Taranaki tornadoes

_{^{[Photo courtesy of Taranaki Daily News]}}

Several small but intense tornadoes struck Taranaki in early July, including New Plymouth and Oakura. These remarkable natural phenomena, known for occurring in the southern United States, happen in New Zealand too. NIWA says there have been 52 tornadoes in Taranaki since 1956, or about one a year. New Zealand tornadoes are generally weaker than American ones and last for a few minutes, track across land for 2 to 5 kilometres and are 20 to 100 metres wide. Wind speeds are typically between 115 and 180 km/h. NIWA reports that 10 percent of New Zealand tornadoes have wind speeds higher than 180km/h.

David Hopkins and Graeme Lawrance from the Department's Building Standards Group briefly visited Taranaki to survey the damage and to see what lessons could be learnt from how buildings withstood the tornadoes. There was major damage to some properties. Roofs were ripped off, windows were broken or sucked out, trees were shredded and power poles were blown over. One tornado hit New Plymouth ripping 500 m2 of roof from the local PlaceMakers store. The Oakura tornado caused an entire storey to be torn from a beachfront house, and a local kindergarten (fortunately unoccupied because of school holidays) had roof and extensive impact damage. Several homes were damaged by flying debris and force of the wind.

Newer buildings generally with-stood the force of the tornado reasonably well. Given that the basic design wind speed for Taranaki is 170km/h and the small areas involved, there seemed to be no cause to introduce specific design requirements for tornadoes. However, the Department's team found it would be worth considering debris impact when revising the glazing standard, NZS 4223, especially for schools and pre-school facilities.