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| Titel |
Wave-flume experiments of soft-rock cliff erosion under monochromatic waves |
| VerfasserIn |
Vincent Regard, Dominique Astruc, Bastien Caplain |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250154282
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| Publikation (Nr.) |
 EGU/EGU2017-19357.pdf |
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| Zusammenfassung |
| We investigate how cliffs erode under wave attack. Rocky coast erosion works through
cycles, each one corresponding to three successive phases: (i) notch creation at cliff toe by
mechanical action of waves, (ii) cliff fracturation leading to collapse, and (iii) evacuation of
scree aprons by waves and currents. We performed experiments in a 5m x 14cm x 25cm wave
flume (15 cm water depth) to investigate how waves are eroding a rocky coast. The cliff is
made of wet sand and models a relatively soft rock. We used 3 different grain size (D50 =
0.28-0.41-0.48 mm), changing the cliff rheology. Waves are monochromatic; their
height and period differ for the various experiments. Actual wave parameters are
estimated by capacitive probes located offshore. The experiments are monitored by two
video cameras both on the side and above the flume. Pictures are taken at a rate
of 1Hz during the first 4h and then the rate is decreased to 0.1Hz till the end of
experiment (about 1 day). The monitoring ensure a confident characterization of
experiments in terms of waves (surf similarity parameter ξ and the incident wave
energy flux F) and in terms of sediment (Dean number Ω and Shields number θb at
breakers).
Experiments begin by an initial phase of quick cliff retreat. Then the system evolves with
slower cliff retreat. We focus on bottom morphology which we characterize in function of
wave forcing (ξ, F). We show that the bottom morphology mainly depends on ξ. For our
reference sediment (Dm = 0.41 mm), we observed: (i) surging breakers on a steep terrace
(type T1) for ξ > 0.65; (ii)collapsing breakers on a bared profile attached to the
inner platform (type T2) for 0.55< ξ <0.6; (iii) spilling breakers on gentle terrace
(type T3) for F < 1.3 W/m and 0.55< ξ <0.6. Another bottom morphology, type
T4, displays two sub-systems, an outer system with a double-bar profile where
breaking waves are plunging, and an inner system with a T1, T2 or T3 profile.
Some of these bottom morphologies are unsteady with sandbar oscillation. When
changing sediment grain size, we observed that the bottom typology is similar but
evolves in function of the Ω value. Finally, we observed that the cliff recession is
proportional to F, is not monotonic with ξ and decreases with the sediment grain
diameter. |
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