Reconstruction of transport modes and flow parameters from coastal boulders

Coastal boulders potentially provide very useful information to reconstruct hydraulic characteristics of extreme waves such as tsunamis or storm waves that struck shores in historical and prehistoric eras. Boulder transport models, which are strong tools to reconstruct the hydraulic characteristics during boulder transport, can be classified into inverse and forward models for identification and size estimation of tsunami or storm wave boulders. An inverse model can estimate the minimum wave height necessary to move a boulder and the minimum wave velocity necessary to slide, rotate, or saltate the boulder. A forward model can estimate precise hydraulic parameters such as the maximum wave velocity or wave runup height by reproducing the boulder transport distance. While some models are useful for practical purposes, few parameters are included in these models because they have been developed by simplification of actual phenomena. Therefore, it is noteworthy that hydraulic parameters estimated from the boulder transport model still include large error and uncertainty: the models need to be improved. Future work must be conducted to estimate the tsunami source or the storm size based on the tsunami or storm wave boulder distribution. Such estimation results are expected to be useful for coastal risk assessments.