Geological Survey of Denmark and Greenland Bulletin
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Nr. 4, Review of Survey activities 2003, pp. 89-92 |
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89
A major cliff collapse took place at Store Stejlebjerg in the
southern part of Møns Klint on 5 July 2003 (Fig. 1). This
cliff collapse was one in a number of rock falls that has
affected Møns Klint with a frequency of about one per five
years.
southern part of Møns Klint on 5 July 2003 (Fig. 1). This
cliff collapse was one in a number of rock falls that has
affected Møns Klint with a frequency of about one per five
years.
Geological investigations of the rock fall at Store Stejle-
bjerg were carried out by the Geological Survey of Denmark
and Greenland (GEUS) after the Danish Forest and Nature
Agency had asked for advice and help concerning security
regulations for public access to the site. GEUS was prepared
for this type of investigation due to the Survey's engagement
in the European Union project PROTECT, which aims at
prediction of chalk cliff collapses. In this project a number of
sites in northern Europe have been selected for detailed inves-
tigation, among which two are situated at Møns Klint, south-
east Denmark (Fig. 1). This report provides a short description
of the 2003 cliff collapse at Møns Klint and a brief descrip-
tion of the PROTECT project and its practical implications
for cliff collapse evaluation.
and Greenland (GEUS) after the Danish Forest and Nature
Agency had asked for advice and help concerning security
regulations for public access to the site. GEUS was prepared
for this type of investigation due to the Survey's engagement
in the European Union project PROTECT, which aims at
prediction of chalk cliff collapses. In this project a number of
sites in northern Europe have been selected for detailed inves-
tigation, among which two are situated at Møns Klint, south-
east Denmark (Fig. 1). This report provides a short description
of the 2003 cliff collapse at Møns Klint and a brief descrip-
tion of the PROTECT project and its practical implications
for cliff collapse evaluation.
The Store Stejlebjerg cliff collapse
Møns Klint is a 4 km long NS-trending coastal chalk cliff
up to 130 m high bordering the east side of the island of Møn
(Fig. 1). This cliff provides an instructive structural cross-sec-
tion through a large glaciotectonic complex, the southern
part of which can be characterised as an imbricate fan with
thrust sheets consisting of c. 60 m upper Maastrichtian chalk
overlain by 10
up to 130 m high bordering the east side of the island of Møn
(Fig. 1). This cliff provides an instructive structural cross-sec-
tion through a large glaciotectonic complex, the southern
part of which can be characterised as an imbricate fan with
thrust sheets consisting of c. 60 m upper Maastrichtian chalk
overlain by 10
-15 m glacial deposits of Weichselian age
(Surlyk & Håkansson 1999; Pedersen 2000). Store Stejle-
bjerg forms a nearly 90 m high vertical cliff section that prior
to the rock fall had an irregular overhang from about 25 m up
to about 70 m a.s.l. (Fig. 2). This overhang formed the basal
boundary of the collapse unit, which is estimated to have a
volume of 8000 m
bjerg forms a nearly 90 m high vertical cliff section that prior
to the rock fall had an irregular overhang from about 25 m up
to about 70 m a.s.l. (Fig. 2). This overhang formed the basal
boundary of the collapse unit, which is estimated to have a
volume of 8000 m
3
. The large debris fans formed by the rock
fall, constituted a southern 70 m long seawards projecting
peninsula consisting of large blocks and a northern 90 m
peninsula comprising finer-grained breccia (Fig. 3).
peninsula consisting of large blocks and a northern 90 m
peninsula comprising finer-grained breccia (Fig. 3).
The site was reinvestigated in August 2003. No fractures
were observed at the head of the rock fall, but fractures
related to the syn-sedimentary slump deformation were seen
in the lower part of the cliff with a direction parallel to the
escarpment surface. Slickenside surfaces on some of the
related to the syn-sedimentary slump deformation were seen
in the lower part of the cliff with a direction parallel to the
escarpment surface. Slickenside surfaces on some of the
largest (5 m
3
) blocks indicated displacements along more
than one fracture plane direction. The triggering of the rock
fall was interpreted to be due to a very dry spring followed by
heavy rainfall immediately preceding the cliff collapse.
fall was interpreted to be due to a very dry spring followed by
heavy rainfall immediately preceding the cliff collapse.
PROTECT
The aims of the EU-project PROTECT (PRediction Of The
Erosion of Cliffed Terrains), EU Contract No. EVK3-CT-
2000-00029, are to develop predictive tools that will identify
sections of coastal chalk cliffs that are approaching a state of
imminent collapse and allow accurate forecasts to be made
concerning the timing of the collapse. The monitoring tools
Erosion of Cliffed Terrains), EU Contract No. EVK3-CT-
2000-00029, are to develop predictive tools that will identify
sections of coastal chalk cliffs that are approaching a state of
imminent collapse and allow accurate forecasts to be made
concerning the timing of the collapse. The monitoring tools
Geological Survey of Denmark and Greenland Bulletin 4, 8992 (2004) © GEUS, 2004
Prediction and risk evaluation of chalk cliff collapse:
the PROTECT project
the PROTECT project
Stig A. Schack Pedersen and Ingelise Møller
Fig. 1. Location map of the five test sites selected for the investigation of
chalk cliff collapse in the EU-project PROTECT.
focused on are azimuthal resistivity measurements, micro-
seismicity and acoustic emission, and the project also aims at
contributing to the understanding of the physical properties
of rock masses which lead to unstable cliffs and failures. The
project works closely with user communities in order to
ensure that development is adapted to user requirements.
This includes issuing informed hazard warnings in areas
around cliffs, providing information for land-use planning in
the coastal zone and conservation regulations, and maximis-
ing the use of the cliffed coastline as an amenity (Busby et al.
2002).
seismicity and acoustic emission, and the project also aims at
contributing to the understanding of the physical properties
of rock masses which lead to unstable cliffs and failures. The
project works closely with user communities in order to
ensure that development is adapted to user requirements.
This includes issuing informed hazard warnings in areas
around cliffs, providing information for land-use planning in
the coastal zone and conservation regulations, and maximis-
ing the use of the cliffed coastline as an amenity (Busby et al.
2002).
In the PROTECT project nine partners are involved: (1)
British Geological Survey project coordinator; (2) University
of Brighton, England; (3) Bureau de Recherche Géologiques
et Minières, France; (4) Geological Survey of Denmark and
Greenland; (5) Institut National de l'environement Industriel
et des Risques, France; (6) Isle of Wight Centre for the
Coastal Environment, England; (7) Direction Departementale
de l'Equipement de la Seine Maritime, France; (8) Urzad
Morskiw Gdyni, Poland; and (9) Consorzio Ferrara Ricerche,
Italy.
of Brighton, England; (3) Bureau de Recherche Géologiques
et Minières, France; (4) Geological Survey of Denmark and
Greenland; (5) Institut National de l'environement Industriel
et des Risques, France; (6) Isle of Wight Centre for the
Coastal Environment, England; (7) Direction Departementale
de l'Equipement de la Seine Maritime, France; (8) Urzad
Morskiw Gdyni, Poland; and (9) Consorzio Ferrara Ricerche,
Italy.
The main scientific topics addressed by the project are dis-
cussed below.
Detection of fracture dilatancy
Temporal azimuthal apparent resistivity measurements are
made at the five research sites at bimonthly intervals (Figs 1,
4). The parameters required to monitor variations in the rock
mass are determined and fracture orientations are calculated
for each research site.
made at the five research sites at bimonthly intervals (Figs 1,
4). The parameters required to monitor variations in the rock
mass are determined and fracture orientations are calculated
for each research site.
Detection of cracking
Five accelerometers and five geophones were installed at the
test site in France (Mesnil Val; Fig. 1) and acquisition began
in January 2002. Initial investigations of the microseismic
activity shows that a microseismic event can be recorded on
one transducer. As a consequence, the microseismic network
is strongly recommended compared to the waveguide system,
which has also been considered (Busby et al. 2002).
test site in France (Mesnil Val; Fig. 1) and acquisition began
in January 2002. Initial investigations of the microseismic
activity shows that a microseismic event can be recorded on
one transducer. As a consequence, the microseismic network
is strongly recommended compared to the waveguide system,
which has also been considered (Busby et al. 2002).
Influence of rock and external parameters
Detailed rock mass data and erosion data are collected from
the research sites, and rock sampling for investigation of
physical properties and strength of the chalk is carried out.
Meteorological and water level data, including external tem-
perature, barometric pressure, wind velocity and direction,
and precipitation are recorded for the study of influence of
external parameters on the behaviour of the rock mass.
the research sites, and rock sampling for investigation of
physical properties and strength of the chalk is carried out.
Meteorological and water level data, including external tem-
perature, barometric pressure, wind velocity and direction,
and precipitation are recorded for the study of influence of
external parameters on the behaviour of the rock mass.
90
Fig. 2. Oblique aerial photograph of the c. 90 m high chalk cliff at Store
Stejlebjerg prior to the rock fall. The photograph was taken on 13 March
2003 as part of a systematic photogrammetric survey aiming at an aero-
triangulation of the Møns Klint cross-section for future examinations of
structural geology and landslide activity.
Fig. 3. The broad 90 m long peninsula formed by the rock fall seen from
the top of Store Stejlebjerg on 29 August 2003. The preservation poten-
tial of these breccias is not high and the peninsula will probably be lost
to erosion in about three years.
91
Fig. 4. Maps of the five test sites established for the PROTECT investigations. All locality maps drawn with north at top. Contour lines are in metres.
Each grid point is provided with a vector bar representing the displacement of the grid point based on repeated measurements. The displacement is
relative to the first measurement. Horizontal displacements shown by length of bar (upscaled by a factor of 100). Vertical displacments are shown by
colour.
92
Interpretation and integration of data
A database has been established for the data sets collected by
the PROTECT project. All surveys of fracturing and rock
strength are stored for comparison and integrated interpreta-
tion. A rock fall at the French site in June 2002 destroyed all
the connecting wires and conductors installed in January, and
had to be replaced. However, the signal obtained from this
rock fall gave a very marked precursor anomaly, which indi-
cated that the collapse could be predicted about eight hours
before it happened.
the PROTECT project. All surveys of fracturing and rock
strength are stored for comparison and integrated interpreta-
tion. A rock fall at the French site in June 2002 destroyed all
the connecting wires and conductors installed in January, and
had to be replaced. However, the signal obtained from this
rock fall gave a very marked precursor anomaly, which indi-
cated that the collapse could be predicted about eight hours
before it happened.
During the first project period a monitoring system for
direct verification of precursor movements in the terrains or
actual collapse of the cliff was established. Five field sites were
selected for the investigations: two at Møns Klint in Den-
mark (Jættebrinken and Dronningestolen), two sites at East-
bourne on the south coast of England (Beachy Head and
Birling Gap), and one site at the French coast in Normandy
(Mesnil Val; Fig. 1). Subsequent reporting from the moni-
toring system has been submitted regularly, and measure-
ments of the test grids established in the field have been
carried out at an interval of about 4 months.
actual collapse of the cliff was established. Five field sites were
selected for the investigations: two at Møns Klint in Den-
mark (Jættebrinken and Dronningestolen), two sites at East-
bourne on the south coast of England (Beachy Head and
Birling Gap), and one site at the French coast in Normandy
(Mesnil Val; Fig. 1). Subsequent reporting from the moni-
toring system has been submitted regularly, and measure-
ments of the test grids established in the field have been
carried out at an interval of about 4 months.
For testing the dislocations in the terrain a grid has been
established at each research field (e.g. Fig. 4). The grid con-
sists of a number of fixed points that have been measured
with a theodolite. In general the deviation of the measure-
ments is within
sists of a number of fixed points that have been measured
with a theodolite. In general the deviation of the measure-
ments is within
± 1 cm on the z-coordinate. The x and y
coordinates have a somewhat larger deviation, which is
mostly below
mostly below
± 3 cm (Pedersen et al. 2002). One of the
results from the test grid measurements is that the volume
involved in the collapse at Mesnil Val in June 2002 could be
calculated to 2750 m
involved in the collapse at Mesnil Val in June 2002 could be
calculated to 2750 m
3
. Moreover, it is evident that initial
creep along the fault zone displacing the Birling Gap field has
been documented by the test grid measurements (Fig. 4).
been documented by the test grid measurements (Fig. 4).
Prediction and risk evaluation of landslides
and rock falls
and rock falls
In order to predict rock falls the sites for potential collapse
must be identified and the triggering mechanism has to be
understood. The sites with most potential for collapse are of
course overhanging cliffs; however, some of the collapses are
also related to older fault and fracture systems, which
must be identified and the triggering mechanism has to be
understood. The sites with most potential for collapse are of
course overhanging cliffs; however, some of the collapses are
also related to older fault and fracture systems, which
demand a structural analysis to provide a structural model for
prediction of risky sites. GEUS will continue its mapping of
Møns Klint, and when the potential sites for collapse have
been identified it will be possible to install monitoring equip-
ment for collapse warning. The experiences from PROTECT
suggest that acoustic emission is a promising tool, and the
Survey intends to continue cooperation for developing
improved prediction equipment.
prediction of risky sites. GEUS will continue its mapping of
Møns Klint, and when the potential sites for collapse have
been identified it will be possible to install monitoring equip-
ment for collapse warning. The experiences from PROTECT
suggest that acoustic emission is a promising tool, and the
Survey intends to continue cooperation for developing
improved prediction equipment.
Considering the risk evaluation, one evaluation parameter
is the past frequency of rock falls. The record of landslides at
Møns Klint over the last 100 years indicates that a major
landslide or rock fall will occur about every fifth year
(Pedersen 2003). Another parameter could be to estimate the
erosion rate. At Møns Klint the erosion rate varies from zero
at Jættebrinken to nearly 50 cm per year north of Dronninge-
stolen (Pedersen 2003). The average erosion rate is about 35
cm per year, which is relatively small compared to the erosion
rate of about 70 cm per year along the English Channel (data
from the PROTECT data files). In spring 2003, GEUS was
asked to provide a landslide risk analysis for a projected exhi-
bition centre at Møns Klint. Based on a structural model for
the location of the centre and the erosion rate estimates the
main conclusion was that the centre would not be threatened
by coastal landslide erosion in the foreseeable future
(Pedersen 2003).
Møns Klint over the last 100 years indicates that a major
landslide or rock fall will occur about every fifth year
(Pedersen 2003). Another parameter could be to estimate the
erosion rate. At Møns Klint the erosion rate varies from zero
at Jættebrinken to nearly 50 cm per year north of Dronninge-
stolen (Pedersen 2003). The average erosion rate is about 35
cm per year, which is relatively small compared to the erosion
rate of about 70 cm per year along the English Channel (data
from the PROTECT data files). In spring 2003, GEUS was
asked to provide a landslide risk analysis for a projected exhi-
bition centre at Møns Klint. Based on a structural model for
the location of the centre and the erosion rate estimates the
main conclusion was that the centre would not be threatened
by coastal landslide erosion in the foreseeable future
(Pedersen 2003).
References
Busby, J.P., Gourry, J.C., Senfaute, G., Pedersen, S. & Mortimore, R. 2002:
Can we predict coastal cliff failure with remote, indirect measure-
ments. In: McInnes, R. & Jakeways, J. (eds): Instability, planning and
management, 203208. London: Thomas Telford.
ments. In: McInnes, R. & Jakeways, J. (eds): Instability, planning and
management, 203208. London: Thomas Telford.
Pedersen, S.A.S. 2000: Superimposed deformation in glaciotectonics.
Bulletin of the Geological Society of Denmark 46, 125144.
Pedersen, S.A.S. 2003: Vurdering af skredrisiko for området oven for
Maglevandsfaldet på Møns Klint. Danmarks og Grønlands Geologiske
Undersøgelse Rapport 2003/50, 35 pp.
Undersøgelse Rapport 2003/50, 35 pp.
Pedersen, S.A.S., Møller, I. & Gudmunsson, L. 2002: Test grid established
for the EU-project PROTECT at cliffed terrains in Denmark, England
and France. Danmarks og Grønlands Geologiske Undersøgelse
Rapport 2002/30, 30 pp.
and France. Danmarks og Grønlands Geologiske Undersøgelse
Rapport 2002/30, 30 pp.
Surlyk, F. & Håkansson, E. 1999: Maastrichtian and Danian strata in the
southeastern part of the Danish Basin. In: Pedersen, G.K. &
Clemmensen, L.B. (eds): Field trip guide for 19th Regional European
Meeting of Sedimentology (August 1999), 2958. Copenhagen: IAS.
Clemmensen, L.B. (eds): Field trip guide for 19th Regional European
Meeting of Sedimentology (August 1999), 2958. Copenhagen: IAS.
Authors' address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sasp@geus.dk
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sasp@geus.dk
