Flood waters roared through the business district of Kaikoura on the evening of Thursday December 23, sucking 300 kg industrial refrigerators through shop windows “like torpedoes” according to one eyewitness. Rocks from a retaining wall uprooted petrol pumps set in concrete at the BP station. Civil Defence sailed a 7.5 m charter yacht down the main street to rescue shop owners as flood waters rose.
Silt and debris 30 cm deep was left through much of the town, and the refrigerators, taken out to sea by the flood, were blown back by the wind and strewn along the high tide mark as far as 10 km away.
The same depression brought heavy rain to most eastern areas of the South Island and a record flood in Otago’s Taieri River, where newly completed floodwater control gates worth $150,000 collapsed into the river.
In central Otago, a flash flood brought thousands of tons of boulders, mud, and silt down Slaughterhouse Creek near Roxburgh. When the wall of water hit the culvert under State Highway 8, spray shot more than 10 metres into the air, causing power lines to arc and cutting power to the district. A 30-ton boulder was left in the middle of the road.
Two weeks later, torrential rain in Fiordland and Westland spilled across the Alps into western Otago and Canterbury. Massive slips cut the roads to Milford and Haast, isolating hundreds of tourists who were later brought out by boat and plane. Lightning, which lasted 16 hours along the Alps, cut electricity in many areas and started a number of small fires. A power surge as far away as Christchurch caused the Park Royal Hotel to be evacuated when fire alarms were triggered.
One of the worst-hit farms was Makarora Station above Lake Wanaka. Some 1300 sheep were drowned in a riverside paddock when the Makarora River burst over its stopbank, and a bull worth $6000 was killed by lightning. Hundreds of cubic metres of silt and gravel washed down from the mountains, destroying 20 km of fencing as it covered paddocks and yards and replacing 15 tonnes of superphosphate in a shed.
A fortnight later,Milford had its highest ever 24-hour rainfall with 538 mm on January 22, and Colliers Creek near Hokitika had 682 mm—more than the average Christchurch rainfall for an entire year.
On the plus side, the hydro lakes received plenty of water and are expected to have no trouble supplying electricity this winter. Also, floods in the Arrow River eroded fresh gravel into the riverbed, producing a minor gold rush with $3000-worth of gold being taken in a few weeks.
The January rains on the West Coast were caused partly by a large slow-moving anticyclone east of the orth Island that extended a ridge to the north of the Tasman Sea.This produced persistent northwest winds over the Tasman Sea which transported warm air with high water vapour content from the tropics to New Zealand.The water vapour condensed to rain as the
air was lifted over the Southern Alps.
The persistent northwest winds also helped to raise the sea surface temperatures west of New Zealand. This in turn helped raise the air
temperatures over New Zealand.
The average temperature for all New Zealand in December had been 1° below average at 14.6°C. However, January was 0.4° above average at 17.3°C.
Temperatures were especially high in the east of the North Island, where daily maximums were often above 30°C. Water had to be trucked to places in the Gisborne area, and in Hawkes Bay apples suffered from sunburn and baking. As the Waipawa River dried up, fish trapped in pools had to be rescued by staff from the Hawkes Bay Fish and Game Council.
In the last week of January and the first half of February, the anticyclones shifted further poleward and moved slowly east across the South Island, bringing periods of easterly winds, clear skies and high temperatures to the Manawatu.
Railway lines buckled near Hunterville on February 15 when air temperatures are estimated to have exceeded 30°C in sheltered places, and the temperature of the steel rails is likely to have reached around 45°C. Thirteen freight wagons were derailed, damaging 519 metres of track and closing the main trunk line. Freight traffic was diverted to the Stratford line, but that was closed by another derailment near Wanganui the following afternoon.
A period of dry weather set in over the West Coast, and after three weeks without rain water was having to be trucked to some residents in Westport.
February temperatures continued to be above average over much of the country. In Wellington the average daily maximum temperature was 1.4° above normal, and New Zealand Breweries reported its packaged beer sales rose by almost 12 per cent.
Severe hailstorms were also a feature of the summer, particularly in Canterbury, Nelson and Hawkes Bay. The most damaging occurred in Hawkes Bay, where hailstones the size of golf balls destroyed most of the fruit in hundreds of orchards. Vineyards were luckier, with only five out of 120 affected. Damage is expected to be around $50 million, and an estimated 1000 jobs have been lost.
Although hail occurred in the last two summers, large hail was rare, partly because both air and sea temperatures were below average as a result of cooling from the combined effects of an El Nifio event and the ash cloud from the Mount Pinatubo eruption.
This summer, El Niflo had finished and the Pinatubo ash cloud had retreated poleward, so temperatures were up. Warmer air temperatures and higher water vapour content because of warmer seas both contribute significantly to the energy available to intensify a developing thunderstorm.
They are further intensified by cold air in the middle atmosphere, and a number of the hailstorms occurred with extremely cold temperatures at 6000 m. This cold air aloft spread over New Zealand from the south, and therefore came from a region that may still be under the influence of the Pinatubo ash cloud.
Another outbreak of hail on March 7 affected Canterbury, Marlborough and Wellington. Thunder and lightning lasted several hours in the capital. Only minor damage was reported, but one lightning bolt struck the Shelly Bay Airforce Base, blowing a hole in some concrete and setting the wind sock alight.