INTRO | HIGHLIGHTS | FEATURES | INTERVIEWS | PERSPECTIVES
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EMERGENT BEHAVIOUR
is needed. ‘We’re trying to look at those spatial relationships
and also how fast that happens through time’, says Rosser.
IN THEIR RESEARCH PAPER published in the journal
Geology, Rosser’s team reported on sequences of rock falls
that occurred at the coastal cliffs of the North York Moors.
‘Rock falls cluster in space, but they cluster in time as well’,
says Rosser. The study identified precursors to the largest
rock falls. ‘Even though the monitoring is high-resolution,
the data is very noisy’, says Rosser, ‘there is rarely a perfectly
clear signal’. This is because there are many different things
happening at the cliff face including weathering, but also
variations in the rock, such as its strength, along with the
shape and structure of the cliff itself.
Siobhan Whadcoat is developing a model that will use the
long-term data set on the North York Moors coastal cliffs
to simulate these patterns for the first time. This model
incorporates failures around joints in the rocks, how failures
interact, and how rock falls develop over time.
Identifying risk indicators for rock falls before the ‘big one’
is not as straightforward as it would seem, because the cliff
is undergoing so many changes at once at different scales.
However, Rosser and his research team have managed to
identify patterns in the ‘noisy’ cliff.
Whilst smaller rock falls at the cliff face may appear to occur
randomly, successive terrestrial laser scan data demonstrate
‘clustering’ of rock fall events in space and time. Small rock
falls are always much more frequent than large rock falls.
Previously Rosser and his team recorded over 500,000
rock falls, but less than 100 of those made any noticeable
difference to the cliff line observable by the human eye,
and the changes were too small to be picked up by aerial
photography or in maps.
/// KEY MESSAGES FOR POLICY
-
The cause of rock fall hazards
at coastal cliffs cannot only be
attributed solely to environmental
drivers, such as rain or marine
erosion. Rock falls evolve over time
and respond to a range of preparatory
and triggering factors.
Whadcoat’s PhD research aims to account for the internal
processes that cliff erosion models have not represented
well in the past. She will also investigate the ways rock
falls cluster together, which seem to hold clues as to how
other rock slopes may also behave. ‘Overarching failure
patterns could also apply to rock fall failures in non-coastal
environments, in areas such as Yosemite in California’,
says Whadcoat.
-
Hard rock cliff faces that appear
solid and stable are likely to be
experiencing an ongoing reduction in
rock mass strength which may in time
result in failure.
In researching the mechanisms that control rock falls,
Whadcoat hopes to uncover what triggers events to occur
suddenly, and to develop over long periods of time. This is
important because some of these unique characteristics
of rock cliff behaviour are most likely universal, and worth
knowing for any community that lives near a coastline or in a
mountainous region where adaptation or mitigation of rock fall
hazards is of paramount importance.
- rock fall model that is based on
A
physical data and is inclusive of
rock mechanics that determine the
collapse of cliffs would improve
understanding of how rock falls occur
along coastlines.
-
Precursors may exist for large rock
falls, a sequence of smaller rock
falls for example, and these provide
a warning before the main rock fall
event takes place.
Most of the failures the team monitored with TLS were
shallow in depth, but would they have occurred regardless
of any external environmental drivers, such as wave erosion?
With more data the research team will be able to confirm
whether small sections of the cliff falling one after the other is
indeed an emerging pattern of rock falls. Further investigation
/// REFERENCES AND
FURTHER READING:
Brain, M.J., Rosser, N.J., Norman, E.C. &
Petley, D.N. (2014) Are microseismic ground
displacements a significant geomorphic agent?
Geomorphology, 207, pp. 161–173.
Norman, E.C., Rosser, N.J., Brain, M.J., Petley,
D.N. & Lim, M. (2013) Coastal cliff-top ground
motions as proxies for environmental processes.
Journal of Geophysical Research – Oceans, 118
(12) pp. 6807–6823.
‘IMAGINE THE CLIFF AS EQUIVALENT
TO A BIG GAME OF JENGA,YOU PULL
A CHUNK OUT OF THE BOTTOM,
THEN AFTER A CERTAIN AMOUNT OF
TIME THE BIT OF THE JENGA BLOCK
DIRECTLY ABOVE IT MIGHT FALL,
BECAUSE IT’S WEAKENED’.
Dr Nick Rosser
Rosser, N.J., Brain, M.J., Petley, D.N., Lim, M.
& Norman, E.C. (2013) Coastline retreat via
progressive failure of rocky coastal cliffs. Geology,
41, pp. 939–942.
For further information about this research
from the Coastal Behaviour and Rates of
Activity (COBRA) project visit: http://www.
dogweb.dur.ac.uk/cobra/. The research is
supported by Cleveland Potash Ltd. Contact
Dr Nick Rosser: [email protected]
Credit: Camila Caiado.
More than weathering causes coastal cliffs to collapse.