**1. Introduction**

Hydrodynamic processes are important factors in coastal zone evolution. The off- and onshore relief of sandy beaches is deeply bound with the wave regime. The wave climate and its variations are main mechanisms of cross-shore sediment transport in the coastal zone; for instance, the formation and movement of underwater longshore bars, which are observed on many sandy coasts [1–3].

Around 10% of sea coasts have underwater bars [4]. The timescale of longshore bar formation and movement can vary from days to months [5]. According to laboratory experiments [3], under weak or moderate waves, the underwater bar moves shoreward until it joins the coast and disappears. Stronger waves switch the direction of the bar movement seaward. With changing wave conditions, the underwater bar can stay approximately at the same place and be considered stable [3].

Underwater bars are specific features of the bottom relief, so that they affect a wave transformation process within a coastal zone. As a result, the bars have influence on a cross-shore sediment transport and shoreline deformations. The wave transformation over barred profiles of sandy beaches and the corresponding morphodynamical features are a challenging and intensively studied topic [1–3,6].

Nevertheless, the role of underwater bar positioning in shoreline dynamics is still not obvious. From an engineering point of view, this issue is important for coastal defense, and it should be clarified [7]. Artificial underwater bars and reefs that imitate natural structures have become popular in coastal engineering [8]. Such constructions (breakwaters) are installed in order to decrease the wave load on the coast and reduce erosion. The decline of wave energy occurs due to breaking and shortening of the mean wave period by non-linear dispersive wave transformation over bars [9]. A similar effect is also detected in studies devoted to the impact of wave farms on nearshore wave

conditions and coastal protection [10]. It is crucial to find an appropriate position and an optimal shape for artificial underwater structures, in order to obtain the maximum benefits. Thus, a detailed study of the influences of the bar position on wave transformation, the corresponding sediment transport in the coastal zone, and the rate of wave-induced shoreline erosion, is a very important scientific and coastal engineering task.

The goal of this work is to investigate the influence of the underwater bar position (off a non-tidal sea coast) on the transformation of waves above it, and on corresponding cross-shore sediment transport, on the timescale of a strong storm.
