Bottom Stress in the Inner Surf and Swash Zone

Laboratory measurements of free surface elevations and fluid velocities were made to investigate the inner surf and swash zone hydrodynamics for one case of irregular waves on a rough, fixed, impermeable bed. The vertical profiles of the tangential velocity near the bed were measured at seven locations in the surf and swash zone using a laser-Doppler velocimeter and precision traverse. The temporal variation of the bed stress was estimated from an assumed logarithmic velocity distribution. The logarithmic velocity profile was established in the inner surf and swash zone for many phases of each runup and rundown event. The average of the absolute bottom stress was largest just below the still water shoreline and was dominated by the rundown bed stress at that location. The quadratic drag law with the friction factor fitted for each wave at each location gave good estimates of the measured bed stress. The friction factor averaged over all waves at each location was not constant across the inner surf and swash zone. The use of separate friction factors for the runup and rundown cycles improved the bed stress estimates above the still water shoreline.