Bottom shear stress in the surf zone

To investigate the bottom shear stress in the surf zone, detailed laboratory measurements were made of the free surface elevations and velocities for the case of regular waves spilling on a rough, impermeable 1:35 slope. The velocity profiles were measured at several vertical lines in the cross-shore direction to include the shoaling region seaward of breaking, the break point, the transition region, and the inner surf zone. Each vertical line included measuring points at a fraction of the grain height above the rough, fixed bottom. A logarithmic layer was found to exist in the bottom boundary layer for most of the phases over a wave period seaward of the break point and in the surf zone. A regression analysis was used at each phase to estimate the shear velocity and bottom roughness from the phase-averaged horizontal velocities in the lower portion of the bottom boundary layer. The bottom friction factor was estimated from a quadratic friction equation based on the measured horizontal velocity above the bottom boundary layer together with the estimated shear velocity. The quadratic friction equation with the fitted friction factor was shown to predict the temporal variation of the bottom shear stress within a factor of 2. The bottom roughness estimated from the grain size assuming rough turbulent flow was shown to agree qualitatively with the measured values. The cross-shore variation of the friction factor estimated from an empirical formula developed for nonbreaking waves was shown to agree within a factor of 2 of the measured values.