Experimental determination of the hydrodynamic forces responsible for wave impact events

•The largest force imposed on intertidal organisms is often impingement.•Impingement magnitude scales with object area, drag coefficient, and flow velocity.•Our results suggest that impingement is simply drag, rather than a novel force.•Impingement is unlikely to limit organisms' size in the intertidal zone.

Breaking waves can impose enormous forces on intertidal plants and animals. While some hydrodynamic forces (drag, lift, and the acceleration reaction) are well-studied, the factors affecting the magnitude of a fourth force – the impingement force – remain unknown. Characterized by a sharp, transient spike in force at the instant of wave arrival, impingement is often the largest hydrodynamic force imposed on intertidal organisms, yet the variables affecting its magnitude are unstudied. To delineate the factors influencing impingement, we tested a variety of objects (encompassing a range of areas, volumes, and drag coefficients) in simulated waves using a high-speed water flume. Impingement magnitude is proportional to objects' frontal area and drag coefficient, and occurs concomitantly with spikes in water velocity at the front of simulated waves. We conclude that impingement is a brief spike in drag caused by an increase in velocity at the wave front, rather than a novel hydrodynamic force. Consequently, previous hypotheses regarding impingement's ability to limit organism size in the intertidal zone are rejected.