Boulder displacements along rocky coasts: A new deterministic and theoretical approach to improve incipient motion formulas

Several impacts of high-energy marine events – tsunamis and storms – which have occurred along the Mediterranean coasts, have determined boulder displacements in different ways, spanning from sliding and overturning to saltation. The dynamics of these transport modes to the incipient motion of boulders, strictly connected to the pre-setting scenarios, has been extensively studied considering the boulder size and the hydrodynamic parameters of high-energy marine events, such as the wave flow velocity. Notwithstanding this, known hydrodynamic relationships provide some unrealistic flow values concerning the real wave flow velocity registered along the Mediterranean coasts. In this work, a new deterministic and theoretical approach has been developed to assess the minimum flow velocities required to move coastal boulders. The proposed approach is based on the introduction of modified parameters, which consider the real dimensional features of the boulders assessed by means of modern geophysical survey techniques, into the incipient motion formulas. To accurately define these parameters, we created a database composed of boulders located along the coasts of Apulia and Sicily (Italy) that were surveyed in previous studies through photogrammetric and Terrestrial Laser Scanner techniques. Results highlighted that the incipient motion formulas, revised with the new proposed parameters, provided minimum flow velocity values for boulder transport initiation, which were significantly lower compared to those calculated using the previous approach. Thus, the proposed modifications are a step forward towards defining the dynamics of boulder displacement.