A study entitled “Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan” (http://www.earth-surf-dynam.net/3/543/2015/esurf-3-543-2015.html) was recently published in the European Geosciences Union’s open-access journal Earth Surface Dynamics. The paper is the result of a collaboration between the Geoarchaeology, Coastal Geomorphology, Geochronology Working Group of the University of Cologne and researchers from DOST – Project NOAH (Nationwide Operational Assessment of Hazards) and the University of the Philippines Marine Science Institute (UP MSI). Simon Matthias May, Max Engel, Dominik Brill and Helmut Brücknerfrom the University of Cologne designed the study while Camille Cuadra, Mahar Lagmay, Joy Santiago and Kenneth Suarez did the Delft3D modelling. Fieldwork was first done in Eastern Samar in February 2014 with Michelle Reyes of UP MSI. A second field survey was done in March 2015 with the help of the Project NOAH Storm Surge Component.
The study presents evidence for onshore block and boulder dislocation on the coast of Eastern Samar, Philippines during Typhoon Yolanda – one of the strongest tropical cyclones ever recorded. Field surveys in Hernani, Eastern Samar were conducted to measure the elevation of dislocated clasts using differential global positioning system (DGPS). The clasts were classified as “transported”, “not transported”, or “possibly transported” during Haiyan based on vegetation cover, weathering patterns and color of rock surfaces, the freshness of buried plant debris, and satellite imagery. Longshore transport of blocks reaching up to 180 t and boulders up to 23.5 t that were shifted upslope to elevations of up to 10 m above mean lower low water level during Yolanda were documented and the required minimum flow velocities needed to transport the clasts were computed.
A high resolution storm surge model coupled with a wave model was created using Delft3D software to determine the flow velocities during Typhoon Yolanda and results show that modeled flow velocities are insufficient to account for the transport of the documented clasts. Results of hydrodynamic and wave modelling support the hypothesis that infragravity waves, which are not resolved in phase-averaged storm surge models, were responsible for the remarkable flooding pattern in Eastern Samar.
The paper provides insights about the hydrodynamic and sedimentary processes during a tropical cyclone using sedimentary parameters of the clasts collected on field, bi-temporal satellite images, characteristics of the storm surge and waves inferred from local nucolmerical models, and inverse modelling of minimum flow velocities required to initiate boulder movement.
Regardless of the mechanisms responsible for the exceptional coastal flooding pattern, the sedimentary findings presented in the paper give striking evidence of high run-up and storm wave and surge accompanying sustained currents along the coast of Eastern Samar capable of transporting block-sized clasts over horizontal distances up to ~40 meters and producing spatially randomized clast distributions, both of which are often associated with tsunami deposition thus demanding a careful re-evaluation of storm-related transport. Dislodged boulders and blocks record catastrophic coastal flooding during strong storms or tsunamis and play an important role in coastal hazard assessment.