Geomorphic responses of coral coasts to tropical cyclones. Recent examples from French Polynesia and
This talk will present the diversity of geomorphic responses of coral coasts to high energy event such as tropical cyclones. Two recent cases will be examined: the impacts of Tropical Cyclone Oli in French Polynesia and those of TC Tomas in
. Both storms formed during the
2009-10 South Pacific cyclone season. Fiji
Tropical Cyclone Oli struck the western part of
French Polynesia in February 2010.
Submarine reef erosion is quantified through coral colony degree of destruction
and massive coral colony displacement. Sediment transport and beach retreat are
quantified, and flow velocities at the coastline are estimated through boulder
analysis. In this particular event, outer reef slope angle appears as a major
control factor for coral destruction with vertical submarine cliffs relatively
shielded compare to gentle slopes (Tubuai, Tahiti).
Submarine boulder measurements provide valuable estimates of flow velocity
profile with depth. Beachrock slabs measurements provide also estimates of flow
velocities at the reef/beach junction. Combining these different geomorphic
markers might be a way to apprehend the flow velocity variation when the
cyclone waves cross the coral reef.
Tropical Cyclone Tomas, a category-4 intensity system, battered northern and eastern islands ofThe study of geomorphic impacts of modern high energy events on coral coasts helps to improve the interpretation of ancient sedimentary deposits associated with coastal flooding. A better knowledge of marine inundation history (frequency and intensity) is crucial in the perspective of natural hazard mitigation.
2010. The striking impression overall was the range of cyclone constructional
imprints that nourished existing coastal sediments. Fresh coral boulders strewn
across reef platforms indicate the power of TC Tomas was sufficient to deliver
new coral blocks (mostly derived from fore-reef sources rather than the reef
surface). Flow velocities of the storm surge at the shoreline were estimated
through mathematical modeling of angular beachrock slabs plucked out of in situ exposures. Coralline shingle
tongues encountered on the coastline, together with isolated beach ridges and
sand-sheets in both beach and back-beach locations indicate how the variable
and discontinuous nature of cyclogenic accretionary features poses a greater
challenge in terms of identification and interpretation in the sedimentological
record compared with tsunamigenic deposits on affected coastlines. Fiji