South island storms and floods of December 2010

摘要

An unusually severe and spatially extensive storm over the northern and central South Island of New Zealand on 26-28 December 2010 caused severe flooding. Synoptic charts show that markedly warm moist maritime air from subtropical regions adjacent to northern Queensland moved into New Zealand, where it met a series of frontal systems. Much of the rain occurred in an interval of about 24 hours. Daily rainfall totals exceeded 25 mm over most of the South Island, although totals were below 25 mm for the east coast from about Timaru north, and, surprisingly, in an apparent rain-shadow between Nelson City and Motueka. Exceptional daily totals, with greater than 1/10 annual exceedance probability (AEP), occurred patchily from the most northern extremity of the South Island to Central Otago, but rainfalls exceeding 1/50 AEP quantiles occurred at only a few gauges, mainly located in the northwest Nelson Ranges. Flood flows in the Aorere and Anatoki Rivers in northwest Nelson were at, or above 1/100 AEP levels, and record high flows occurred in a number of rivers draining the central portion of the Southern Alps, notably the Hokitika and Rakaia. Earlier rainfall between 21-25 December recharged soil moisture levels over much of the island, with the result that losses to soil infiltration were lower and runoff rates higher than average for the 26-28 December storm. This contributed to the severity of floods experienced in a number of rivers. A comparison between the December 2010 storm and seven historic storms, as measured in 31 different catchments, shows that this storm caused the most widespread flooding in the past 30 years. The storm and the earlier events were nationally significant because they filled hydroelectric storage lakes in what was expected to be a summer with unusually low inflows because of the prevailing La Ni?a conditions.