Unsteadiness in longshore currents

Abstract

Classically, field and laboratory investigations have relied on averaged results from Lagrangian measurements to establish longshore current velocities. Likewise, conservation equations have been time averaged in the formulation of longshore current theories. Recent experimental measurements by the authors indicate that at a fixed point in the surf zone variations in excess of 150 percent of the mean current velocity occur over time periods from three to eighty seconds. These unsteady motions in longshore currents persist horizontally across the surf zone and vertically from the surface to the bottom. This persistence and magnitude of the observed velocity fluctuations from this investigation imply that time dependent analytic treatments of conservation equations are necessary in order to properly determine longshore current velocity.