A quasi-experimental setting of coarse clast transport by the 2010 Chile tsunami (Bucalemu, Central Chile)

On 27th February, 2010 a MW 8.8 earthquake occurred in central Chile, causing a tsunami that severely affected the central Chilean coast. In this study we present sedimentological data of the 2010 Chile tsunami from the coastal town of Bucalemu in Central Chile. The tsunami deposited both fine grained sediments as well as cobbles and boulders. The location offers a quasi-experimental setting for coarse clast transport because an artificial pile of cobbles and boulders was piled there for construction purposes some days before the tsunami. Therefore, the pre-tsunami transport setting and transport distances are well defined. Additionally, a local flow depth of ca. 2 m and flow speed of ca. 5.6 m/s are deduced from eyewitness reports and video recordings. The cobbles and boulders have been transported up to 155 m from the pile in landward direction. The size of the quarried clasts was restricted to max. 0.3 m3 and 720 kg. Bigger boulders were not available at the location, even though calculations show that the tsunami would have been capable to transport them. Calculations of flow speeds required for the initialization of transport in different modes, such as sliding, rolling or saltation, indicate that all clasts that were moved by the tsunami in landward direction, were entrained by the flow given a minimum observed speed of 5.6 m/s, with 65% being transported by rolling and 35% by saltation. The transported clasts are arranged in three groups positioned in morphological depressions on the coastal plain. A landward fining trend is not present. The arrangement of clasts originating from a pile and being deposited in distinctive groups, entails that the landward transport of cobbles and boulders is a function of repeated entrainment, transport and deposition by several waves of the tsunami wave train. Consequently, we interpret the clast distribution as a result of a combination of the number of individual waves of the tsunami wave train, their respective energies, the local topography, and the pre-transport setting of a clast pile which releases wedged clasts with each successive wave.

► Tsunami runup deposit which is not modified by backwash.
► Well known pre-transport scenario.
► Known inundation parameters.
► Example of coarse clasts transport over a flat coastal plain.