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| Titel |
Monitoring the degradation of weathered volcanic man-made escarpments by close-range photogrammetry in Tahiti, French Polynesia |
| VerfasserIn |
Thomas Dewez, Aude Nachbaur, Olivier Sedan, Emilie Nowak, Emmanuel Des Garets |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048627
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| Zusammenfassung |
| Urban growth on the extinct volcanic island of Tahiti, French Polynesia, induces developers
to build houses on steep mountain sides. To increase horizontal building space, workers end
up cutting extensive horizontal benches delimited by steep taluses. Such man-made
escarpments have a typical gradient of 5/2 (i.e. 70-) and heights reaching locally 15 m, some
are protected by nets and spray-crete but most remain unprotected. Such steep artificial
slopes, dug in tropically weathered volcanic material and exposed to tropical rainy climate
have become a source of concern to inhabitants and land planning authorities: the
scarps are progressively degraded through time, thus diminishing usable horizontal
surfaces and eroded sediments eventually pollute the lagoons downstream. BRGM
was put in charge by French Polynesia’s Land Planning Authority to develop a
practical means of monitoring talus degradation and quantify talus erosion rates. In the
absence of very costly terrestrial laser scanning equipment, we proposed the rather
less costly technique of ground-based photogrammetry. A Pentax K200D with
21-mm-calibrated lens was used along with Photomodeler Scanner version 6 to compute 3D
models of escarpments. Two strategies were followed : first a set of 17 different
escarpment profiles originally surveyed in 1991 were measured anew in January 2009 to
address decadal scarp profile evolution; and second, the monthly evolution of a
fresh section cut into so-called Mamu 3-4, a heavily argilized basaltic section with
pockets of scoriae. This presentation addresses the field photogrammetric setup
used to measure the topographic evolution of escarpment profiles, and discusses
the erosion rates gathered from this study. 75% of observed profile erosion rate
are contained between 5 mm/a and 56 mm/a. Monthly surficial evolution of the
monitored road cut showed that the escarpment behaved as a detachment limited system. |
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