Failures of Germany's largest cliff coast sensed by seismometers
Dry summers are increasing the risk of large cliff collapses

Date:
August 28, 2020
Source:
GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
Summary:
In a study carried out over more than two years, scientists were
able to draw a new and surprisingly detailed picture of coastal
cliff failure activity.



FULL STORY ==========================================================================
The 10-kilometer-long, bright white coast of Germany's largest island,
Ru"gen, is shaped by episodically occurring failures. These failures were typically assumed to happen due to strong rain storms. In a study carried
out over more than two years, scientists of GFZ German Research Centre for Geosciences were able to draw a new and surprisingly detailed picture of coastal cliff failure activity. The study is based on continuous seismic measurements and high resolution aerial images, and has been published
in the Journal of Geophysical Research.


==========================================================================
The chalk cliff coast of Ru"gen, with the 118-meter-high Ko"nigsstuhl rock column, is a renowned landmark of the National Park Jasmund. The cliff is formed by fossil calcareous algae and retreats by up several decimeters
per year on average. That retreat is spectacularly accomplished by the break-off of large chalk chunks (in this study up to 5000 m3) that cause
strong impacts on the beach. These impacts generate small earthquakes
that were sensed by the scientists using seismometers; a network of such sensors was installed along the coast between spring 2017 and 2019. Along
with meteorological data and drone-based 3D models of the cliff, the
seismic data enabled the routine collection of essential data about
failures that were previously available only in rare cases: timing,
duration and evolution of single events with sub-second resolution,
position and volume of detachment and deposit zones, and properties of
the ground around the occurrence of an event.

Based on more than 80 detected failures, the scientists were able
to identify several interconnected drivers. "Sufficient wetting of
the chalk by water is the primary reason," says Michael Dietze, lead
author of the study. "However, the water is not only provided by rain
onto the cliff surface, but also by ground water flow and condensation
of air moisture during the colder night time hours. Consequently, the
cliff is preferentially active at night, as well as a few hours and
also a day after rain events." Failures predominantly happen in winter,
because during that season the trees are not able to withdraw soil water, causing wetter ground conditions. The study found a dramatic effect of
wetter and drier than average summers. In 2017, when 126 % of the typical
rain amount was measured, a total of 65 failures happened. In contrast,
the exceptional drought summer of 2018 (51 % of the average rain amount) resulted in only 11 cliff failure events during the following winter.

The long term effects of such dry summers for the cliff remain a matter of debate. Generally, it is the small failures that provide the beach with
enough loose material to prevent the waves from cutting into the basal
parts of the cliff. If that sediment is lacking, wave driven undermining
at the base will prepare the cliff for large scale sector collapse. The scientists have now instrumented the neighboring island of Hiddensee,
to investigate the direct effect of waves in more detail. In addition,
the northern part of this island is periodically struck by massive slope failures, most recently in winter 2019, when a block of several hundred
meter length moved downslope by more than 20 cm.


========================================================================== Story Source: Materials provided by GFZ_GeoForschungsZentrum_Potsdam,_Helmholtz_Centre. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. M. Dietze, K. L. Cook, L. Illien, O. Rach, S. Puffpaff, I. Stodian,
N.

Hovius. Impact of Nested Moisture Cycles on Coastal Chalk
Cliff Failure Revealed by Multiseasonal Seismic and Topographic
Surveys. Journal of Geophysical Research: Earth Surface, 2020;
125 (8) DOI: 10.1029/ 2019JF005487 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200828115354.htm

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