Wet, wet, wet
6 August 2008
Wet, wet, wet
landslides, full to overflowing rivers and more than few
despondent faces are courtesy of Wellington’s wettest
month in the past year, with many parts of the region
receiving up to twice their average rainfall totals,
according to Greater Wellington’s hydrology summary for
A series of low pressure systems brought frequent storms throughout the month with the worst coming courtesy of an easterly low pressure system on July 29 and 30 bringing sustained rainfall to the eastern Wairarapa hills, Wairarapa plains, eastern Tararua Range, Orongorongo Range, Wainuiomata and the north eastern Hutt Valley.
Greater Wellington’s monitoring site ‘Tanawa Hut’, in the north eastern Wairarapa hills, experienced 120 mm of rain in 24 hours – its highest one day total since a storm in August 2004.
If the easterly front wasn’t enough, a north westerly airflow on July 31 brought further rainfall that mostly affected the Tararua Range, Kapiti Coast and Hutt Valley.
The Angle Knob site in the Tararua Range received more than a metre of rain for the month, well above its average of 600mm, and Karori’s total of 237mm was one and a half times its July average.
July’s rains boosted annual totals significantly; throughout the region we have now received about, or more than, average rainfall for the time of the year - in stark contrast to the drought situation experienced in much of the region during summer and early autumn.
Not surprisingly river flows were also above average for the time of year. Floodwarning alert levels were exceeded in the Hutt and Ruamahanga rivers on July 11-12, while the storm on July 30 resulted in particularly high flows in the Ruamahanga River and its eastern tributaries – the river rising 3m during the height of the flood.
The Tauweru River had a peak flow of nearly 300 m3/s, which has an estimated return period of 5 years. Flow in the Ruamahanga River was high enough for the floodway system to operate, and in both the Ruamahanga and Tauweru rivers it was the highest flow since the floods of July 2006.
Water levels in the region’s shallow aquifers have also increased and, assuming the remainder of winter stays wet, they bode well for both maintaining flows in groundwater fed streams and water levels in wetlands, and leaving enough for irrigation for next summer.
The hydrological summary is based on data from selected monitoring locations in the Wellington region. See http://www.gw.govt.nz/section2534.cfm ?
Greater Wellington monitors rainfall, river flows, groundwater levels and soil moisture at many locations that may not be mentioned in this summary report. Maps of site locations and up-to-date data can be found at www.gw.govt.nz/monitoring