APPLICATION OF AN UNDERTOW MODEL TO IRREGULAR WAVES ON PLANE AND BARRED BEACHES

An undertow model calibrated for regular waves on plane beaches is app lied to predict the irregular wave induced undertow for both plane and barred beaches and for both laboratory and field data sets. The model combines a logarithmic profile in the bottom boundary layer with a co nventional parabolic profile in the interior. The height and period of the irregular waves are represented by the local root-mean-square wav e height and spectral peak period, and the measured mean volume flux b elow trough level is used as input to the model. The model is capable of predicting the undertow profiles both inside and outside the surf z one, provided that the empirical coefficient associated with the mean bottom shear stress is adjusted at each measuring line. The model appe ars to give reasonable predictions of the bottom boundary layer thickn ess and shear velocity, although these predictions could not be verifi ed due to a lack of data. To develop a predictive undertow model, a si mple relationship with an adjustable coefficient is applied to predict the measured volume flux below trough level using the local wave heig ht and water depth. The calibration coefficients involved in the predi ctive model are not universal among the lab and field conditions possi bly due to the effects of wave directionality and longshore currents i n the field measurements, which are neglected in this paper.