Staged response to Te Kūiti flooding

Waitomo Mayor John Robertson has told Te Kūiti flood victims there are some things the council can do immediately, but other changes will take longer.

Immediate measures included constructing additional cess pits in places where there were not enough, and work on these began after the January flood.

Such matters as the intensity of housing and flood zones were addressed in the district plan. The last one of these came out 15 years ago and a new one was in the final year of being upgraded.

“This (plan) tries to work out and guides us where flood prone areas are and if somebody wanted to buy a house, they could go to it for details.

“There are things the council can do, and you should (reasonably) expect us to do right ... What we don’t have control over is what is happening in the weather.  

“We could talk about climate change world-wide.”

John said unexpected weather events this year were part of this.

In January, Te Kūiti had a state of emergency, following about 170 millimetres of rain in 24 hours, that fell on already wet ground.

The river came right up to a few mm short of the level of the 1958 flood (the largest one known). That event was a lot of rain over 24 hours and the stormwater had to cope with trying to get out over a flooded river.

“But the event we had on Sunday was 45mm of rain in one hour and a quarter. Now that’s huge.

“What happens if the rain falls on the hills and it came down in torrents and quickly overwhelmed some the underground systems could not cope with that amount of water,” John said.

“They were two very different events . . . what we don’t know is, what is the climate really doing?

“Is this going to continue or is it going to get worse?

“We’re trying to understand how to build a stormwater network that can cope with this.”

Council’s general manager for infrastructure, Shyamal Ram, outlined short, medium and long-term measures the council was taking to mitigate the flooding risk in Te Kūiti:

He said with so much surface runoff it did not matter how big the pipes were, they would not cope.  

Even a system designed for a one-in-100 years storm event would not have coped with the intensity of the rain on Sunday.

n Short term (from one to six months) the council would begin to install more cess pits in some streets that did not have enough of these. These would be larger than before with grills less prone to getting blocked by debris.

Double cess pits would be installed at seven locations, others would be upgraded. Wing walls and weirs would also be installed to guide water to existing drains.

Manholes in low lying areas would be replaced with “scruffy domes”, less prone to being blocked by debris.  

Council staff would continue to take video the existing pipes to see where maintenance was needed.  

Shyamal appealed for people to clear rains and cess pits on their own properties when they saw they were blocked.

n Medium term (six to 24 months) A lot of water came from the hills and needed to be intercepted.

The aim would be to delay how long water that fell on the hills took to end up in the river.

This would give the network more of a chance to work. To do this detention ponds would be constructed at critical points.

These would be dry most of the time but “when the rain really falls” they would act like holding tanks, reducing the volume of water flowing down from the hills.

Enough detention ponds built at strategic locations could mean the present pipe network would cope, without needing to be (entirely) replaced.

n Long term, council had begun to do a modelling exercise to upgrade its Geographical Information System, which showed all the networks.

This needed updating and the public could help by reporting pipes which were sometimes now shown on council plans.

This built up a better picture of what happened in storm events. The idea was to see where the bottle necks were, in one- in-two-year, one-in-50-year, and one-in-100-year flood events.

It took into account what had happened in the past.  

The model which, for example, could show that a certain street was flooding by half a metre of water, would enable better options analysis to help solve the issues by constructing the right pipes in the right locations.