Review of council’s flood response
More than one millimetre of rain was falling per minute at the peak of the May 14 flooding in Porirua and Tawa, an official report reveals.
Porirua City Council and Wellington Water have produced an event summary for the flooding. It summarises what happened, the impact, response and levels of service in the Porirua basin.
The report says the highest intensity rainfall was on the eastern slopes of Colonial Knob, in Porirua’s CBD and the lower end of Porirua East. At the peak, between 11am and 11.30am, 33.6mm of rain fell in 30 minutes – a rate that would be expected to fall in a 1-in-100 year storm.
The next highest rainfall was recorded in Tawa, where 22mm fell in that 30-minute period, and 39mm in an hour.
Peter Bailey, Porirua City Council’s general manager of asset management and operations, said having so much rain fall in such a short time was always going to put massive pressure on the city’s stormwater network.
The council has a performance measure in its long-term plan to have no residential, commercial or public buildings affected by flooding, but the measure applies to 1-in-10 year storms.
‘‘ Just to have that much localised flooding, on that scale, made life difficult,’’ Bailey said.
‘‘What’s important for us now is to look at the things our staff did and what provisions we can make in the future to limit the damage.’’
Making a curb in Rangituhi Cres, Elsdon, was an example of the measures the council could undertake to achieve that, Bailey said.
The report summary said debris, including sediment and vegetation, washed into streams and created issues where streams meet the stormwater pipes.
Takapuwahia Stream, Te Araroa Trail Stream and Mitchell Stream all suffered blockages that led to properties being flooded.
At the peak, the Porirua Stream had a flow rate of 66 cubic metres per second, the heaviest rate recorded since 1980.
Thirteen properties in Rangituhi Cres were inundated with water, including four where water entered the houses. Water also entered a home in Thompson Grove, Porirua East.
Flooding in Porirua’s CBD was caused by pipes becoming blocked.
Bailey said the cost of the flood- ing and future upgrades on the stormwater network would likely be absorbed in current budgets. Some houses in Porirua East might have to be raised, he said.
Meanwhile, Bailey said the closure of Raiha St for two weeks was due to soil under the road being unstable after the floods. It has since re-opened.
Flooding in Takapuwahia was particularly bad on May 14.