtain insurance for crops, even if they had gone searching for it before these events.

Given that the roads were so badly affected by slips and dropouts (even on the state highways), there is a lot of head scratching going on in government – can these roads all be upgraded to survive the next big weather event? Is there even enough time to get them repaired before the next weather event? Where is the money going to come from?

Cutting back on safety projects seems to be one answer! And a lot of nice to have, vanity projects will be kicked into the long grass.

I am sure that the sequence of events didn’t help as we had three major ones in quick succession, but we do now need to seriously revise what a 1:100 year storm event looks like. Back to the drawing board for many stormwater modellers I guess.

With roads on steep hillsides, water rushing over the road undermines the ground of the hill face supporting the road and erodes the ground under the road. Cracks form and eventually the road itself slips and the seal can no longer protect the base and subbase. It washes away under the action of flowing water and debris and the highway is gone.

I counted 11 sections of state highway that are currently closed due to flooding or slip damage and bridges were lost. For a while some townships were cut off and the residents could only get out by boat or helicopter, demonstrating that even the road network can end up providing no redundancy in a major flood.

Many more local roads will have been damaged and will be unusable, as local roads are often built to a lower standard (narrower and a lower standard of drainage).


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