Mediterranean Storms
(Proceedings of the 4th EGS Plinius Conference held at Mallorca, Spain, October 2002)
 2003 by Universitat de les Illes Balears (Spain)
LANDSLIDE RAINFALL TRIGGERS IN THE SPANISH EASTERN PYRENEES
J. Corominas, J. Moya, M. Hürlimann
Department of Geotechnical Engineering and Geosciences. UPC. Jordi Girona 1-3, D-2 building. 08034 Barcelona. Spain
e-mail: [email protected]
ABSTRACT
Three different rainfall patterns for triggering landslides have been observed in the Spanish Eastern Pyrenees: (a)
high intensity and short-lasting rains trigger shallow landslides, debris flows and some rock falls; (b) moderate
intensity long lasting rainy episodes reactivate mudslides and both mid-size rotational and translational slides;
and (c) regular rainfall episodes produce surges in continuously moving landslides. The geological and
hydrological features for such a behaviour are briefly discussed.
1. INTRODUCTION
Rainfall is the most frequent landslide-triggering factor in many regions in the world. Many attempts have been
made to predict landslide occurrence through rainfall records. However, there is no agreement on how rainfall
thresholds should be defined and on the role of the antecedent rainfall. In one of the first comprehensive reviews,
Caine (1980) proposed the following intensity-duration rainfall threshold for both shallow landslides and debris
flows:
I = 14.82 D-0.39
(1)
Where I, is the rainfall intensity in millimetres per hour and D is storm duration in hours. Equation (1) can be used
for rainy periods lasting between 10 minutes and 10 days. The concept implicit in this equation is that the average
rainfall intensity needed to trigger debris flows is much higher for short lasting rains than for rainstorms extending
several days. These results have been confirmed by other researchers (Wolle & Hachich, 1989; Kim et al. 1991).
However, the applicability of such a threshold in pervious soils has been questioned (Brand 1995; Au 1998). For other
climatic regions, different rainfall thresholds have been proposed (i.e. Wieczorek, 1987; Larsen & Simon, 1993) which
differ significantly from that proposed by Caine.
2.
RAINFALL TRIGGERS OF PYRENEAN LANDSLIDES
Our experience studying recent landslides in the Eastern Pyrenees has shown that the relationship between
rainfall and occurrence of landslides is complex. We have reconstructed the temporal activity of landslides based on
technical reports, field reconnaissance and dendrogeomorphological analysis. According to the nature and
permeability of slope materials, we have observed several patterns of landslide-triggering rainfall events.
2.1
Shallow landslides
In slopes covered with pervious colluvium and weathered bedrock formations, high-intensity short-lasting
rainfall is able to trigger debris slides, debris flows and rock falls. The analysis of the isohyets and of the areal
extent of the shallow slope failures in three recent events (November 6-7, 1982; September 8, 1992 and December
17-18, 1997) has allowed the establishment of a rain threshold of 190 mm in 24-36 h for the Eastern Pyrenees
(Figure 1). In all of the cases, no significant rainfall was recorded during the weeks previous to the occurrence of
the failures. Instead, long lasting rain episodes of low to moderate intensity do not trigger shallow landslides. This
is due to the presence of large interparticle voids in colluvium and macropores (animal burrows, decayed root
channels, soil pipes) in weathered argillaceous soils that drain water infiltrated from moderate-intensity rains very
rapidly. Only high-intensity rains allow the concentration and the build up of pore water pressures that lead to the
slope failure.
6 - 7 / nov / 1982
8 / sep / 1992
17 - 18 / dec / 1997
Figure 1. Isohyets (in mm) and area affected by shallow slides and debris flows in three recent rainfall episodes at the
Spanish Eastern Pyrenees. Rain gages are indicated as solid triangles.
2.2
Mudslides and mid-size landslides
Mudslides and both rotational and translational slides, with volumes of some tens to few hundreds of thousand of
cubic meters, are reactivated by moderate rainfall episodes of 40 to 100 mm in 24h if, at least, 90 mm of antecedent
rain have fallen during the previous days (Corominas & Moya, 1999). This type of landslides take place on clayey
and silty-clayey geological formations which are of low permeability. In such formations, water infiltration from the
surface is restricted by the hydraulic conductivity of the materials, which is mostly controlled by the grain size and, to
a lesser extent, by soil fissures and recharge through pervious layers such as interbedded sandstone layers.
All the rainfall episodes over 40 mm that have occurred during the last 40 years in the Eastern Pyrenees, have
been plot in figure 2. In this figure, all the events related to the reactivation of landslides have been indicated with an
open circle and a rainfall threshold has been established. The threshold has the following form:
I = 66.1 D-0.59
(2)
Where I, is the rainfall intensity in millimetres per day and D is storm duration in days. Equation (2) is valid for
rainfall episodes lasting for more than one week. In figure 2, however, it is observable that four reactivation events
occurred with a recorded rainfall below the threshold. Three of these events correspond to La Nou mudslide which
was moving for three consecutive years (Moya, 2002). Instead of being reactivated every year, it is more appropriate
to say that La Nou mudslide did not stop during all that time. The causes of such a behaviour correspond to that of
active landslides and will be discussed in the following chapter.
130
120
R eactiv atio n ev en t
N o reactiv atio n
T h resh o ld
110
M ean in ten s ity (m m /d ay )
100
90
80
70
60
50
40
30
20
10
0
0
7
14
21
28
D u ratio n (d ay s)
35
42
Figure 2. Rainfall threshold for reactivation of mudslides and mid-size slides in the Eastern Pyrenees.
Beside the events shown in Figure 2, there exist some few cases in which large (several hundreds of thousand to
some millions of cubic meters in volume) rotational and translational slides can be reactivated by heavy rainfalls
lasting 24-48 h, as it occurred in November 1982 in La Coma, Gosol and Pont de Bar. These were very particular
cases in which a karstic network favoured rainfall to rapidly infiltrate over a large area and to concentrate at the
landslide bodies (Corominas & Alonso 1990).
Finally, one may expect that, wet seasonal episodes, lasting for several months or years, might reactivate large
dormant landslides. In fact, local reactivation of some large landslides in Switzerland has been associated to prolonged
periods of wetness extending over decades (Noverraz et al. 1998). However, evidences for such a behaviour have not
been found yet in the Pyrenees.
2.3
Active landslides
Some landslides are permanently in motion, as the Vallcebre landslide (Corominas et al. 1999). Active
landslides keep in motion because driving forces have not been balanced by resisting ones. These movements keep
going until the stability is reached, for instance, by changing the geometry of the slide mass. Even though active
landslides often involve clayey materials, they are very sensitive rainfall events of few tens of mm which cause
sudden surges and acceleration of the movement. In such active movements, rainfall is able to quickly penetrate into
the landslide body through open tension cracks that appear at the landslide head and in stretching zones (graben). As a
consequence, the build-up of pore water pressures takes place instantaneously (Figure 3).
3.
CONCLUDING REMARKS
The actual behaviour of the landslides in the Eastern Pyrenees shows that the relationship with the rainfall
triggers is not simple. Both lithology and hydrology of the slope exert an strong control on the rate of rainfall
infiltration and on the build-up of pore water pressures that lead to instability. Under such circumstances, one may not
expect any rainfall threshold expression that could work for landslides all over the world. Such a expression can not
be obtained either in the same region, as Pyrenean landslides have shown. Hydrological triggers for shallow, mid and
large landslides are different. Shallow failures are very sensitive to short-lasting high intensity rainfall events. Instead,
reactivation of larger landslides is sensitive to prolonged periods of wetness. For the largest landslides, short-term
responses to rainfall are expected in cases in which groundwater is quickly transferred into the landslide body through
12 0
0
10 0
1
2
80
3
60
4
40
5
20
6
0
7
D e p th o f g ro u n d w a te r ta b le (m )
R a in fa ll (m m in 2 4 h )
karstic conducts or by open fissures present at the ground surface. Overlooking of these features may lead to a
misconception of the role of climate in triggering landslides.
V e lo c ity (m m /d a y )
20
15
10
5
0
2 1-N ov -9 6
20-J an -9 7
21 -M a r-9 7
20-M ay -9 7
1 9-Ju l-9 7
17-Sep -9 7
Figure 3. Above: rainfall record (bars) and groundwater level changes at piezometer S-2 of Vallcebre landslide.
Below: rate of landslide displacement. Notice how both groundwater changes and displacement rates occur shortly
after the rainfall episodes (Corominas, et al. 1999)
4.
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