Why there were no Tauranga floods

After years of dealing with city flooding following every heavy rain event, council staff have been slow to take the credit for the Tauranga result following last week's rain which cause floods on the Coromandel, Kawerau – but not Tauranga.

“It's important to note though that we didn't get the level of rainfall that Auckland and Waikato got,” says Tauranga City Council drainage services team leader Wally Potts.

“Our rain gauges show that we received about 220mm to 250mm over six days. This is about 20 per cent of our annual rainfall. However, it was fairly evenly spread and didn't have the peak intensities experienced in Auckland. They had 50mm in one hour whereas we had 100mm fall over 24 hours.”

Over March 16-18 there was 136mm of rain recorded by MetService at the Tauranga Airport; 48.6mm Thursday, 55.6mm on the Friday and 32.4mm on the Saturday.

The week before from March 7-11 there was 179.4mm of rain in Tauranga over four days with the peak 67.2mm on March 8.

Since floods in April 2013 Tauranga City Council has changed its approach to city flooding, buying and removing flood prone Matua houses and significantly investing in the Mount Maunganui stormwater projects.

The council also voted to spend $10 million a year for three years on stormwater projects and computer modelling of the city's catchments. The capital works programme in the 2015 2025 is continuing at a rate of $6m per year.

“We have made some gradual improvements that have reduced low level, but frequent flooding in some areas, so last week's rain didn't cause the sort of problems that we might have seen in previous years,” says Wally.

“One of the more significant areas of improvement is in Matua where we've built a new overland flow path. That area is performing well and letting excess water drain away safely without incident. In previous heavy rain events, a lot of water would have flowed straight across a couple of private properties. It is always satisfying to see good designs working.”

The city council has also done a lot of improvement work in Totara, Maru and Te Maire streets. A bigger capacity stormwater system was installed along The Mall when the Pilot Bay boardwalk was constructed during 2013. This made a difference to previous flooding hotspots like Commons Avenue, although Wally says Commons Avenue is still going to face difficulties in larger rain events.

“The Mount Maunganui area sits quite low so tides have a major influence on the ability of the stormwater system to perform well because some of the outlets are submerged at high tide.

“Quite often people will see our reserves filling with water and call it flooding, but it's actually the stormwater system doing its job by giving the water a safe place to go until it can disperse.

“Places like Carmichael Reserve and the reserves along the Wairakei Stream and even open drains at the Otumoetai golf course are designed this way.”

Leaf litter is a real problem for stormwater inlets, says Wally. All 200 or so problem areas, are cleaned prior to a storm event and again afterwards.

“Residents can do their bit to protect their property by clearing gutters as much as possible during heavy rain. It all helps to maintain a resilient system and community,” says Wally.

More stormwater work will be undertaken in the Nikau Crescent area and the council has completed minor drainage improvements in Leinster Ave. Papamoa is well designed for major events with the exception of a couple of the older areas that rely on soakage alone, says Wally.