Geoscience Reference
In-Depth Information
to the north of the mountain followed by the
eruption. Erupted debris and the collapse of
the north side of the mountain, together with
melting of the summit icefields, resulted in pyro-
clastic flows off the summit cone (lateral flow
of hot gases and unsorted volcanic fragments).
This initial surge deflated and was translated
into a ground-based debris flow, which continued
as a debris avalanche down the North Fork of
the Toutle River. This became a debris flow and
gradually evolved into a hyperconcentrated flow
as it moved downstream (Tables 2.7 & 2.8).
2.3.3 Natural and climatically induced
slope failures
Mass wasting of slopes in mountain environ-
ments proceeds by a combination of small-scale
processes and infrequent large-scale events (see
Case Study 2.3). In the Swiss Alps debris flows
are a widespread phenomenon occurring in all
altitudinal belts, although they are especially
common in the periglacial zone. Debris flow start-
ing zones tend to occur in poorly consolidated
debris on slopes or in gullies. The association
Case study 2.3 Impact of an extreme climate event on the hillslope sediment system - Cyclone Bola
(New Zealand)
In 1998 Cyclone Bola had a major impact on much of the North Island, New Zealand with the
worst affected area being on the east coast (Case Fig. 2.3c). Cyclone Bola was the largest storm
event in the Waipaoa catchment that had occurred since European settlement in the 1830s.
Between 6 and 9 March up to 900 mm of rain fell in the north-east of the catchment (Page et al.
1999). The Waipaoa is soft-rock (Cretaceous greywackes and Miocene-Pliocene siltstones and
sandstones) hill country on the east coast of the North Island, New Zealand (Case Fig. 2.3b).
This is a tectonically active area with an uplift of approximately 3 mm yr
−1
. The storm was
estimated to be a 100 year return period event. The main impact of the storm was extensive
erosion of hillslopes and massive sediment transfer along the valley systems. This is not the only
major erosional event in this area. Deforestation and conversion to pasture in the wake of
European settlement initiated a major phase of instability in the catchment. Similarly large
storms over the past 100 years have also had major impacts on erosion and sedimentation in
the watershed (Trustrum et al. 1999).
A short-term sediment budget was constructed by Page et al. (1994) to assess the response of
Cyclone Bola on the smaller Tutira catchment (3208 ha) to the intense rain storm event (Case
Fig. 2.3a). The budget quantifies the total amount of sediment generated during the event and
the relative contribution from different erosion processes. Sediment storage is also estimated
along with the amount of sediment discharge into two lakes within the catchment. A total of
1.35 (
0.13) million cubic metres of sediment was moved during the storm (420 m
3
ha
−1
). Of this
total, 21% was stored on the hillslopes, 22% deposited on the valley floors, 51% was deposited
in the lakes and the remaining 6% was discharged at the catchment outlet. Approximately 89%
of the sediment generated during the storm was from landslide erosion on the slopes. Channel,
gully and sheet erosion was only a minor component of the budget (Case Fig. 2.3d).
Because of the dominance of landslides in the Cyclone Bola event work has been undertaken
to characterize the significance of this in sediment delivery (Page et al. 1999; Trustrum et al.
1999). Page et al. (1999) developed a method for assessing sediment production from land-
sliding in the Waipaoa catchment and applied this to the Cyclone Bola event with the aim
of determining the contribution of landslides to suspended sediment output from the event
(Case Fig. 2.3b). Using a geographical information system (GIS) containing the distribution of
±
