Testing the hypothesis for tsunami boulder deposition from suspension

Tsunamis are known to deposit boulders but it is not clear if they are transported solely as bed load or whether suspended load can be involved. It has been reported that numerous individual boulders and imbricated boulder stacks at elevations above normal storm tide levels along the New South Wales (NSW) coast of SE Australia, have been deposited out of suspension by tsunamis. The NSW coast therefore provides an important natural laboratory to test the hypothesis that tsunamis can deposit large boulders from suspension. Using standard hydrodynamic equations to formulate the relationship between boulder transport and flow depth, we provide a test of the suspension hypothesis. We show that the tsunami flow depths required for deposition from suspension are most probably physically unrealistic. The NSW boulders are therefore not indicative of deposition from suspension and an alternative explanation must be sought for their mechanism of emplacement. This analysis has important implications for the emerging sub-branch of tsunami boulder geology.