Block and Boulder Transport in Easterm Samar (Philippines) during Supertyphoon Haiyan

Fields of dislodged boulders and blocks record catastrophic coastal ?ooding 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 180t and boulders (up to 23.5t) shifted upslope to elevations of up to 10m above mean lower low water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion approaches indicate that boulder dislocation occurred with ?ow velocities of 8.9– 9.6ms-1, which signi?cantly exceeds depth-averaged ?ow velocities of a local coupled hydrodynamic and wave model (Delft3D) of the typhoon with a maximum <1.5ms-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 ?ooding pattern in Eastern Samar, which are not resolved in phase-averaged storm surge models. Our ?ndings 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.