Flood waters washed away the banks of the Waipoua River near the bridge into Lansdowne. PHOTO/MARY ARGUE
As heavy rain doused the region on Tuesday, the Ruamahanga River flow near the Waihenga Bridge reached a peak of 863 litres of water per second, the second-highest flow rate this year.
A South Wairarapa District Council [SWDC] spokesperson said the bridge remained closed yesterday, but there was yet to be an update from Waka Kotahi New Zealand Transport Agency on whether it had suffered structural damage.
The lowest flow the river had seen this year was about 7.8 litres of water per second on February 4.
Only 66 millimetres of rain fell in Masterton on Tuesday, however, 161mm of rain was recorded at Bannister Basin.
About 134mm fell at Mangatarere Valley Hill, which recorded its wettest day in October 1998, when 403.5mm was recorded at the site.
Groundwater stores rapidly filled across Wairarapa on Tuesday, jumping from 17313mm stored in the ground near Waihenga Bridge to 18777mm by yesterday evening.
River flow on February 14 at Waihenga reached 1171 litres of water per second – the highest flow recorded this year.
At the time, groundwater stores filled from 16847mm on February 12 to a peak of 18724mm on February 14.
In the historic May 1981 flood, the Ruamahanga River reached a peak flow of 1171 litres of water per second. This was its highest flow since records began in 1956.
Niwa said Upper Hutt and southern Wairarapa received 155mm of rain in 1956, a one-in-100-year event.
Masterton farmer Mike Butterick said he thought Tuesday’s flooding took longer to drain because the groundwater stores were already so full.
A Niwa report assessing climate change in Wairarapa said no statistically significant trend in extreme rainfall had been observed in the region yet.
It said it was important to note that it could take some time before statistically significant trends emerged in various climate and weather extremes due to the noise variability in weather data and the rarity of the extremes.
The report said by 2040, projected annual rainfall changes for some areas ranged from negative two to eight per cent.
It said by 2090, annual rainfall changes of up to eight per cent were projected.
“Larger and more extensive changes to rainfall are projected at the seasonal scale.”
Annual “wet days” are expected to decrease under the two more extreme climate change scenarios, but small increases could be expected under the least extreme scenario.
It said up to 21 fewer wet days could be expected with the most extreme climate change scenario by 2090.
Centre for Climate and Emergency Solutions said extreme precipitation events had produced more rain and become more common since the 1950s in many regions of the world.
“Warmer air can hold more water vapor. For each degree of warming, the air’s capacity for water vapor goes up by about seven per cent.”
It said an atmosphere with more moisture could produce more intense precipitation events, which was exactly what it had observed.
“Increases in heavy precipitation may not always lead to an increase in total precipitation over a season or over the year.”
Metservice said almost the entirety of New Zealand was hidden by a huge mass of clouds on Tuesday.
“This is the cloud associated with the strong winds and wet weather of Tuesday, it’s now pulling away to the southeast.”
It said there were hints of another system due to arrive on Thursday.
Metservice said to keep a close eye on the severe weather warnings and watches.
