JANUARY 1998 DEBRIS FLOWS:
At the time of the field survey, slides were still active and were mapped both along slopes modified for human use and also on slopes under dense vegetation away from roads.
However, due to blocked roads these sites could not be visited.
Most of the debris flows originated in the 1st and 2nd order steep channels which appear to have been the sites of previous slope failures.
Debris flows generally developed from failures within the colluvium and weathered bedrock, and were shallow.
Slide cross-section
However, there were exceptions.
These slides ranged from small events that traveled about 5 m, to flows that traveled down the streams for 20-30 m.
A common feature of these events were their typical occurrence during intense storm rain on 4th January, flow slurry, generally rapid movement, and sudden impact, such as Grants Pen.
These slides originated on slopes between 20-40 degrees and left empty scars, 1-2 m deep and 2-10 m across.
It appears that some of the channels, or debris chutes, were choked by rock and soil debris and boulders, and organic debris which created landslide dams. These dams were short-lived and appear to have breached within about 6 hours releasing water laden with sediment slurries, boulders and organic debris causing flooding and destruction of property and infrastructure.
Debris flow deposits at the time of survey were wet/moist with the consistency of a cement slurry, ranging from non-plastic sandy soil to plastic soil.
The inherent geologic factors controlling the slides include:
· altered and weathered andesitic rocks and thinly layered shale/mudstone sequence,
· intensely fractured bedrock (faults and joints),
· colluvium covered slopes, often rich in clay content, and
· permeable and impermeable materials along slopes facilitating the building up of a local perched water table.
Geologic cross-sections of two slides