Articles | Volume 3, issue 4
Research article
01 Dec 2015
Research article |  | 01 Dec 2015

Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan

S. M. May, M. Engel, D. Brill, C. Cuadra, A. M. F. Lagmay, J. Santiago, J. K. Suarez, M. Reyes, and H. Brückner

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.

Short summary
Block and boulder fields record catastrophic coastal flooding events and play a pivotal role in coastal hazard assessment. After Supertyphoon Haiyan on 8 Nov 2013 the transport of extremely large blocks of up to 180 t in E Samar (Philippines) was documented, indicating that hydrodynamic conditions induced by tropical cyclones, including infragravity waves, and resulting coarse-clast transport patterns may be comparable to tsunamis.