**4. Conclusions**

We carried out a study on the influence of the underwater bar location on the transformation of the waves above it, and on the corresponding cross-shore sediment movement in the coastal zone, on a time scale of one storm. This allows us to conclude on the following:

1. The position of the underwater bar affects the shoreline degradation and the distance of seaward sediment transport. The maximum transfer of sediments towards the sea is within the distance between the shoreline and the underwater bar crest. A minimum of sediment movement occurs when the bar is located away from the shore, at a distance of less than half the wavelength, in deep water. The coastline retreat is minimal if the bar is located away from the coast, at a distance of 0.7-0.82 from the wavelength in deep water. In these cases, the underwater bar will have a more protective effect on the shore, compared to a profile without the bar.

2. The presence of the underwater bar located at specific distances from the coast may lead to an increase in shoreline degradation. If there is a longshore underwater bar that is located at an angle to the coastline, the non-uniformity of the coastline retreat, and the formation of festoons are possible.

3. The greatest difference in coastal retreat rate associated with the underwater bar location is observed within the first hour of the storm. Regardless of the location of the underwater bar on the initial profile, the equilibrium profile is formed after 6 h, for the selected wave conditions. The resulting equilibrium profile contains an underwater terrace, and it is close to the classical equilibrium profile. At the same time, the erosion rate slows down significantly and becomes identical for all profiles.

4. Changing the parameters of the waves during their transformation over different profiles has a significant impact on the degree of transformation of the underwater beach profile. It has been established that there is an inverse relationship between the retreat of the coast line and the low-frequency wave heights near the coast. The decrease in the mean wave period, which is associated with the growth of higher harmonics during the passage of waves above the bar, reduces shoreline erosion. The distance of seaward sediment transport transfer is directly related to the significant wave height.

5. When waves propagate over profiles with underwater bars that are located at different distances from the coast, the wave asymmetry changes differently. According to the modelling results, the increase in wave asymmetry near the shore due to the existence of the bar leads to a decrease in the influence of waves on the coastal retreat.

**Author Contributions:** Both authors took part in field works on Shkorpilovtsy study site. XBeach modelling was conducted by O.K. in frame of post-graduate study (Y.S. was a scientific advisor). Analysis of modelling data was carried out by O.K. and Y.S.

**Funding:** This research was performed in the framework of the state assignment theme No. 0149-2019-0005, and supported in part by RFBR project No. 18-55-45026.

**Acknowledgments:** Authors appreciate collaboration with colleagues from IO BAS (Varna, Bulgaria) and Zenkovich laboratory of the sea shelf and coasts (IO RAS, Moscow, Russia).

**Conflicts of Interest:** The authors declare no conflict of interest.
