Comment on ‘Reassessment of hydrodynamic equations: Minimum flow velocity to initiate boulder transport by high energy events (storms, tsunamis), by N.A.K. Nandasena, R. Paris and N. Tanaka [Marine Geology 281, 70–84]

Studies of coastal boulder deposits have dramatically increased in number during the last decade. These have been accompanied by modeling efforts to understand the initiation of transport and the transport itself. A recent article by Nandasena et al. (2011) has attempted to improve equations developed by Nott (1993, 2007) for the movement initiation of boulders, but we argue that these equations are not appropriate for the tsunami data (at least in North Sumatra) used to reassess Nottʹs set of equations. Indeed, the field work on which the discussed article is partially based is inconsistent with the findings. nd-beach context and the distribution of these boulder deposits may invalidate the modeling approach of the authors. It is very unlikely that water alone was able to initiate the transport of the limestone clasts. The role of the smaller fractions must be assessed as these were an important component of the 2004 Boxing Day tsunami waves. Moreover, the spatial distribution of the boulder deposits, which part of the discussed article relies on and that has been described in a previous paper, is not consistent with the inferences drawn in the present article. The deposit has been described as a mix of boulders of various sizes and weight concentrated in space, without landward fining. This suggests an ‘en masse’ deposit rather than a deposit related to a progressive reduction of water velocity. Such ‘en masse’ deposits have been related in different environments to ‘debris-flows’. we suggest that this modeling needs to take into account smaller sediments and eventually move towards a Bingham flow concept for the case of tsunami waves.