Extreme water velocities: Topographical amplification of wave-induced flow in the surf zone of rocky shores

Water velocities as high as 25 m s21 have been recorded in the surf zone of wave-swept rocky shores—velocities more than twice the phase speed of the breaking waves with which they are associated. How can water travel twice as fast as the waveform that initially induces its velocity? We explore the possibility that the interaction of a wave with the local topography of the shore can greatly amplify the water velocities imposed on intertidal plants and animals. Experiments in a laboratory wave tank show that interactions between bores refracted by a prowlike beach can produce jets in which the velocity is nearly twice the bore’s phase speed. This velocity can be further amplified by a factor of 1.3–1.6 if the jet strikes a vertical wall. This type of topographically induced amplification of water velocity could result in substantial spatial variation in wave-induced hydrodynamic forces and might thereby help to explain the patchwork nature of disturbance that is characteristic of intertidal communities. Physical disturbance caused by wave-induced forces is an important determinant of ecological structure and community dynamics in the intertidal zone of wave-swept rocky shores (e.g., Dayton 1971; Paine and Levin 1981; Denny and Wethey 2000). As a means of quantifying spatial and temporal patterns in surf zone water velocities, several studies have used recording dynamometers to measure the maximum hydrodynamic forces imposed on small objects in the intertidal zone (e.g., Jones and Demetropoulos 1968; Denny et al. 1985; Bell and Denny 1994; Vogel 1994; Gaylord 1999; Denny and Wethey 2000). If it is assumed that the force imposed on each object (usually a sphere) is attributable solely to drag (see Gaylord 2000), these measurements can then be used to estimate Umax, the maximal wave-induced water velocity that occurs in the surf zone (Eq. 1). 2F U 5 (1) max !rACd Here, F is the imposed force, r is the density of seawater (1,025 kg m23), A is the projected area of the object, and Cd is a dimensionless drag coefficient (which might vary slight1 Corresponding author ([email protected]). 2 Present address: Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, California 92037. 3 Present address: Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208.