 |
| Titel |
Five seasons of chalk cliff face erosion monitored by terrestrial laser scanner: from quantitative description to rock fall probabilistic hazard assessment |
| VerfasserIn |
T. Dewez, G. Chamblas, E. Lasseur, R. Vandromme |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026345
|
|
|
|
|
|
| Zusammenfassung |
| Coastal cliffs are ubiquitous landforms around the world and cliff recession is a major subject
of concern to coastal communities. These elements argue for a better understanding of
processes occurring along coastal cliffs. The purpose of the study is to carefully describe the
phenomenology of erosion on an evolving chalk coastline. Six repeated terrestrial
laser scanner surveys were carried out for 2.5 years on a 700-m-long coastal chalk
(Lewes Chalk – Upper Cretaceous) cliff site at Mesnil Val in Normandy, a setting
representative of about a quarter of the English Channel’s chalky coast. We describe
the methodology used to process digital surface models and the novel technique
developed to discriminate erosion from noise and to build a catalogue of eroded patches.
Terrestrial Laser Scanner surveys permit the detection of any erosion patch thicker
than ca. 3 cm. The catalogue of erosion patches contains more than 8500 objects
with volumes spanning eight orders of magnitude, from 10-4 m3 up to 104m3.
The average erosion rate is 0.13 m/a if one excludes a rare single massive cliff
collapse event of 70Â 000 m3that occurred in March 2008, or 0.94 m/a if this event is
included. A scaling power law relationship exists between event volume and chalk
production that implies that erosion is not dominated by any characteristic event.
This scaling relationship is different for summers and winters showing clearly that
more chalk is eroded in small patches in the winter than in the summer, whereas
erosion of blocks larger than 10m3 are just as probable in the summer as in the
winter.
Probabilistic block fall hazard relationships were derived in order to predict the
recurrence of events of a given size. The relationship between block fall volume and
frequency takes the shape of a negative power law : F = a V -b, with F being the frequency, V
the volume of a block, a = 53.568 and b = 0.51.
We draw the conclusion that repeated topographic surveys with terrestrial laser scanners
are appropriate tools for assessing block fall hazard on a given cliff face, much like
seismometers are for seismic hazard assessments. |
|
|
|
|
|
|