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Preliminary Conclusions
- Prolonged heavy rainfall between 15-20 February 2004 caused widespread
landsliding over about 7500 km2 of the lower North Island. The regions
most affected included hill country in the Manawatu-Wanganui (~4000-6000
km2 ) and Wairarapa (~1200 km2 ), and the wider Wellington area (~300
km2 ).
The most severely damaged areas were in the Turakina, Whangaehu, and
Mangawhero valleys, where thousands of smaller (10-1000 m3 ) shallow
(1-2 m deep) soil and regolith slides and flows occurred, along with
some larger (~1000-100,000 m3) deep-seated landslides in Tertiary mudstone,
some forming small landslide-dams in streams (Photos
1 and 2). The floods removed blockages
by landslides observed in larger rivers.
- Most of the landslides occurred on steeper (~25-40O) pasture-covered
hill slopes, gullies and terrace edges. Some of the shallow landslides
were so numerous and extensive to produce areas of coalescing soil slides
affecting slopes covering many hectares (Photo 5 and 5a). The debris from many of these landslides
did not reach permanent stream channels, but remains on the slopes.
Gully and river bank failures, however, contributed considerable sediment
and trees to flooded rivers, with the latter causing some bridge failures.
- Landslide occurrence was clearly related to land use. Hill slopes
covered with bush and exotic forest were much less affected by landsliding,
with only a few isolated landslides observed compared to grassland areas (Photos 8 and 9). Forest
cover clearly provided good protection against landsliding during this
event. However, most recently milled forest areas were severely affected
by landsliding.
Damage to farmland was extensive across the region, and many roads were
severely damaged and closed by slips. SH 3 in the Manawatu Gorge is
still closed by several large landslides (Photo
10). Some slips came close to houses and buildings, but as far
as we are aware none were significantly damaged (Photos
3 and 4).
- Soil and debris flows exhibit scar to debris ratios of 1:3 to more
than 1:10 in some of the more mobile flows, somewhat higher than the
ratios reported for Cyclone Bola (~1:2 to 1:3). This will affect calculations
of land lost to production in the longer term. Ground surveys are still
to be conducted to confirm these initial conclusions.
None of the many pre-existing, deep-seated very large bedrock slides
in the area - like the Ohorea landslide (~175 million m3) in the Mangawhero
Valley (Photo 11) - was obviously reactivated
by the floods, and no new landslides of this type and size were formed.
These very old (prehistoric) features were probably initiated by other
factors (such as river down cutting and large earthquakes) several thousand
years ago.
For more information contact
Graham Hancox -
Contact
G.Hancox
M.McSaveney
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in the map below to view a photograph of the damage to that area.