**2. Study Site**

The study site is located in the southern part of Kamchia-Shkorpilovtsi beach, which is the largest and longest (≈14 km) sandy beach along the Bulgarian Black Sea coast with well-developed dunes and the presence of two rivers—Kamchia and Fandakliyska (Figure 1).

**Figure 1.** Description and location of the study site. The satellite view shows the location of the field site on the Bulgarian Black Sea coast and indicates the positions of the research station 'Shkorpilovtsi', the pier, and the state of the crescentic bars (dashed grey line) along the coastline on 24th September 2009; white diamond—extraction point of the climatic wave data, depth 17 m; white star—same for wave time series, depth 4.4 m; data sources: the Black Sea basin map was taken from free maps platform [43] and the satellite image was acquired by Maxar Technologies, USA [44].

The coastal area is open to the winds and waves of the eastern half. Strong seasonal variability is the most remarkable feature of the wind and wave climate. Winds from the northeast quarter are the most frequent in the western Black Sea. Having the largest fetch, they trigger the most severe storms. The northeastern winds prevail over the study area. In general, the southeastern winds are less significant in terms of storm intensity. Following the wind pattern waves approach the shore mostly from the northeast, east and

southeast as the impact of waves coming from northeast and east is much more pronounced. Thus, a large amount of the waves approach the shore in the normal generating intensive cross-shore water circulations as compared to relatively weak long-shore wave-driven currents [45]. Based on wave hindacast data the maximum significant wave height can reach up to 5 m at depths of about 20 m [46]. The beach is formed due to the accumulation of erosive and fluvial sediments and has a positive sediment budget since it is constantly fed by sediments from both rivers, as well as from a well-developed ravine and gully system [47]. Morphological conditions of the adjacent coastal zone are rectilinear shoreline with almost parallel isobaths and nearshore crescentic bars (Figure 1).

The study site area near the mouth of Fandakliyska River is equipped with two field research facilities: a research station 'Shkorpilovtsi' and a 230 m-long pier constructed perpendicularly to the shore, thus covering the most dynamic part of the coastal zone (≈200 m). It rises 7 m above the mean sea level and water depths at its sea-end vary between 4–5 m (Figure 1). Therein, at depths 0–10 m the mean slope of the seabed is 0.02–0.025, while for depths 10–20 m it is 0.006 [48]. Reflection coefficient estimated for different parts of the shore is of the order of 10−2–10−3, which means that the reflection of the waves at this dissipative coast has practically no effect on their heights [49]. The shoreface around the pier usually features one inner sand bar at distances 40–60 m and depths 0.5–2 m [50] and an outer crescentic sand bar at depths 3–5 m and distances 110–200 m. On the upper submerged profile (depths <2.5 m), over 95% of bed sediments are composed of coarse to medium-sized sands (0.3–0.76 mm), which mostly consist of quartz (96%), while the remaining 4% are represented by fine shell particles (CaCO3). As depth grows, their content decreases, and at 8–10 m depth over 90% of the sediments consist of particles less than 0.25 mm in size [45,47].
