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ROCK MECHANICS
MUCH OF THE RESEARCH on coastal cliff hazards such as
rock falls is on cliff retreat – the movement of the coastline –
and not erosion itself. Whilst erosion at the toe is important,
it is not the only contributing factor to the development of
the whole cliff via rock fall. This may be especially true when
it comes to the larger rock failures when a larger part of the
cliff suddenly gives way.
A rock fall recorded at one of the field sites on the North
York Moors coast caused the cliff to step back 13 metres,
generating a local tsunami. Rock falls on this scale are not
well represented in current models because of the degree
of complexity involved and the lack of monitoring data that
includes such infrequent events. The timing and triggers of
this rock fall and similar failures remain tricky to identify.
An important finding from this research has identified that
‘the development of the rock fall is primarily driven by
external processes, such as wind and rain, but beyond this,
internal controls, such as the fracturing and cracking of the
Staithes low tide.
Credit: Emma Norman.
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rock itself, control when the rock fall occurs, its size and
nature’, explains Siobhan Whadcoat, who is modelling rock
falls identified in research for her PhD in the Department
of Geography. Evidence to support this idea comes from
sequences of smaller rock falls which occur before larger
events; these smaller events can be precursors to a larger
collapse later.
There is currently debate over environmental changes that
may affect coastlines in the future, particularly those that
can potentially be accelerated by the impacts of climate
change. By definition, the previous focus only on the external
mechanisms that affect coastal rock cliffs overlooks the
internal processes that cause cliff rocks to break down
over time.
Similar to landslides observed elsewhere, which sometimes
occur without an immediately obvious trigger, coastal cliff
rock falls are highly unpredictable, making them challenging
to model conventionally. A rock fall may, for example,
occur according to the gradual fracturing of the cliff rock,
via a mechanism termed ‘progressive deformation’, where
microcracking takes place within the cliff rock over time.
Research in rock mechanics is useful for understanding
Staithes mid tide.
these processes, and might apply to coastal cliff failure,
showing the influence of structure, strength, size, and
tendency to fracture.
Waves continue to influence the cliffs during low tide:
breaking waves on the foreshore, but also from further
offshore, both transfer energy to the sea bed, which
propagates as seismic waves directly to the cliff. Such effects
are typically at their most intense during high spring tides
where high water allows larger waves to reach the toe of the
cliffs where they break. These processes cause the cliff to
vibrate, a process that holds some influence on occurrence of
observed cliff rock falls. This observation is one of the main
findings from research by Dr Emma Norman, who recently
completed her PhD on coastal cliffs erosion on the North
York Moors coast.
‘A small transfer of energy, say, from ocean waves,
could be enough to trigger a rock fall, but the
magnitude of the energy delivered by the waves
doesn’t appear to directly relate to the magnitude
of the rock fall that happens at that point in time’,
says Norman.
Staithes high tide.
During storms, as large waves hit the cliff face, there may be
a cumulative effect that gradually or progressively weakens
the cliff rock. In addition, if a small rock fall occurs, it can
weaken the area around it by removing support, initiating a
sequence of rock falls to occur over time. This would explain
why a cliff along the coast that appears perfectly stable
one moment collapses suddenly without warning the next,
without an obvious trigger.
‘When a fragment of rock falls away there’s a change in the
stress distribution through the cliff rock which then may
cause further fracture, and, eventually, failure’, says Norman.
Processes affecting the rock cliff may include strong wind
action, salt crystal growth, heavy rain or, during a storm,
large waves crashing into the cliff, transferring large amounts
of energy that can cause the cliff to shake. During large
storms, local residents who live near the cliff have claimed
that they could ‘feel’ su ch ground motions.
While it is clear that waves transfer energy to the cliff face, it
is unclear whether they alone are causing rock falls to occur.
Rock falls are complex, but there may be indicators that
scientists can look for in order to predict when they occur.