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| Titel |
Low retreat rate sea cliff monitoring by photogrammetric methods |
| VerfasserIn |
F. Marques, P. Redweik, R. Matildes, L. Santos |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028296
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| Zusammenfassung |
| Sea cliff evolution is dominated by the occurrence of mass movements of different types
and sizes, being a considerable source of natural hazard in coastal areas. Hazard
assessment requires the use of substantially complete inventories of cliff failures
occurred in the past, which are usually produced by multi-temporal aerial photo
based studies, where photogrammetric methods still combine the advantages of
high accuracy and the possibility of monitoring periods of time in excess of half a
century.
However, the study of low retreat rate cliffs still involves problems, because parts of the
cliff top contour remain unchanged during the monitoring period and retreat is restricted to
the places where cliff failures have occurred. This situation creates problems related with
the inherent difficulties of a multi-temporal processing of aerial photos (different
flight directions and scales, insufficient ground control for older photos and poor
radiometric quality) and requires the use of refined photogrammetric approaches to
prevent the generation of false mass movements, which cannot be included in the
inventories.
Because cliff retreat is discontinuous in space and time, better sampling of the process
requires the widening of the monitoring time window, by use of older aerial photos, which
involve further problems as unavailable camera calibration and photo distortions. To address
these problems, the sea cliffs of Burgau-Lagos coast (Southwest Algarve, Portugal) were
studied using digital photogrammetry methods, which involved several aerotriangulation
steps, generation of pseudo-camera data for the older aerial photos, stereo plotting of the cliff
top, ridges and toe and automatic generation of digital terrain models. Inconsistencies in the
results were solved by systematic stereo photo interpretation, supported by oblique aerial
photos.
This study enabled the detection and characterization of 137 cliff failures that occurred
between 1947 and 2007 along the 13 km long cliffs, causing the loss of 10,234 m2 of
horizontal area at the cliffs top. The cliff failures identified correspond to planar slides (58%)
mainly in Cretaceous alternating limestone and marls, toppling failures (17%) mainly in
Miocene calcarenites, slumps (15%) in Plio-pleistocene silty sands that infill the karst in the
Miocene rocks, and the remaining 10% correspond to complex movements, rockfalls and not
determined cases.
The space distribution of cliff failures is quite irregular but enables the objective
separation of sub sections with homogeneous retreat behavior, for which were computed
mean retreat rates varying within one order of magnitude from 5x10-3m/year in strong
sandstones to 5x10-2m/year in Miocene calcarenites with frequent karst sinkholes filled with
Plio-pleistocene silty sands, reflecting the strength variations of the rock masses that compose
the cliffs. The maximum value of local retreat of the cliff top was up to 33m, with more
frequent values in the range 2m to 6m. The relation between the magnitude expressed by the
maximum local retreat and frequency follows an inverse power law for values higher
than 2m with exponent of 1.8, close to the values recently proposed (Marques,
2008).
The photogrammetric methods used in this study, and specially the strategies to obtain
reliable information from old aerial photos, provided an accurate characterization of the cliffs
evolution and supported a reliable compilation of a systematic inventory of cliff failures
occurred in a 60 years period. These methods provided a confirmation and enhancement of a
previous inventory made with simplified methods, providing a more detailed picture of the
space and time distribution of cliff failures, but without changing substantially the general
pattern of the studied phenomena.
The use of these methods requires a considerable degree of expertise and resources and,
for reliable compilation of an inventory data set, a final systematic checking of each cliff
retreat event is still required.
The specific topic of DDEM generation for reliable assessment of volumetric data is still
in development and may provide a significant improvement of the outputs of these methods in
the particular topic of low cliff retreat rate monitoring.
Reference:
Marques, F.M.S.F. (2008) Magnitude-frequency of sea cliff instabilities. Nat. Hazards
Earth Syst. Sci., 8, 1161-1171. |
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