## 3 Discussion of Results
The undertow at 100-200 m from the coast is not very large and possibly affects the transfer and accumulation of sand material in the vicinity of the bar top, i.e., the periodic energy exchange can lead to a shift in the symmetry of the bar. Moreover, the influence of the undertow or sand avalanching may affect the erosion observed at 110 m from the coast (Figure 2). The growth of the second harmonic, as waves propagate closer to the coast, leads to the erosion of the bottom profile and the accumulation of sand material close to the coast. When wave-induced cross-shore sediment, which already has a significant value, interacts with the undertow directed towards the sea, a well-defined underwater bar is formed (sand accumulation at 60 m from the coast, Figure 2). This confirms the ideas in the literature about the influence of wave non-linearity on the underwater bar formation (for example, [25; 27]). The underwater bottom profile averaged over the day of 28 September and the profile of 29 September, formed by a weak wave regime (Figure 2), show that the underwater bar shifted significantly towards the coast. This happened due to changes in the wave regime and corresponding changes at points where the biphase changed its sign. The resulting depth difference (the deformation of the sea bottom profile) corresponds to the calculated trends of bottom deformation, taking into account the influence of the undertow. Thus, the shoreward slope of the bar is smoothed out even more (130-150 m) due to the transfer of sand material from its seaward slope (160-200), which occurs against the backdrop of the energy exchange between harmonics. The contribution of the undertow in this vicinity is minor (Figure 7). The erosion at 70-100 m from the coast is associated with another growth of two harmonics, and the accumulation of sandy material at distances less than 60 m from the coast is associated with the beginning of its decrease and a change in the sign of the biphase. This causes bar movement towards the shore. In this case, the periodic energy exchange is a mechanism of the underwater bar movement. The undertow, being directed towards the sea, weakens the process of accumulation and supports the shape of the bar. However, in cases when the influence of undertow is minor, the cross-shore wave-induced transport and accumulation process may prevail and, subsequently, this will lead to the adjoining of the bar to the shore, the disappearance of the bar as a morphological form and the smoothing of the underwater bottom profile in general. This analysis shows that when the wave regime changes, the scenario of wave transformation changes as well, i.e., the spatial period of energy exchange between harmonics, the position of the maxima of amplitudes of the second harmonic, the shift in the biphase and the resulting bottom deformations are all subject to alterations. This explains why some wave regimes lead to the erosion of the underwater slope, and others lead to sediment accumulation on it. Over the gentle bottom slope for \"small short waves\", more spatial periods of energy exchange between the first and second non-linear harmonics can be expected and, accordingly, a larger part of the underwater profile will be subject to deformations due to non-linear wave transformation. Sandy material will be redistributed in relatively small spatial scales along the whole underwater slope, and will form, for example, several bars, or will not make significant changes to the underwater profile. With an increase in the period (height) of waves and a decrease in the number of spatial oscillations of amplitudes, for example, if only the initial stage of growth of the second non-linear harmonic is observed, it can be expected that the underwater slope will be subject to storm erosion due to the predominance of the undertow over the wave transport of sediments.