Geoscience Reference
In-Depth Information
Case Table 2.3
Summary of sediment production and delivery produced from landslides in the Waipaoa catchment
(North Island, New Zealand) during Cyclone Bola 1988. Estimates based on storm isohyet data. (Source: Page et al. 1999.)
Land system
Percentage area of
Percentage
Sediment generation
Percentage of total
Percentage contribution to
total catchment
pasture*
(m
3
)†
suspended sediment load
Te Arai
23
96
20,053,000
61
39
Wharerata
12.5
75
6,809,000
21
13
Waihora
3.5
76
577,000
2
1
Wharekopae
20.5
92
1,978,000
6
4
Makomako
6
84
2,493,000
7
5
Waingaromia
1.5
89
948,000
3
2
Total
148,480 ha
32,858,000
100
64
*As of 1988 - excludes indigenous forest, scrub and exotic forest
>
8 years old.
†Adjusted for forest, scrub and soil conservation plantings.
Shallow landslides are responsible for approximately 64% of the load at the exit from the
catchment. In respect of longer term suspended sediment yield, however, landslides contribute
only 10 -19% of the load (Page et al. 1999). Erosion of stored sediment in tributaries is an
important control on longer term sediment production (Marutani et al. 1999). In the few years
immediately following an event of this magnitude suspended sediment concentrations are 100%
greater than in the years preceding the event owing to continued erosion of landslide scars and
stored sediment (Marutani et al. 2001).
Relevant reading
Marutani, T., Brierley, G.J., Trustrum, N.A., et al. (2001)
Source-to-sink Sedimentary Cascades in Pacific Rim
Geosystems
. Matsumoto Sabo Works Office, Ministry of Land, Infrastructure and Transport, Japan.
Page, M.J., Trustrum, N.A. & Dymond, J.R. (1994) Sediment budget to assess the geomorphic effect of a
cyclonic storm, New Zealand.
Geomorphology
9
(3), 169-88.
Page, M.J., Reid, L.M. & Lynn, I.H. (1999) Sediment production from Cyclone Bola landslides, Waipaoa catch-
ment.
Journal of Hydrology (New Zealand)
38
(2), 289-308.
Trustrum, N.A., Gomez, B., Page, M.J., et al. (1999) Sediment production, storage and output: the relative
role of large magnitude events in steepland catchments.
Zeitschrift für Geomorphologie
N.F. Supplement
115
, 71-86.
between debris flow activity and ground ice means
that the formation of debris flows is susceptible
to climate change, especially in marginal peri-
glacial areas (Zimmermann & Haeberli 1992).
In particular, since the Little Ice Age (AD 1450
to 1890) glacier retreat, disappearance of per-
ennial snow patches and degradation of low-
lying permafrost have resulted in the exposure of
thawed debris on steep unstable slopes. Continued
warming will result in an expansion of this zone.
Changing climate also influences storm charac-
teristics and changes the temporal and/or spatial
pattern of precipitation and snow deposition.
An increased incidence of large-scale debris flows
from within the Alpine periglacial belt poses a
threat to mountain villages and infrastructure as
many debris flows will be large enough to reach
the valley floor.
In the summer of 1987 the Alpine coun-
tries were devastated by a number of floods. In
Switzerland eight people were killed and damage
exceeded 1.3 billion Swiss Francs (
c
. 1 billion
US$). Heavy summer thunderstorms followed
a long winter of extended snow cover and a
cold/wet spring. The worst affected areas were
south-east Switzerland, on 18 -19 July and again
