Middlemarch susceptible to flash flooding — ORC
WAITAKI
THE Middlemarch landscape is susceptible to flash flooding after high intensity heavy rain, the Otago Regional Council says.
The flooding at Middlemarch to start the year was not caused by the Taieri River overtopping, the council said.
It was due to heavy rain falling on short, steep tributaries of the Taieri River that flow from the Rock and Pillar range, it said.
Council natural hazards manager JeanLuc Payan said the town was situated at the point a steep gully merged on to the flatter valley floor.
Flooding on these areas, known as alluvial fans, could lead to ‘‘debris flood events’’ when the tributaries carrying water to the river were overwhelmed, Mr Payan said.
‘‘These tributaries are prone to blockage and debris, which can cause the streams to break out of their existing channels at times of higher flow,’’ he said.
From the start of the year until the rain finally let up on January 3, more than 100mm fell on Middlemarch.
Snow Ave, near the centre of town, was under water on the Saturday and then, as the floodwater subsided, another bout of heavy rain inundated buildings a second time on Sunday.
It was the second major flood since November 2018 when one disrupted life in the town of about 200 people that lies within Dunedin city boundaries.
Otago Regional Council regulatory general manager Richard Saunders said the council completed its regular March Creek river management, vegetation maintenance and debris clearing work in November.
But there were no other flood protection works for the town, Mr Saunders said.
He said the council expected to be involved in coming discussions with the community and the Dunedin City Council about how to address the flooding.
In a recent media statement, the regional council noted that Middlemarch was not the only
Otago town in this type of landscape.
Like Middlemarch, Roxburgh was near a steep, narrow catchment and alluvial fan.
Roxburgh has been hit in recent years by severe flooding, including a flash flood in November 2017 which caused major landslips and affected all three routes into the town.
THE Waitaki District Council is expecting at least a sixfigure sum for its flood recovery expenses, Waitaki Mayor Gary Kircher says.
The district was hampered by last week’s rainfall, and while the council had not received any official figures, it would be ‘‘certainly quite appreciable’’.
‘‘It will be into the six figures — NZTA’s cost alone is probably $200,000 or $300,000,’’ Mr Kircher said.
Several of the region’s roads were closed last week, including State Highway 83 after Parsons Creek Bridge, near Otematata, partly collapsed due to flood damage.
It had since reopened. Mr Kircher praised contractors and the NZ Transport Agency’s quick response to the situation.
‘‘It was really pleasing to see both there, and on State Highway 82 on the Waimate side, where there was also that road closed for a bit, that they got on and got those problems sorted, and the roads were reopened in time for the [last] weekend.’’
Two sections of the Alps 2 Ocean Cycle Trail, near Kurow and Duntroon, were badly affected, and had diversions or extreme care warnings in place.
‘‘Some of the places the trail goes through are through floodplains and while they may be all right 98% of the time, occasionally you have these events where it means they need a bit of fixing afterwards.’’
The Duntroon Domain was closed, as the building and campsites had been inundated during the downpour.
‘‘It’s been a major cleanup, but it’s a community that really pulls together — when talking about recovery you get some really proactive communities.’’
All boil and conservewater notices were lifted from Tokarahi, Awamoko, Otematata and Duntroon water supplies yesterday.
The council was reviewing its district plan and would update its information regarding flooding, planning manager Hamish Barrell said.
‘‘The rainfall event over New Year that brought misery to the holiday season has again reminded us of how important it is that we plan ahead, thereby reducing risk to the community and emergency services,’’ Mr Barrell said.