South Island’s flooding nightmare

In early June, transport links on SH1 in the South Island were severely affected after extreme flooding in Canterbury resulted in the closure of the Ashburton River/Hakatere Bridge.

The bridge across the Ashburton River on SH1 – the South Island’s main link road – was closed on 1 June after drivers reported slumping on the deck at the Ashburton end. The damage was caused by a build-up of debris in the river around the piers immediately to the north. As flow dropped from 1500 cumecs to about 400 cumecs of water, debris got caught up in adjacent piers, and the water was directed to the pier that sustained the damage.

The NZ Transport Agency load-tested the bridge that evening using a crane and 14-tonne weights. NZTA system manager Pete Connors said based on monitoring data received, the damaged pier, which sunk about 15cm during the recent one-in-100-year floods, was stable, and the bridge was reopened to light traffic.

Load-testing for heavier traffic (trucks) was carried out on the evening of 2 June, with the NZTA taking heavy trucks across the bridge carrying up to 40 tonnes of load to measure deflections. On 3 June, the bridge was opened to more vehicles, including 50MAX and HPMV.

The heavier vehicles were given bridge access for the first week from 7am to 7pm – daytime hours – but any trucks with an overweight permit had to take the alternate routes on local roads to get south of Ashburton.

Connors said the pier testing, using heavily laden trucks, showed negligible movement, indicating that the pier had stabilised, and the structure was robust.

Monitoring and data analysis of the bridge during the following week showed it was safe to take trucks (up to 50MAX and HPMV). As from 10 June, it was reopened around the clock to all traffic, other than overweight permitted vehicles, with speed limit of 30kph in place.

More monitoring and data analysis were carried out on 16 and 17 June, during which time electronic monitoring equipment was installed to allow remote monitoring of the structure.

The initial closure of the bridge created huge disruption to supply chains, with lengthy detours in place for trucks. Heavy vehicles were directed via SH77 to Methven and onto the Inland Scenic Route 72 to get through the district.

Connors said many roads needed patching and repairs after being under water and damaged by debris for 48 hours, and there would be temporary speed limits in several places around the Canterbury region while crews undertook that work.

“There’s been some damage because the roads got quite soft with all-day torrential rain sitting on both sides of the road. And now, with the number one state highway going through Mayfield, they’re getting wrecked pretty quick. The roads in our county are shocking to start with, and like a lot of other people, we’ve got an issue with our roaduser charges not being spent in the right places. It’s a bit frustrating when you see all the projects going on – if we could spend our road-user charges where we actually earned them, I think we’d all be better off,” said Mayfield Transport dispatch manager Mike Farnell.

The NZTA is still working through its detailed repair methodologies for this bridge pier. The fix will likely involve scour-protection works, re-levelling and underpinning the pier. Connors estimated repairs could be completed in six to eight weeks. (Around the time you read this – Ed.)

“If we have more high river events, that could affect that timeline, but we are working as fast as we can to expedite this repair.”