**2. Study Area**

The section investigated is a part of Trzebiatów Coast, itself a part of the Southern Baltic Coastal Zone, Poland. It features cliff- and dune-type shores [32,45,46]. The study covered the western part of the sandbar–dune Rega Gate area from coastline kilometer 363, the entire Rewal Upland cliff shore section, and the eastern part of the sandbar–dune Dziwna Gate area to the River Dziwna mouth (391.4 km).

Research carried out in the area by various authors [5,32,38,47–53] demonstrated the nature of geodynamic processes and developmental trends. This study builds on research conducted by Jahn [54] on beach pebbles of the Polish Baltic coast, as well as on comprehensive coastal zone research (including beach gravels) in the Niechorze–Trz ˛esacz section of the coast, carried out in the early 1970s [42].

Owing to the specificity of the sea–land interactions, the Trzebiatów Coast has been divided into the following morphodynamic regions (Figure 2a,b) [4,55]:


The entire area of study, including the shallow foreshore, represents a morainic upland built mainly of morainic tills, occasionally separated by glaciofluvial sediments [37,58,59]. The sandy-gravelly deposits of the foreshore are located only in the central part of the Dziwna Gate. The sandbar areas show older valleys of different origins, with distinct horizons of organic sediments, mainly of lake and lacustrine origin [51,60].

The Rega Gate area is a part of a riverine floodplain valley covered by sandy spit and aeolian deposits. The Rewal Upland area is a fragment of a Pleistocene upland with variable denivelation, the depressions of which are frequently covered by aeolian sands [38,61].

The Dziwna Gate area is a fragment of a "bridge" connecting Pomeranian Bay with Kamie ´n Lagoon, remodeled by sandbar deposits and intersected by the River Dziwna mouth [60].

The Pogorzelica–Dziwnów coast section shows a sandbar–dune backshore built by Holocene sands, and cliffs formed by Pleistocene morainic and glaciofluvial deposits. The entire section (except for its westernmost part with the old River Dziwna bed) features an erosional platform built mostly by morainic tills or locally by glaciofluvial sediments, and frequently covered by thin layers of marine sands. During storms, these sands (or gravelly sands) work as an active layer and their movement erodes the platform [58,62].

The estimated rate of cliff sections erosion during last 100 years erosion has varied from 0.5 to 2 m/year. Due to the presence of embankments along Rewal cliff, the accumulation along the dunes slowed to <2 m/year, sometimes indicating even erosion (~0.7 m/year), especially around the Dziwna spit, where intensive erosion dominates [56].

According to meteorological conditions, the study area is dominated by southwestern and western winds annually, with mean wind speeds of 2 to 4 m/s, and frequency reaching 20 to 25% (light to gentle breeze). The frequency of 6–8 m/s winds varies 15 to 20% (gentle to fresh breeze). Frequency of strong winds and storms (>8 m/s) is only about 1–2%. The highest wind speeds occur during autumn and winter, usually 0.6–1.4 m/s and more intense compared to summer and spring [6].

The frequency and intensity of storm events in the study area increased during last 70 years. Storms induce an increase of the wave splash zone, which usually reach approximately 2–2.5 m above the seasonal mean high water. The frequency of waves with heights <0.5 m, depending on the season,

exceeds 90%. The annual mean sea level increase induced by global climate forcing varies from 0.04 to 0.08 cm/y [63,64].

The dune and cliff foot positions in the study area are positively correlated, especially in the area between Rega and Rewal (correlation coefficient >0.75; significance level α = 0.05). In the western part of the study area, the correlation is less (<0.25; α = 0.1). During the last 40 years, the correlations decreased, mainly due to more embankments and the increasing mean sea level, which heavily disturb circulation of pebbles in the study area [36]. In the western part, longshore currents distribute particles towards Wolin Island. Meanwhile, in the eastern part, bedload transport towards Kołobrzeg dominates. In the middle part, the divergence zone was observed, with no visible processes of gravel redeposition (temporal erosion–accumulation) [43].

**Figure 2.** (**a**) Morphodynamic types of the coast in the study area, (**b**) representative geological cross-shore profiles 1–4 (adapted from [65]).

Lithodynamic processes in the study area are strongly related to seasonal variability of meteorological and hydrodynamic conditions. The grain size composition along sandy coast sections can be characterized by individual groups of so-called "lithodynamic belts". The transformation of initial grain size depends on stormy conditions and intensity of aeolian processes. These differences are related to (i) grain size variability in the surf zone and backshore, (ii) intensity of wave storms, (iii) spatiotemporal changes of winds and air streams, (iv) different rates of seashore drying or wetting by rainfall, (v) influxes of the water from the surf zone, and (vi) marine aerosols or dew [66]. Deposition and redeposition of coarse sedimentary material may be attributed to three stages:


The beach sections in the study area are composed mainly of fine and medium sands. The amount of gravel is generally low, especially during periods between storms. Among the gravels, the 2–5 cm subfraction dominates. During the wave storms the relative amount of coarse grains increases, mainly due to the removal of fine fractions, or by redeposition of foreshore gravels [42,44].
