Re­view of coun­cil’s flood re­sponse

Kapi-Mana News - - NEWS - By KRIS DANDO

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.

Photo: FAIR­FAX NZ

Flood­ing in Taka­puwahia was par­tic­u­larly bad on May 14.

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