DYNAMICS, VELOCITY AND RUN-OUT OF THE GIANT STOREGGA SLIDE
F. V. DE BLASIO1, D. ISSLER1,2, A. ELVERHØI1, C. B. HARBITZ2, T. ILSTAD1,
P. BRYN3, R. LIEN3 , F. LØVHOLT2
1
Institutt for geologi, Universitetet i Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
2
NGI, POB 3930 Ullevaal Stadion,N-0806 Oslo, Norway
3
Norsk Hydro, N-0246 Oslo, Norway
Abstract
A huge slide (volume of 2400 km3 and run-out 450 km) was released in the Storegga
area off the western coast of Norway during early Holocene, followed by numerous
smaller debris flows. We perform numerical simulations of the giant slide using a
Bingham model for the clay material. Agreement with present deposit distribution and
run-out is found by assuming that the shear resistance between the debris flow and the
seabed decreases during the flow, and we suggest sediment remolding or hydroplaning
as possible explanations. Debris velocities are predicted and possible applications to the
associated tsunami event are investigated.
Keywords: Submarine slide, debris flow, turbidity current, Storegga
1. Introduction
The giant Storegga slide that occurred along the western coastline of Norway in the late
Holocene is one of Earth’s largest known gravity mass flows. Due to the plans for gas
field developments, the Storegga slide area has been intensively investigated, especially
in the upper parts close to the headwall (the region of the Ormen Lange gas field). Detailed sea floor surveying combined with high resolution seismic, sediment coring and
drilling have provided an extensive database and detailed maps of the various slide
phases (Haflidason et al. 2002). Lobes from more than 70 individual slide events have
been identified, ranging from the first major phase of 2400 km3 with a run-out distance
of 450 km (termed phase 1), to minor slides of about 0.01 km3 or less located along the
headwall.
In a companion article (Issler et al., 2003) we present numerical simulations of the debris flows in the Ormen Lange area, where the typical run-out distance of debris flows is
15–20 km. The present paper focuses exclusively on the large Storegga slide “phase 1”
as defined by Haflidason et al. (2002), see Fig. 1. The aim is to simulate the dynamics of
the flow and velocity based on the only constraint of post-slide deposit location, thickness and run-out. As for the debris flows in Ormen Lange (Issler et al., 2003), basis for
our simulation is the BING model for a Bingham visco-plastic fluid (Huang and Garcia,
1999; Imran et al., 2001), supplemented by a front and a surface drag. Some variants of
the basic model are also introduced.
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De Blasio et al.
Initial deposit
20 kPa
0.5 kPa
Present deposit
10 kPa
seabed profile
Figure 1. Simulated profiles of the final deposit. The material is modelled as a
viscoplastic (Bingham) fluid with the indicated yield stress. The present deposit
profile is also shown for comparison.
2. Simulations of Storegga slide, phase 1
In a first approximation we use a Bingham (visco-plastic) model with yield stress of 10
and 20 kPa. The Bingham rheology rather than a granular model was selected because of
the high clay content of the sediments (Huang and Garcia, 1998, 1999; Coussot, 1996).
Fig. 1 shows the final deposits from some calculations. The results are at variance with
the field data: the calculated run-out distance is much smaller than the observed one and
the deposit shapes do not reflect the present distribution. Such values of the yield stress
result in an overloading of the upper part of the slope located between 100 and 200 km,
much more than observed, and absence of deposition at larger distances (400–450 km)
where material from phase 1 is observed to be abundant.
In order to understand the flow of the phase 1 slide, one needs to identify a mechanism
capable of increasing the run-out distance, i.e., decreasing the bottom friction. We consider three candidate processes: remolding, wetting and hydroplaning.
Geotechnical measurements of sediment static resistance in Storegga indicate a shear
strength of the order of 50–100 kPa, at least five times larger than the dynamic yield
stress needed to give an accurate prediction in minor debris lobes in the Ormen Lange
area. This marked decrease of the sediment resistance can be attributed to a sudden rearrangement of clay particles during the flow in these sensitive clays. A crucial question is:
Could this process have led to a further reduction of strength in the phase 1 slide? A
reduction of yield stress to 1–2 kPa due to strength loss is indeed observed in another
region within the Storegga scar, called the northern flank, where sediments have been
Dynamics and material properties of the giant Storegga slide
225
extensively remoulded and mobilised (Haflidason et al. 2002). Assuming the material of
phase 1 and of the northern flank to be similar, we simulated the slide with a very small
yield stress and found that a value below 0.5 kPa is necessary for the run-out distance to
be compatible with field data (see Fig. 1). The plateau between 200 and 400 km plays an
important role in that only simulated debris flows with yield stresses lower than 0.5 kPa
can traverse it. In fact, even for a value of 2 kPa we find that the debris flow stops before
the plateau. However, such a small yield stress (<0.5 kPa) results in too little deposition
in the upper part of the basin. There are also sedimentological problems for this explanation. Phase 1 and North Flank sediments should differ strongly. The latter involve materials from the upper 30–50 m of the column, while phase 1 includes sediments that have
been buried much deeper.
An even more efficient mechanism for a progressive decrease of yield stress during the
flow is wetting, i.e., water incorporation into the soil, a process promoted by the high
shear rates at the interface between the flowing sediment and water. The difference with
respect to the previous case is that wetting leads to a progressive (i.e., increasing with
time) loss of strength. This process is not sufficiently understood at present to develop a
well-founded model; rather, an empirical approach combining back-calculations of comparable slides and qualitative arguments to determine the direction of deviations from
other slides is called for. In the following, we loosely refer to remoulding as the combined effect of clay particle rearrangement and wetting. We tentatively adopt a simple
model for the eYROXWLRQRI WKHUKHRORJLFDOSDUDPHWHUVDOORZLQJWKH\LHOGVWUHVV y(t) to
vary as
τ y ( x, t ) = τ y (∞) + [τ y (0) − τ y (∞)] exp[−Γγ ( x, t )] ,
(1)
where •y(0) and •y(•) are the initial and residual (i.e., completely remoulded) yield
stresses, • is the total shear deformation, • is a dimensionless remoulding efficiency and
x is the co-ordinate parallel to the sea bed. This model is reminiscent of the KomamuraHuang rheological model in soil mechanics (Komamura and Huang, 1974). The effect
proposed here, however, results from wetting rather than rearrangement of soil grains.
The total shear deformation at the base of the debris flow at time t is calculated using a steady state approximation for the Bingham fluid as
t
γ ( x, t ) = ∫ dt ’
0
∂U ( x, y = 0; t )
U ( x, y = D; t )
= 2∫ dt ’
∂y
D
0
t
(2)
where D is the depth of the shear layer and U(x,y;t) is the flow velocity parallel to
the seabed as a function of the height y.
As shown in Fig. 2, progressive remoulding leads to long run-out distances even for
a high initial yield stress. The simulation also indicates that the front of the debris flow
can potentially cross the plateau. Sediment accumulates more uniformly along the flow
path, both on the steeper slopes and on gentle slopes in the distal part. In fact, in the upper part of the basin the mobilised material still has a relatively high yield stress, resulting in thick deposits, in agreement with our simulations for the Ormen Lange area,
226
De Blasio et al.
which require a yield stress of 10 kPa or higher. When reaching the deeper parts of the
basin, the material becomes softer and can easily cross the plateau. In short, a more homogeneous distribution of the deposit is obtained, in better agreement with the observations.
Initial deposit
10 kPa with remoulding to 500 Pa
10 kPa with remoulding to 100 Pa
5 kPa with
hydroplaning
Seabed profile
Figure 2. Simulated deposits with remoulding and with hydroplaning.
In Fig. 3, the yield stress at the front is shown as a function of the position of the
VOLGHIURQW:LWKDYDOXHRI Ú–4 in Equation (1), the yield stress is substantially
decreased after about 50 km and the effect is significant already after only 20–25 km. A
YDOXHRI LQWKLVUDQJHZRXOGH[SODLQZK\RQO\WKHODUJHVWOREHVRI2UPHQ/DQJHZRXOG
be partially affected. However, this value is purely empirical and cannot presently be
confirmed or refuted on the basis of laboratory measurements or theory.
Hydroplaning. As the average velocity U of the debris flow front increases, the dynamic
2
pressure at the front grows rapidly as U . At a critical velocity, the pressure is sufficient
for a thin water layer to be intruded underneath the debris flow, producing a lubrication
effect. So far, hydroplaning has been observed only in experiments (Mohrig. et al.,
1999). However, some puzzling features of submarine debris flows like out-runner
blocks are strong evidence that hydroplaning can occur naturally in the sub-aqueous environment, under conditions still poorly understood. In Fig. 2 we also show the final
deposit from a simulation with hydroplaning. During hydroplaning, the sediment flows
in a plug-like fashion above a lubricating water layer even at low slope angles (Harbitz
et al., 2002) and may be transported over a large distance until either the debris flow
decelerates below the critical velocity for hydroplaning, or water is expelled from underneath the debris flow or is absorbed by the sediment. Fig. 2 shows that including hydroplaning, most of the debris stops on the plateau, although the front part may hydroplane
Dynamics and material properties of the giant Storegga slide
227
a little further. Unfortunately, the model for hydroplaning depends on unknown parameters, such as the minimum thickness of the water layer for an efficient lubrication,
Figure 3. Change in the
yield stress for the same
model as in Fig. 2. The
material starts with a
yield stress of 10 kPa and
is remoulded to a minimum value of 100 Pa or
3D:LWKDFRQVWDQW
= 0.0005, remoulding
becomes very pronounced
after about 50 km of flow.
or the stress between the water layer and the debris flow. Even apparently innocuous
quantities such as the initial water velocity profile can affect the results of the computation.
We conclude that a simple model for hydroplaning can reproduce the run-out distance of
phase 1 with a yield stress one order of magnitude higher than for a non-hydroplaning
simulation. In principle, both remoulding and hydroplaning may have played a role during the Storegga phase 1 debris flow. They might be two closely related rather than distinct, mutually exclusive processes. Considering the high pressure in the water layer and
the shear stresses between the water layer and the debris flow (on the order of 1 MPa
and 1–10 kPa, respectively) one concludes that water incorporation into the sediment at
the bottom of the flow must be very effective. Hydroplaning might be favoured at the
beginning of the flow, when the material is still sufficiently compact to prevent water
from seeping into the sediment. During the flow, water will be incorporated in the sediment with increasing efficiency, partly because of the pores in the sediment and partly
because of cracks and rupture planes created by the large shear stresses. Note also that
for the dynamics, one does not need the whole debris to be fully wetted. From Newton’s
equations, it follows in fact that the resistive forces determining the change in the average momentum of the debris flow depend only on the shear at the top (drag force) and at
the bottom, while the material properties in between can only redistribute locally the
velocity.
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De Blasio et al.
3. Velocity of the giant Storegga slide and tsunami generation
After the initial acceleration, the forces acting on a debris flow are close to equilibrium
between the component of the gravity force parallel to the seabed, the drag force exerted
by water and the friction at the base. In the high-velocity and low-yield stress regime
relevant to the phase 1 slide the drag force becomes the most important resistive force,
1/6
and it follows that the velocity U scales like U ~ V where V is the volume. One can
thus expect the flow of the huge mass of Storegga to be associated with very high velocities. The velocities without and with hydroplaning are shown in Figs. 4. Interestingly, the
top velocities are approximately the same in all cases (about 60 m/s or more than 200
km/h), but the flow without remoulding stops after only 1.5 hours. The case with remoulding and hydroplaning are more similar to each other, with flow duration of about
4–5 hours and also comparable top velocity. The critical velocity for hydroplaning
(which depends on the debris flow thickness and is of the order of 25 m/s) is reached
early during the simulation. The case with hydroplaning is sensitive to the details of the
seabed. Since the seabed profile is partly hypothetical, the spikes in the velocity were
probably absent or smeared out when and if the Storegga phase 1 was hydroplaning. The
impact pressure P exerted by the front of the debris flow of density ρ on a static object
2
can be estimated as P ≈ ρ V and would be of the order of 5–8 MPa.
Tsunami surface elevations of about 5 m were probably produced in the source area
during the slide (Harbitz, 1992). Sedimentary traces of tsunami deposits contemporary
with the Storegga slide have been tracked for example in Scotland and off the western
coast of Norway (Dawson et al., 1988, 1993; Long et al., 1989; Bondevik et al., 1997).
The Norwegian Geotechnical Institute (2002a, 2002b, 2002c) has produced a set of new
simulations for tsunamis generated by submarine gravity mass flows in the Storegga/Ormen Lange area. Owing to the uncertainties related especially to volume and
dynamics of potential future slide events, a series of 19 potential mass flow volumes
moving with simple and prescribed velocity profiles have been studied for a parameter
sensitivity analysis.
The simulations reveal complex wave dynamics with wave amplification due to bottom
topography. They show prevailing large-scale longitudinal oscillations in the fjord systems, rather insensitive to local run-up effects. While the large slide scenarios have a
regional effect, the smaller slides may generate the same maximum surface elevations,
but only with local effects. The maximum surface elevations correlate best with the
mean kinetic energy of the slide or with the product of initial acceleration and volume of
the slide (i.e. mean kinetic energy normalised with respect to run-out and density).
Hence, the results show that both the initial acceleration and the volume are important
for determining the maximum surface elevation, but that the run-out is of minor importance.
For the largest slide volume (2400 km3), the simulated surface elevations vary from
about 50 m in the most exposed inner fjord locations to about 10 – 15 m along the coast.
The wave current speeds are large (up to 2.6 m/s) for the open sea locations close to the
wave generation area. The wave current speeds are about 1 m/s and less in the open sea
locations outside the wave generation areas. The wave current speeds can be signifi-
Dynamics and material properties of the giant Storegga slide
229
cantly larger in the coastal and fjord domains. The dominating wave periods are above 2
hours for the largest slide volume. For the smaller slides producing shorter waves, the
maximum surface elevations are larger at the coastal locations than in the fjords.
=10 kPa with remoulding
=20 kPa with remoulding
=5 kPa Bingham
=5 kPa with hydroplaning
Figure 4. Front velocity of the debris flow as a function of the position with the different models
explained in the main text. Average velocities are approximately 60 % of the value at the front.
Slide volumes less than 5 km3 with initial acceleration of 0.033 m/s2, maximum velocity
of 10 m/s, and run-out distance of less than 6 km do not give significant impact on land,
i.e. the tsunami inundation level added to the mean tidal high water does not surpass the
highest recorded sea level. The velocity profiles are calibrated versus retrogressive slide
run-out analyses (Norwegian Geotechnical Institute 2002c).
4. Conclusions
Our study of Storegga slide phase 1 uses the BING model for a Bingham visco-plastic
fluid including mechanisms such as high-degree remoulding and hydroplaning. Our
simulations indicate a decrease of the resistive shear forces between the debris flow and
the seabed during flow, and we suggest remoulding and hydroplaning as possible
mechanisms. At present, our understanding of the physics of remoulding and hydroplaning and a more strict determination of the relevant parameters is far from complete. The
most significant gaps in our present understanding of the Storegga phase 1 and similar
slides are: (i) the break-up rate of overconsolidated clays, leading to a dramatic decrease
of strength, (ii) the rate of mud entrainment and mixing with water in the shear layer,
(iii) hydroplaning and (iv) the relation between hydroplaning and wetting. To produce
more reliable flow simulations, dedicated experiments and theoretical work on these
topics should be carried out.
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De Blasio et al.
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