Bell Rd drainage system re-think

Papamoa dairy farmers are calling for a re-think on the drainage design for the area after two weeks of flooding that has forced them to dry off their herds early and lose about a month's worth of production.

Two cyclones in two weeks left the Bell Rd farms underwater, with drainage relying on pumps.

“Over 30 years we've been milking cows here and its gets flooded, but not this bad,” says farmer Graham Thompson. “The last flood was worse than this one last week, but this is getting up there.

“It's got worse since the Tauranga Eastern Link highway went through and all of the subdivisions up there. There's more water coming down a lot quicker.

“A lot of computer modelling has been done about all this and I think its miles out.”

An issue is the TEL culvert over the Bell Rd drain. The farmers see the culvert as a choke point that backs up the water in the drain, which then floods neighbouring farmland.

“We told the TEL people three years ago we were not happy with that culvert down the road. And nothing's been done since then. It's probably only 3.5m wide, it wants to be double that,” says Graham.

“The drain's this wide and culvert's only this wide. You don't have to be an expert to figure out something's not right there.”

David Hurst has been milking cows in Bell Rd for 30 years. He says here were similar flood levels in 2005.

“But the real issue is the whole bottom end of the scheme is under-designed. TEL has had a huge effect on it and the water just can't get away, as you can see.

“A huge amount of the water now comes from the developments at Papamoa East, you know yourself what's happening along there.

“Pasture will absorb one inch of rainwater an hour, bush will absorb four inches, and concrete nothing. “Now there is more concrete than there is pasture and bush.”

They need bigger pump systems and more of them, says David. His pasture has been underwater for 10 days.

“You couldn't avoid the rainfall, but you could improve the drainage system,” says David, who reckons regional council staff who brought in extra pumps are doing a “marvellous job”.

BOPRC's works co-ordinator for the Bell Rd pump station Arty Rangihika says two cyclones in two weeks is too much for the pump station.

“It's just the rainfall we've had,” says Arty. “Debbie was a cyclone so, we got that in one night. If it's over a week it's fine, but to have something happen like with the last two – Debbie and Cook – it's just too much to handle.”

All of the flood waters and all of the run off from the TEL is pumped into the Kaituna River by the Bell Rd pumps. The two pumps each have a capacity of 1500 litres a second; with two of them operating that's three cubic metres of flood waters a second.

“They were predicting 300-400mm to fall from this last one Cook,” says Arty. “It actually got downgraded to 150mm. It's just hard to predict what you are going to get – 150mm we can handle, but 300mm you are starting to push it a bit.”

Flooding data used for answers

Recent Papamoa floods are providing the first opportunity in several year for engineers to calibrate the flood model for the changed landscape following the construction of the Tauranga Eastern Link, says New Zealand Transport Agency project manager Wayne Troughton.

The TEL stormwater and flood control measures were constructed according to the Bay of Plenty Regional Council's existing model. “We designed everything and over-designed it,” says Wayne.

Over time the regional council also repaired the stop banks, rebuilt the road and changed the drains, cleaned them out and made a whole lot of changes.

“The regional council initiated a new model that was more detailed, but they needed to calibrate the model against a storm,” says Wayne. “Since that 2014 event there haven't been any big storms to calibrate it against.”

The flood from recent cyclones Debbie and Cook mean the model can now be calibrated and engineers will be able to see where the issues are coming out.

 “We are working with them. Once we get the results from the model calibrated, see where the bottlenecks are, then there will be some discussions about what needs to be looked at going forward,” says Wayne.

“Until we get that data back that we have been waiting for, for quite some time, we're really flying blind.”