Abstract
Abstract. Fields of dislodged boulders and blocks record catastrophic coastal flooding during strong storms or tsunamis and play a pivotal role in coastal hazard assessment. Along the rocky carbonate coast of Eastern Samar (Philippines) we documented longshore transport of a block of 180 t and boulders (up to 23.5 t) shifted upslope to elevations of up to 10 m above mean lower low water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion approaches indicate that boulder dislocation occurred with flow velocities of 8.9–9.6 m s−1, which significantly exceeds depth-averaged flow velocities of a local coupled hydrodynamic and wave model (Delft3D) of the typhoon with a maximum < 1.5 m s−1. These results, in combination with recently published phase-resolving wave models, support the hypothesis that infragravity waves induced by the typhoon were responsible for the remarkable flooding pattern in Eastern Samar, which are not resolved in phase-averaged storm surge models. Our findings show that tsunamis and hydrodynamic conditions induced by tropical cyclones may shift boulders of similar size and, therefore, demand a careful re-evaluation of storm-related transport where it, based on the boulder's sheer size, has previously been ascribed to tsunamis.
Highlights
We present evidence for onshore block and boulder dislocation at the carbonate coast of Eastern Samar (Philippines; Fig. 1) during Supertyphoon Haiyan, one of the strongest tropical cyclones (TCs) on record
Regardless of the mechanisms responsible for the exceptional coastal flooding pattern, the sedimentary findings presented here give striking evidence of very high run-up and strong wave- and surge-accompanying sustained currents along the coast of SE Samar during Supertyphoon Haiyan. They were capable of transporting blocksized clasts over horizontal distances of up to ∼ 40 m and producing spatially randomized clast distributions, both of which are often associated with tsunami deposition. Based on their SE–NW trajectory and a surge-perpendicular orientation of their longest axis (Fig. 3), we conclude that the exceptional flooding pattern, caused by wave setup and infragravity waves, induced the transport of the largest clasts rather than the high breaking waves alone
The shore-parallel orientation of the slabshaped boulders on top of the carbonate platform may suggest that superimposed waves, having reached heights of more than 5 m (Bricker et al, 2014), contributed to their trajectory as well
Summary
Fields of dislocated boulders and blocks are among the most impressive sedimentary evidence of catastrophic coastal flooding (Williams and Hall, 2004; Scicchitano et al, 2007; Goto et al, 2010, 2011; Etienne et al, 2011; Nandasena et al, 2011; Richmond et al, 2011; Engel and May, 2012; Terry et al, 2013) and are widely used to infer the most extreme magnitudes of marine flooding events (tsunamis, storm surges) over large timescales (Etienne et al, 2011; Engel and May, 2012; Terry et al, 2013). Storm transport was ruled out based on their large size, elevation and distance from the coast, and local extreme storm wave conditions (Scicchitano et al, 2007; Engel and May, 2012), while the long wave period of tsunamis has been associated with a higher transport competence (Lorang, 2011). Questions about the reliability of the report on the storm wave transport in 1912 have been raised, citing pre-1912 photographs of the boulder in its present position (Cass, 2002; Scheffers et al, 2008)
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