## 3 Discussion of Results
Calculated deformations of the bottom relief and moving averaged difference in measured profiles between 28 and 29 September (**d**). site, the sand sizes vary from 0.30 to 0.70 mm and the corresponding sediment fall velocities will be in the range from 0.039 to 0.1 m/s. Thus, the coefficient before the second term will vary from 0.2 to 0.51, increasing with a decrease in grain size. For a sand size of 0.30 mm, the transport of suspended sediments will in 2.5 times more than that of bedload sediments, but for a sand size of 0.70 mm, the contribution of suspended and bedload sediments will be equal to the given qualitative assessments. It can be noted that the periodic exchange of energy encourages the following scenario of sediment discharge spatial fluctuation, and its components describing bedload and suspended sediment transports (Figures 5c and 6c): the energy of the second harmonic grows and decreases along with the discharge. As can be seen from Figures 5c and 6c, the maximum values of the main components of the cross-shore sediment discharge coincide with the maximum of the second harmonic. The cosine of the biphase at this moment also reaches its maximum; it equals one, since the biphase is equal to zero. The minimum values of the main components of the cross-shore sediment discharge can be attributed to the beginning of the growth phase of the second harmonic as well as to its small values, due to the backward transfer of energy to the first harmonic. Such a characteristic change in the sediment discharge during the energy exchange between harmonics leads to the following deformations of the sea bottom: erosion (positive gradient values) occurs when the second non-linear harmonic grows and, in this case, the sand material transfers and accumulates in the part of the bottom profile where the second harmonic decreases, which corresponds to negative gradient values. That is, with non-linear wave transformation during the period of energy exchange between harmonics taking place, the direction of sediment discharge shifts due to a change in the biphase: erosion occurs with an increase in the amplitude of the second harmonic, i.e., the removal of sand material at distances corresponding to its growth to a maximum value, and at distances corresponding to the backward transfer of energy and a decrease in the amplitude of the second harmonic. The sand material accumulation then occurs. The effect of periodic energy exchange on sediment transport is better observed when the waves do not break (Figure 6c,d). As the swell attenuated, the wave period decreased, the conditions of wave transformation altered, and the scenario also changed. When the waves became shorter, the spatial period of exchange decreased, because the detuning \(\delta\) (Formula (1)) increased. Therefore, it is already possible to observe more than one energy exchange period over the existing underwater bottom profile (Figure 6a). The biphase changes its sign twice--at 150 m and 50 m. This leads to additional accumulation, occurring at less than 50 m from the coast, due to sand material being transferred from the area at 50-80 m from the coast. The features which were revealed on the basis of qualitative analysis are confirmed by the experimental data. Differences in depth between the measured profile in the evening of 27 September and averaged profile on 28 September (Figures 5d and 6d, thin line) qualitatively correspond to the calculated bottom deformation, taking into account the effect of the undertow. The undertow calculated by Formula (8), with a significant wave height parameter for two different scenarios of wave transformations, are shown in Figure 7.  
Figure 7: The undertow calculated on Formula (8) for mean wave regime (28 September) (**a**) and wave regime on the 29 September 2007 (morning) (**b**). Under the influence of the periodic energy exchange between harmonics, the erosion occurs on the seaward slope of the bar, with its sand material being transferred to the shoreward slope of the bar.