Detailing agents of physical disturbance: wave-induced velocities and accelerations on a rocky shore

Water motion produced by breaking waves plays a defining role in structuring intertidal communities. However, despite the clear ecological importance of wave action, fine-scale details of the flows generated by breaking waves on rocky shores have not been well-described. This lack of information has hindered the efficacy of mechanistic studies aimed at exploring quantitatively the processes leading to wave-induced damage or mortality of organisms in littoral habitats. In response to these limitations, high-frequency field measurements of intertidal water velocities and accelerations were conducted simultaneously with recordings of inshore wave height at four representative sites in the mid-intertidal zone of a rocky shore. Maximum velocities and accelerations recorded under waves of measured height are presented and placed in the context of simple wave theories. Rates of occurrence of simultaneous velocities and accelerations are computed as a function of wave height, providing a means of linking instantaneous values of drag and lift to concurrent hydrodynamic accelerational forces. These data together provide an improved, high-resolution picture of the nature of flows typical of Littoral environments and their relationship to wave height, and as such serve to reemphasize both the severity as well as the stochastic nature of the fluid motions that characterize wave-swept coasts. (C) 1999 Elsevier Science B.V. All rights reserved.