ISSUES IN THE ASSESSMENT OF GRAVITY MASS FLOW HAZARD IN THE
STOREGGA AREA OFF THE WESTERN NORWEGIAN COAST
D. ISSLER, F. V. DE BLASIO, A. ELVERHØI, T. ILSTAD,
Institutt for geologi, Universitetet i Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
H. HAFLIDASON
Department of Geology, University of Bergen, N-5007 Bergen, Norway
P. BRYN, R. LIEN
Norsk Hydro, N-0246 Oslo, Norway
Abstract
Statistical analysis of the lobes of the Storegga slide reveals a power-law dependence of
the runout distance on the release volume. For small to moderate volumes, visco-plastic
models with a (remoulded) yield strength of about 10 kPa reproduce this dependence
quite well, in contrast to granular-friction models. However, either progressive wetting
of the bottom shear layer or hydroplaning has to be invoked to explain the extreme
runout distance and the sediment distribution of the largest slide phase. Preliminary estimates of the turbidite volume put severe constraints on the formation rate and density
of the turbidity currents accompanying the slide.
Keywords: Submarine slide, debris flow, turbidity current, Storegga, Ormen Lange
1. Introduction: scope of the study and general approach
The Storegga area off the western Norwegian coast (6°E–6°W, 62–69° N including the
turbidite) has been the site of several giant slides, the last of which occurred approximately 8200 y B.P. (Haflidason et al., in press). It involved a volume of some 3000 km3
and reached a runout distance of 450 km for the debris flow and 800 km for the associated turbidity current. The Ormen Lange gas field is located only 3–15 km seaward from
the present-day headwall—a situation raising concerns about the hazard to which the
planned petroleum production installations would be exposed. In an effort probably unprecedented in marine geology, the whole area has been bathymetrically and seismically
mapped in detail and sediment cores have been collected in addition to drilling.
We used this information in a modelling study to determine the conditions under which a
future debris flow from the Storegga headwall endangers the planned installations on the
seafloor. We used models based on the Bingham visco-plastic rheology, but one of the
tested extensions also incorporates aspects of granular behaviour (see Sect. 3). Of particular interest was whether a model correctly reproduces the very pronounced powerlaw dependence of the runout distance on the release volume that is observed in the Storegga slide (cf. Sect. 2). Turbidity currents were disregarded in our study because the
over-consolidated clays in the present-day headwall will at most allow minor slides to
occur that do not easily generate turbidity currents. The present paper summarises methods and results obtained in our study that are of scientific interest beyond the original
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Issler et al.
Figure 1. Overview map of the
Storegga slide. The associated
turbidite (not shown) extends to
about 69°N and 6°W.
purpose. A companion paper (De Blasio et al., 2003) focuses on modelling the earliest
(and largest) of the six slide phases (Haflidason et al., in press).
2. Volume dependence of runout distance and runout ratio
It has long been known that the runout distance R (at fixed drop height H) grows while
the runout ratio H/R decreases as the release volume of a gravity mass flow is increased
(Elverhøi et al., 2002, and references therein). Previously compiled tables of these quantities had to use data from very different sources, representing different soil properties
and topographical settings. Haflidason et al. (in press) were able to identify several dozens of individual debris flow lobes within the Storegga slide and to determine their volumes, drop heights and runout. They grouped them into six major phases of a retrogressive process, in which the size of the slides diminished from one phase to the next. This
rather homogeneous data set offers the rare opportunity of studying the scaling behaviour, through which inferences on the dominant processes can be drawn, over a range of
six and three orders of magnitude in volume and runout distance, respectively.
For simplicity, the runout distance and drop height are measured from the uppermost
point of the slide scar to the toe of the deposit, even though it would be physically more
appropriate to use the centre-of-mass locations. The lobes also display very little lateral
spread; accordingly, we use the release volume per unit width A (the mean area of a longitudinal section) instead of the volume for our comparisons. Figure 2 shows that the
runout distances and runout ratios quite closely obey the scaling relations
R = 31.6 km–0.74 · A0.87 ,
H/R = 0.021 km1.14 · A–0.57 .
(1)
At least 95% of the lobes are within the bounds (19.7…50.5) km–0.74 · A0.87, and for
about 90% of the lobes H/R is in the range (0.013…0.033) km1.14 · A–0.57. The slides that
deviate most from the scaling relations (G10, G12 in Fig. 2) have somewhat unusual
Gravity mass flow hazard in the Storegga area
233
Figure 2. Dependence of the
runout distance (top) and runout ratio (bottom) on the release volume per unit width,
A, for the distinguishable
lobes of the Storegga slide.
Based on data from Haflidason et al. (2003). The middle lines correspond to the
best power-law fit, and the upper and lower lines were chosen to correspond to a confidence level of at least 95%
and 90%, respectively. Phase
1 and 4 are the first (and largest) and fourth phases of the
Storegga slide, respectively.
G10 and G12 belong to a series of small, late-phase lobes
near the Ormen Lange area.
Drop heights were not available for all lobes, therefore
fewer data points appear in the
lower plot.
release zones; the inordinately long runout zone of the slide phase 4 appears to be
caused by a steepening of the flow path shortly before the expected stopping point. Applicability of the scaling relation over such a large range in A is all the more surprising
as the soil in the smallest slides disintegrated much less than in the earlier phases where
also lubrication effects like hydroplaning may have been important.
If it is assumed that the slide conditions in the Storegga area (soil properties, sea floor,
release mechanisms, etc.) have not changed substantially over the past 8200 years, the
relations (1) or the bounds indicated above can be used to estimate the runout distances
for various hypothetical release volumes. We will discuss in Section 6 why this assumption may not be justified. We emphasise that the scaling behaviour (1) is site-specific.
Whereas primarily the dominant flow mechanism and the soil properties govern the
runout ratio, the runout distance also depends very strongly on the path profile. Equation
(1) appears to favour visco-plastic over granular behaviour for the following reasons:
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• If the resistive forces were of purely frictional (Coulomb) type, the runout ratio (for
the centre of mass) would have to be independent of the release volume. In spite of
additional effects1, the data appear to rule out pure Coulomb behaviour.
• FoU D PRGHUDWHO\ UDSLG %LQJKDP IORZ ZLWK \LHOG VWUHQJWK y, one obtains the approximate scaling relation2
H = τ y ⋅lavg..
R g ∆ρ A
(2)
If one assumes that lavg. is a fixed multiple of the initial length l0 and that l0 in turn
1/2
–1/2
scales as A , then H/R ~ A in close correspondence with the data. If lavg. scaled as
R instead (i.e., the flow is assumed to spread over the entire flow path) and we used
0.87
–0.13
the empirical scaling behaviour of R ~ A , we would obtain H/R ~ A in much
less satisfactory agreement with the data.
3. Numerical simulations of the small and medium-size Ormen Lange slides
The high clay content of the Storegga soils called for the application of models emphasising the visco-plastic properties of the flowing material over the granular ones. The
simulations were carried out with the BING model (Imran et al., 2001), based on the
Bingham rheology (Jiang and LeBlond, 1993), and several variants thereof developed
specifically for this investigation, that will be described in detail elsewhere. They capture various aspects such as the presence of solid blocks inside the liquefied soil
(B-BING), the increase of yield strength with depth into the material due to consolidation (C-BING), progressive remoulding and mixing with water along the flow path
(R-BING), and hydroplaning (W-BING). Turbulent drag was added to all models.
For comparison with the empirical volume dependence for R and H/R (see Section 2),
the BING, B-BING and C-BING models were run on a path profile representative of
most later-phase slides in Storegga. All parameters were kept fixed, except for the initial
volume per unit width, A. As shown in Fig. 3, BING (and also B-BING, not shown in
the figure) reproduced the empirical exponents quite well up to A ≈ 1 km2 while
C-BING failed. This lends credit to the scaling arguments developed in Sect. 2 and the
inference that these clayey soils show predominantly visco-plastic behaviour. The clear
deviations found at initial volumes per unit width larger than 1 km2 (De Blasio et al.,
2003) suggest that the largest and earliest slides in the sequence either involved significantly less consolidated material (that had never been subject to glacier loading) or that
some additional, lubricating effects are at work in large flows.
1
Some degree of release-volume dependence will be induced in granular models by hydrodynamic drag,
through the growth of the release-zone length with volume, and through increased longitudinal spreading
during the flow.
2
The energy dissipated at the bed from release to stop is approximately given by WD yÂRÂlavg., where R is
the runout distance and lavg. denotes the average length of the debris flow along its path. WD must equal the
work done by gravity, i.e., WG = HÂAÂg WD ZLWK WKH GHQVLW\ GLIIHUHQFH EHWZHHQ PXG DQG DPELHQW
water.
Gravity mass flow hazard in the Storegga area
235
Figure 3. Dependence of the
runout ratio H/R on the release
volume. Comparison of pure
visco-plastic model (BING,
with yield strengths of 6, 10,
14 kPa) and visco-plastic
model with depth-dependent
yield strength τy(z) (C-BING,
with τy(z) = σzz tan ϕ) against
behaviour observed in the
Storegga slide lobes (cf.
Fig. 2).
The runs mentioned above required a yield strength around 10 kPa to reproduce the observed runout distances. However, geotechnical tests of core samples from near the
headwall revealed much higher unremoulded strength even close to the surface, indicative of the over-consolidation due to glacier loading. In order to understand the severity
of this discrepancy, we applied relationships between the liquidity index and the
undrained remoulded shear strength, developed for very soft and sensitive clays (Locat
and Demers, 1988; Locat, 1997), to the over-consolidated clays of the Ormen Lange
area in order to estimate the effect of remoulding. Notwithstanding the large degree of
uncertainty, the results suggest that the bottom shear layer, in addition to remoulding,
also must have been rapidly mixed with water or water-rich mud—otherwise the yield
strength would have remained too high for a flow to develop. Similar conclusions were
reached by Locat et al. (1996) for debris flows on the Mississippi Fan.
Figure 4. Evolution of the
front velocity along the flow
path from the present-day
headwall to the Ormen Lange
gas field. Comparison of two
hypothetical release volumes
and two values of the yield
strength in the BING viscoplastic model, modified for
turbulent drag at the front and
on the upper surface. Only the
larger mass combined with a
very low remoulded yield
strength would reach locations
more than 15 km from the
headwall. Note that the peak
front velocity is reached after
only 2 km and depends much
more on the release volume
than on the yield strength.
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Issler et al.
In the next step, several values of the yield strength spanning the range indicated by the
calibration simulations were selected. Then the release volume that would be required
for a debris flow to reach selected points of interest was determined; in these simulations, the present-day bathymetry was used for extracting the path profiles. Figure 4
shows an example of front velocity curves along one such path.
Some uncertainty stems from the extremely soft and water-rich hemi-pelagic sediments
that have been deposited on the sea floor since the Storegga event, as they will undoubtedly have some lubricating effect. On the one hand, the flow conditions are in part more
favourable today than they were during the last phases of the Storegga slide because the
earlier phases of this retrogressive process had already removed soft sediments close to
the surface. On the other hand, the earlier phases also left behind strongly remoulded
material that may produce similar lubricating effects. It will be seen in Sect. 4 that entrainment of marine sediments is one of the principal candidate processes for explaining
the long runout distance of the phase 1 slide. Even if a debris flow stops before the offshore installations, it may generate a turbidity current that flows on. Their effect depends
crucially on their mass, suspended from the debris flow or from the seabed. Present
knowledge on the processes involved is not sufficient for a quantitative analysis.
4. Issues in the simulation of the first slide phase
From numerical simulations of the very large first phase of the Storegga slide, the companion paper by De Blasio et al. (2003) arrives at the following conclusions. (i) Viscoplastic models with constant yield strength in the range inferred for the late-phase debris
flows (~ 10 kPa) are not capable of reproducing the observed runout distance. (This
conclusion holds even more for models with predominantly frictional behaviour, as
Sect. 3 showed.) (ii) If the yield strength is much smaller and constant (< 0.5 kPa), the
runout distance can be reproduced, but the simulated deposit distribution strongly deviates from the observed one. (iii) The most plausible modelling is obtained with hydroplaning (Mohrig et al., 1998, 1999; Harbitz et al., in press; De Blasio et al., submitted)
or progressive remoulding and mixing with ambient water or water-rich sediment at the
base of the flow (Talling et al., 2002).
A common property of both mechanisms is their potential for discriminating between the
large, early phases on one hand and the small, late-phase slides on the other hand. The
threshold velocity for hydroplaning is more easily reached by a very large and fast flowslide than by a small one. Similarly, entrainment of bed sediments is likely to be more
intense in a massive, fast, long-runout flow.
5. Turbidity currents associated with the Storegga slide
A large turbidite area exists to the Northwest of the Storegga debris-flow deposits
(Bugge et al., 1988), the uppermost layers of which have the same age as the Storegga
debris-flow deposits. Their depth exceeds 10 m at certain points. According to Haflidason et al. (in press), the mass of the turbidite amounts to about 10% of the mass of the
debris-flow deposit from phase 1—similar figures as those obtained for the 1929 Grand
Gravity mass flow hazard in the Storegga area
237
Banks event (Heezen and Ewing, 1952). Moreover, the Storegga turbidity current (TC)
seems to have been concentrated in six main channels, each between 5 and 20 km wide.
In the laboratory experiments on the mobility of debris flows by Mohrig et al. (1998),
the TC mass did not exceed 1% of the total mass. However, the laboratory slurries were
much stronger in relation to the shear stresses than is the case in typical natural debris
flows, and the suspension rate from a debris flow is extremely sensitive to the shear
stress and soil strength in the interesting velocity range (Parker et al., 1987). It thus appears plausible that the suspended mass could be significantly larger than the 1 per cent
estimated for laboratory situations.
Approximately 200 km3 of suspended sediment flowed through channels with a total
width of about 30–70 km. From laboratory observations, we estimate the length and
height of the TC at the distal end of the debris flow as 150–200 km and 0.2–0.5 km,
respectively. The mean sediment concentration can then only be constrained to the range
from 3% to 25%, comprising both low- and high-density TCs. The numerical simulations indicate that the debris flow lasted for about 5 h. If most of the TC mass was suspended during that period, the erosion velocity averaged over the entire debris flow surface was about 1 mm s–1, corresponding to an erosion rate of 2 kg m–2 s–1. Very likely,
however, suspension mainly occurred at the debris-flow front, at much higher rates.
6. Conclusions
The recognition of a large number of individual lobes of all sizes in the Storegga slide
has offered the unique opportunity of studying the scaling behaviour of characteristic
slide parameters with respect to release volume within a rather homogeneous setting.
Our findings confirm that visco-plastic models provide a reasonable description of such
clay-rich flows. A similar study with more sandy sediments should be very interesting.
In the clayey sediments of Storegga, a high degree of remoulding, reducing soil strength
by an order of magnitude, must have taken place very early in the post-release phase,
otherwise the flows would not have developed. Moreover, progressive remoulding and
especially mixing with ambient water or liquid sediment at the base of the flow appear to
have reduced the soil strength locally by another order of magnitude in the early, largest
phases of the slide. Future research efforts on debris flow dynamics should be directed at
a better understanding of mixing processes at the base and upper surface of debris flows
and the transition from sandy to clayey flows.
More generally, in the assessment of flow-slide related hazard at offshore installations
such as those planned at Ormen Lange, there are three main sources of uncertainty:
(i) Debris flow events that are large enough to reach the installations usually have a very
low release probability that is difficult to quantify. (ii) The degree of lubrication afforded by soft marine sediments is not well understood. (iii) Even if a debris flow does
not reach the installations, the associated turbidity current may do so. Estimating its
stagnation pressure and damage potential requires better knowledge of the range of
probable suspension rates during the debris flow.
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7. Acknowledgements
The authors wish to thank P. Gauer, C. B. Harbitz, K. Høeg, T. J. Kvalstad, J. Locat, J. Marr, D. Mohrig,
G. Parker, and A. Solheim for enjoyable discussions at different stages of the work leading to this paper
and/or for helpful comments on the manuscript. The work presented in this paper was carried out as part of
projects funded by Norsk Hydro ASA, the EU (COSTA – Continental Slopes and Stability, project no.
EVK3-CT-1999-00006), and the Norwegian Research Council (NFR project 1333975 / 431).
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