A laboratory experiment on the incipient motion of boulders by high-energy coastal flows

Studies on coastal boulders transported inland by high-energy events, such as tsunamis, focus mainly on the nature, magnitude and characteristics of past events and contribute to coastal hazard assessments. However, uncertainties on the transport models used in the literature are widely acknowledged. To study the minimum flow conditions for boulders transport, a laboratory experiment was carried out at the Hydraulic Engineering Laboratory (LIDR) of the University of Bologna, Italy. The main objective was to provide experimental data on boulders incipient motion and on the relationships linking boulders weight, geometry and orientation with the flow velocity and flow depth thresholds for transport initiation. The experiments reproduce idealized cases to highlight the contribution of the flow and transport processes, while the complexities due to the environment and irregular boulder shapes are reduced to a simple slope and to cubic and rectangular blocks. The most important result is that boulders move when only partially submerged by the flow, but this case is not even considered in the literature. Even for this oversimplified case, the hydrodynamic approach currently used to predict incipient motion strongly overestimates the minimum conditions for boulder transport. The main conclusions are that the drag and lift coefficients commonly used in the literature are not adequate to correctly estimate the minimum conditions for transport and need to be fine-tuned. The main reason for this discrepancy is that the practical hydrodynamic formulas do not account for the flow variability induced by turbulence that plays a key role in influencing the start of instability. To take into account this intrinsic uncertainty, we suggest determining two dynamic thresholds, separating three distinct regimes: one where transport is impossible, one where it is certain, and an intermediate one where it is possible depending only on the actual turbulence bursts. Data are made freely available (ftp://137.204.48.34). © 2018 John Wiley & Sons, Ltd.