Flood Protection Proves its Worth During Storm

An extensive flood protection system proved its worth when New Plymouth’s CBD escaped serious flooding during the weekend’s storm.

The city’s three flood detention dams and the Bonithon Avenue diversion tunnel were built after serious floods in the 1970s saw businesses awash and goods floating down the roads.

When Huatoki Plaza and Puke Ariki Landing were built, they were designed to flood in a managed way to take the pressure off Huatoki Stream during exceptionally high flows.

“Without those systems in place, the central city would certainly have flooded between Currie and Brougham streets during that storm,” says Manager Water and Wastes Mark Hall.

The three dams were up to half-full during the storm and the diversion tunnel at Bonithon Avenue – which diverted stormwater straight to the Tasman Sea – was “roaring” with flood water, says Mr Hall.

In Waitara, a stormwater pump that forces water from the town’s streets into the river was operating for the first time in 10 years.

“When the Waitara River rises above a certain point, that stormwater outlet is below the waterline,” says Mark Hall.

“The pump makes sure that stormwater still gets removed from the town’s streets even when the river is running high, so Waitara only experienced isolated pockets of surface flooding in low-lying areas due to the volume of rainfall.”

Of concern during the storm was the effect on the New Plymouth Wastewater Treatment Plant, which at one point was receiving a flow of more than 1,180L/s – much higher than the average 300L/s, or even the typical wet weather peaks of up to 700L/s.

“Our biggest worry was that the high inflow would wash away the bugs in the bioreactors that treat the wastewater, which would take us a month to build back up,” says Mr Hall.

“We had to divert some of the inflow to bypass the bioreactors during the highest flows. It meant no swimming or shellfish collection in the discharge area for a few days but we’ve avoided the bigger problem of the plant not operating properly for a month.”

ENDS

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