In particular the East Taieri Drainage Scheme and Lower Clutha Flood Protection and Drainage schemes were under mounting pressure during the flooding across 4-6 Oct, where parts of inland but especially coastal Otago received anywhere from 60mm-210mm of rain over 48 hours, says ORC’s Engineering Manager Michelle Mifflin.

 “There’s still a lot of struggles and distress out there for some people. We’re continuing to work proactively with communities to ensure water drains away as quickly as possible, at this early stage of recovery,” she says.

Last week’s event was widespread across coastal Otago, including substantial flooding around the Pomahaka and Manuherikia Rivers, Kyeburn and Ranfurly and coastal Otago; Clutha district, Balclutha and Dunedin.

While happy in general with the how the flood banks withstood the deluge, Ms Mifflin says ORC is yet to fully assess and understand communities’ concerns, any damage and how schemes performed at a local scale.
While the flood schemes – of which there are more than 200km of ORC flood banks - held up, Ms Mifflin says there have been some reports of floodbank damage, which are still to be fully assessed.

She says ORC’s Engineering’s flood preparedness and response worked well through its existing planning framework, including intelligence captured through water monitoring data which provided live information for management to triage and guide priorities.

Engineering staff began working on flood scheme checks three days before the downpour hit in full and were continually assessing the rivers and flood assets on shifts 24/7 through to late-Sunday.  

“This information gleaned throughout will also transition into our flood repair programme where relevant,” she says.

She highlighted those communications with landowners and wider communities was both proactive and tactical; citing the example of decision-making around opening the Puerua River (a coastal Clutha River tributary), with diggers used to alleviate flooding build-up on land.

“Being able to talk to locals firsthand about local conditions and their concerns was invaluable,” she says.
On the Saturday, Ms Mifflin, ORC’s General Manager Science and Resilience, Tom Dyer, Rural Communities Minister Mark Patterson, Dairy NZ staff, and landowners took part in a community meeting at the Paretai Hall, attended by about 40 people.

“We were able hear their concerns and work directly with the community on solutions to alleviate the extenuating circumstances with flood waters, which included us discussing the risk and consequences around different options,” she says.

Contractors were subsequently given the green light to open a flow path through the Training Line structure Koau Mouth of the Clutha/Mata au River to release water from the Puerua and Barrata Rivers which could have caused more severe disruption around the immediate district.

Ms Mifflin says other coastal river mouths were all open and functioning with staff undertaking some works when able to do so during the event, such as clearing debris and silt removal.

She says the ORC Engineering team pre-prepared then deployed contractors throughout the event to provide 24/7 coverage of ORC’s critical infrastructure, including the numerous pumps required to move floodwaters.

The two portable pumps owned by ORC were operating at both Taieri and Clutha, plus there were additional pumps provided by the Dunedin City Council to assist with floodwaters around the Clutha district.

ORC’s General Manager Science and Resilience, Mr Dyer acknowledged it was a significant event.
A static Low in the Tasman Sea and static High in the Pacific Ocean essentially both stalled and funnelled warmer, moist tropical air down New Zealand’s east coast, principally across coastal Otago and some inland areas.

“This was a very tricky weather event, as its duration and expected rainfall was difficult to forecast. Factors such as snow melt and widespread soil saturation made predicting the effects of the rain uncertain in some catchments,” he says.

Mr Dyer says that in Dunedin while there was some widespread surface flooding and slips in hill suburbs, the ORC’s Leith Flood Protection Scheme was also a focus for ORC’s monitoring staff, although flows peaked at around 70 cumecs, well within the schemes capacity.

“We were pleased with how the Leith scheme operated, especially following earlier redevelopment works which had been completed since the mid-2015 flood event,” he says.

Background

ORC provides flood protection and land drainage for about 43,000 hectares of rural and urban land across Otago, with more than 200km of floodbanks, 14 pumping stations, 42 bridges, 369 culverts and various other infrastructure including weirs, walls, trees and groynes.

Link to Clutha District Council aerial video of South Otago coastal flooding: