The mystery of boulders moved by tsunamis and storms

Boulders are moved differently in storm and tsunami and produce different characteristics of the boulder deposits. This contribution is motivated by two observations. One by Bourgeois and MacInnes (2010), which described that boulders were moved selectively due to different bed roughness during 15 November 2006 tsunami on the island of Matua. The second topic is motivated by the boulder lines on Ishigaki Island by Goto et al. (2010). Both topics are approached with linear wave theory and stability analysis. From both, the safety factor is derived for a spherical boulder with bed roughness and exposure as moment arms, with which we are able to quantify the influence of bed roughness on the incipient motion of boulders. For constant forces, a bed roughness of about 30% of the boulder radius will prevent boulder transport. Furthermore, the comparison between storm and tsunami waves in terms of the amplitude necessary to move boulders revealed that amplitude of storm waves is smaller than tsunami, which we ascribe to the contribution of both velocity components to lift forces. The comparison of total energy and number of waves revealed that storms have a larger total energy and a much larger number of waves, which lead us to the conclusion that tsunamis produce unorganized boulder deposits; whereas, storms are capable of organizing boulders along lines and in clusters.

► Bed roughness is pivotal for boulder movement.
► Lift forces were underestimated for storm waves.
► Storm waves < tsunami for same boulder.