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Reef-top platform coral boulders of Eastern Samar demonstrate the long-term coastal hazard of extreme waves

TitleReef-top platform coral boulders of Eastern Samar demonstrate the long-term coastal hazard of extreme waves
Publication TypeConference Paper
Year of Publication2018
AuthorsEngel, M, Boesl, F, Eco, RN, Galang, JA, Gonzalo, LA, Llanes, F, Quix, E, Oetjen, J, J. Suarez, K, Cuadra, C, A. Lagmay, MF, Schroeder-Ritzrau, A, Frank, N, Schüttrumpf, H, Brückner, H
Conference NameBOOK OF ABSTRACTS - Central European Conference on Geomorphology and Quaternary Sciences Joint Conference of AKG and DEUQUA University of Giessen, Germany September 23 – 27, 2018

The Eastern Visayas region in the Philippines experiences some of the most violent tropical cyclones on Earth, exempli{fi}ed by Typhoon Haiyan (7–9 November 2013) or Typhoon Hagupit (6–8 December 2014). Moreover, strong earthquakes along the Philippine Trench have triggered tsunamis in the past, both implying signi{fi}cant hazards of coastal {fl}ooding through extreme waves for the Paci{fi}c coast of the island of Samar. Due to the very short and fragmented historical record of the region, not much is known about frequency-magnitude relationships and maximum magnitudes on centennial and millennial scales, which can be derived from geological traces and which should be considered in coastal hazard management. We studied a large boulder {fi}eld along the north coast of Eastern Samar distributed over an elevated reef platform to understand mechanisms of boulder transport and to derive implications for the maximum spatial extent, depth and velocity of coastal {fl}ooding. In this paper, we compare the field observations to physical experiments of boulder transport by extreme waves currently undertaken in a flume of the Methods: (i) Documentation of location, shape, morphological features, length, orientation of main axes of >250 boulders (1.5 m<a-axes<11.9 m) in the {fi}eld; (ii) UAV-based 2D/3D- mapping; (iii) creation of SfM-based models of prominent boulders; (iv) interviewing elders of the local community for past events; (v) inverse modelling of coastal {fl}ooding and comparison with Deft3D-based numerical models of Haiyan and Hagupit; (vi) multi-temporal analyses of Pléiades and Worldview-3 scenes to reconstruct boulder movement during recent events; (vii) estimate the age of the carbonate platform and the timing of transport through 230U/Th dating. Preliminary results: (i) the platform{'}s age is mid-/late Holocene and formed through relative sea-level fall; boulder transport occurred over the late Holocene; (ii) Haiyan and Hagupit shifted boulders up to 115 t in steps of <32 m only at the seaward margin of the boulder {fi}eld; (iii) transport during Haiyan and Hagupit clearly re{fl}ects the individual approaching angle of waves; (iv) size-distance relationships of the entire boulder {fi}eld are unclear (r2=0.46 at best) and large clasts are located up to 1.3 km from the platform edge indicating that also major long-period waves (infragravity waves, tsunamis) have occurred in the past; (vi) {fl}ow velocities of up to 6–7 m/s were inferred for Hagupit, while largest clasts more inland (up to 11.9x8.1x4.2 m3; 433 t) require minimum values >10 m/s.

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