en Water-Coastal Water-Coastal Research Paper Research Paper In this paper, a new approach is presented for estimating the vertical and horizontal distribution of undertow in the surf zone for reflective beaches. The present model is a modification of the original model presented by Okayasu et al., (1990) for natural, non-reflective beaches to include the effect of partially reflected waves. The nonlinearity of waves, wave-current interaction and nonlinear mass drift of the incident wave are also included in the present model. The results of experimental investigation and model development show that existence of reflective conditions on beaches results in a reduction in the magnitude of undertow and modifies its distribution across the beach profile. Comparison of the results by those obtained from the experiments clearly indicates that by taking the nonlinearity and wave-current interaction, the predictions of undertow in the surf zone are much improved. In particular, due to the effect of turbulence induced by wave breaking for nonlinear waves, the predicted results show more consistence with the measurements. Nonlinear wave, Reflective structure, Surf zone, Undertow, Wave-current interaction 261 271 http://ijce.iust.ac.ir/browse.php?a_code=A-10-995-1&slc_lang=en&sid=1 N. Abedimahzoon abedi.gu@gmail.com 180031947532846005755 180031947532846005755 No Guilan University A. Lashteh Neshaei maln@guilan.ac.ir 180031947532846005756 180031947532846005756 Yes Guilan University