500-year flood plan for new town

A 12,500-person town planned for Papamoa East will be protected by a stormwater system designed to cope with a flood only expected to hit every 500 years.
New investigations ordered as a result of the storm which devastated Tauranga in May 2005, together with the increasingly erratic climate generated by global warming, has culminated in a hugely upgraded stormwater deal for the planned "live, work and play" town of Wairakei.
Details of the new flood control system were outlined to a planning hearing yesterday to rezone 370ha of flat coastal farmlands. The rezoning is being considered in tandem with a comprehensive stormwater control system for the whole of Papamoa.
Ironically, the flood threat faced by Wairakei from a very big storm would be reduced once the 30,000 satellite city between Wairakei and the Kaituna River was on the map.
Construction of the stage 2 Te Tumu city development was planned to start about 2021.
This future city planned to have new stormwater outlets either into the river or the sea. But in the meantime the joint commissioners hearing panel must rule on a system to protect Wairakei because construction could start as early as next year.
The town will eventually fill a big swathe of farmland east of Parton Rd as far as the existing end of suburban Papamoa. Apart from the man-made beach outfalls at Harrison's Cut and Grant Place, Papamoa's existing stormwater system relied on soakage.
The near dead flat Wairakei Stream, formerly called the Papamoa Main Drain, eventually just soaked away and did not have a natural outlet.
Three solutions had been put up to deal with a worst case weather scenario for a fully developed Wairakei town _ minus a new river or coastal outlet.
Options were outlined by former council stormwater manager and now consultant, John Palmer.
They were to increase the capacity of the Harrison's Cut beach outlet so less stormwater flowed east, increase the capacity of culverts, and create three overland flow paths in which flood waters topped the planned Eastern Motorway and entered the topographically lower Bell Rd catchment.
The flowpaths were safety valves designed for storms more extreme than a 100-year event. Global warming meant 50-year floods might now hit every 10-15 years.
Smaller storms would be handled by a series of stormwater retention ponds surrounded by reserves, linked by overland flowpaths to discharge into Wairakei Stream.
Minor roads and reserves would also serve as ponding areas _ similar to what happened to Papamoa in the storm of 2005.
Meanwhile, Tauranga City Council has decided to act separately to remedy complaints from farmers on the Te Tumu block that their paddocks had become noticeably more boggy from development. A pump station is planned for Wairakei Stream to divert water into a planned stormwater pond which will empty into the Bell Rd catchment.