KINEMATIC UNDERTOW MODEL WITH LOGARITHMIC BOUNDARY-LAYER

A new kinematic undertow profile model is developed to relate the mean horizontal velocity, bottom shear stress, and boundary layer thicknes s in a simple but general manner. The model combines a logarithmic pro file in the bottom boundary layer with a parabolic profile in the inte rior layer. Use of a logarithmic profile is justified using our labora tory measurements for regular waves spilling on a rough, impermeable s lope. Two forms of the model ate presented, each with one calibration coefficient associated with the mean bottom shear stress. By adjusting the calibration coefficient at each measuring line, the model is show n to be capable of predicting the measured undertow profiles both insi de and outside the surf zone for our rough slope case and for smooth s lope cases from the literature. The model does not predict the oversho ot in the bottom boundary layer for the rough slope case outside the s urf zone. The predicted velocity profile for the smooth slope case in the bottom boundary could not be verified due to a lack of data. The p redicted boundary layer thickness agrees with the measurements for the rough slope case and appears to be reasonable for the smooth slope ca ses. The model predicts the shear velocity in the transition region an d inner surf zone reasonably well for the rough slope case, and it und erpredicts the shear velocity outside the surf zone. This model is sho wn to be simple and versatile, but it will need further validation usi ng irregular wave data and varying bottom geometry before it can be us ed in practical applications.