BOTTOM SHEAR-STRESS IN THE SURF ZONE

To investigate the bottom shear stress in the surf zone, detailed labo ratory measurements were made of the free surface elevations and veloc ities for the case of regular waves spilling on a rough, impermeable 1 :35 slope. The velocity profiles were measured at several vertical lin es in the cross-shore direction to include the shoaling region seaward of breaking, the break point, the transition region, and the inner su rf zone. Each vertical line included measuring points at a fraction of the grain height above the rough, fixed bottom. A logarithmic layer w as 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 veloc ity and bottom roughness from the phase-averaged horizontal velocities in the lower portion of the bottom boundary layer. The bottom frictio n 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 wi th the fitted friction factor was shown to predict the temporal variat ion of the bottom shear stress within a factor of 2. The bottom roughn ess estimated from the grain size assuming rough turbulent flow was sh own to agree qualitatively with the measured values. The cross-shore v ariation of the friction factor estimated from an empirical formula de veloped for nonbreaking waves was shown to agree within a factor of 2 of the measured values.