Boulder Transport by the 2004 Indian Ocean Tsunami: Numerical Assessment of Field Observations at Lhok Nga Bay, Sumatra, Indonesia

摘要

Estimating the magnitude of past tsunamis after their deposits, including boulders, is one of the major issues to be developed in studies on tsunami hazard assessment. Hundreds of coral boulders have been transported from the fringing reef and some of them protruded to the land by the 2004 Indian Ocean tsunami at Lhok Nga bay, Sumatra, Indonesia. Eight coral boulders (axes lengths 3.1 m > a > 1.4 m, 2.5 m > b > 1.0 m and 1.8 m > h_b > 0.6 m) whose density is about 1120 kg/m~3 emplaced on the land are selected for numerical simulation. Results reveal that the coral boulders are transported both onshore and offshore directions by tsunami currents, and reproduce field observations within the error of ±40%. However, clast to clast interactions, surface micro-topographical effects and two dimensional current velocity effects are not considered in the numerical model. The onshore transport distance is higher than the offshore transport distance. No landward fining is observed in the field observations and the numerical simulation suggests that the offshore transport by backwash reworks the spatial distribution of boulders. The density of coral boulders affects greatly its transport distance.