Life in an extreme environment: Characterizing wave-imposed forces in the rocky intertidal zone using high temporal resolution hydrodynamic measurements

Abstract

Hydrodynamic forces produced by breaking waves make the rocky intertidal zone one of the most physically stressful environments on the planet, and for many ecological studies, it is important to quantify intertidal flows. However, hydrodynamic measurements with a response time rapid enough to reliably identify “impingement events” – large, transient increases in force occurring at wave arrival – have not been made, although impingement events are hypothesized to be the largest hydrodynamic forces in the intertidal zone. We built a custom force transducer to measure short-duration flows in the intertidal zone, and took measurements of force acting on both a drag sphere and a rigid limpet model. Based on our results, we estimate impingement events occur in 7–14% of waves, and when present, they are the dominant force in the wave. The mean ratio of maximum force to the second largest force within a wave was approximately 1.5 for impingement waves, in contrast to 1.2 for non-impingement waves. However, contrary to previous hypotheses, impingement events are not necessarily the largest forces imposed on intertidal organisms. At our site, non-impingement waves recorded on the drag sphere had a mean maximum force 61% greater than the mean maximum force of waves with impingement events. Our results thus suggest that impingement is not necessarily the greatest hydrodynamic danger to organisms. Future studies should use high temporal resolution measurements to identify the largest transient forces regardless of when they occur in a wave, and to assess how such brief forces might affect intertidal organisms.