Council assessing flood damage after rain

The weather bomb of last weekend has left a cleanup job for the Waitaki District Council.

As the fine, but persistent, rain intensified on October 3, surface flooding and some slips on district roads emerged.

It also caused wastewater to overflow in some residential areas, including northern Oamaru.

Some council water supplies were also affected in Corriedale and Windsor.

The Oamaru Airport weather station recorded 118.6mm between Wednesday and Friday.

But just a few kilometres south, Oamaru had 132.6mm.

It was worse for Palmerston.

Its nearest weather station, at Waikouaiti, recorded 152mm during the same time period.

The average October rainfall in Waitaki is about 50mm.

That meant the district had received well over two months of rain in around 72 hours, Waitaki District Council emergency co-ordinator and Natural and Built Environment director Roger Cook said.

"It also shows that the southern part of the district saw heavier rainfall that the northern areas," he said.

That explained some of the issues the rain had caused in Palmerston and Waihemo.

Council contractors had this week been assessing the damage in its wake.

"Our focus is in the south of the district, which saw heavier rainfall than other areas," he said.

Work to clear slips, remove slash, assess damage to roads and paths and clear mud and other debris was ongoing.

The likely cost of that had yet to emerge, Mr Cook said.

"The first priority is to clear out and clean up affected areas."

Palmerston was particularly affected.

The council had last cleared its culverts and storm drains in Waihemo three months ago, Mr Cook said.

But from the inspections and work done this week, it appeared a lot of material had jammed culverts after washing off private properties.

That had caused blockages in the stormwater system on Friday in particular.

"That’s being cleared now," he said.

The council was grateful to its contractors who had put in long hours.

The efforts of Otago Civil Defence Emergency Management, Fire and Emergency New Zealand and New Zealand Police had been outstanding.

Waihemo residents in Palmerston and Hampden "showed resilience and community spirit" by helping neighbours and giving stranded motorists a place to stay last Friday.

Waitaki District assets group manager Paul Hope said the rain had also highlighted issues in the council wastewater network.

That included stormwater entering the sewerage system from numerous sources at the North End of Oamaru.

Downpipes connected to wastewater plumbing and gully traps and cracks in wastewater pipes were among the sources.

"The wastewater network in the North End catchment is not designed to take such high volumes and consequently fills up and overflows at the lowest points, in this case from private gully traps," Mr Hope said.

"When this happens, there is very little council can do until the rain eases and the network stops surcharging."

The council was committed to fixing the issue and had two projects under way, Mr Hope said.

That included an inflow and infiltration project to identify potential stormwater sources into the wastewater network, both public and private.

Smoke testing by the council on South Hill had already identified "numerous" issues and that testing would be extended.

"Council will work with private landowners to rectify the identified issues, as well as fixing public issues.

"Over time, as the fixes are completed, the volume of stormwater in the wastewater network should reduce."

The second aspect was to study the capacity of Oamaru’s sewerage network.

That would help to plan for the longer term and identify areas below par.

"Upgrade projects will follow such as larger pipes and potentially new pump stations."

— Additional reporting: Jules Chin