**4. Discussion of Results**


As stated in the field site description the nearshore area around the pier features a double bar profile. Bars' morphodynamics over time could be easily traced in a cross-shore plane if presented in chronological order of all field measurements (Figure 2).

**Figure 2.** Coastal morphodynamics constructed from cross-shore profile measurements for all field experiments in October 2007, November 2008–April 2010, October 2016 and October 2018 at Kamchia-Shkorpilovtsi beach; white dashed line contour of the mean sea level.

Considering the surveys in 2007, 2016 and 2018 the inner bar evolution could be observed at offshore distances 40–60 m, as during each of these field campaigns at least one storm event has occurred. In 2008–2010, this zone has widened up to 30–70 m. According to recent studies at the site, the inner bar has a storm origin and commonly exists in the surf zone on a time scale of a few days [60]. Depending on the wave regime, it develops during the storm's initial stage and eventually adjoins the shore while storm attenuates [61]. Analysis of wave and profile data from 2007 and 2016 confirmed the bar formation due to wave breaking by plunging and the combined effect of waves and undertow [50,62], while responsible for its onshore migration are processes of wave breaking switching from plunging to spilling and periodic energy exchange between non-linear wave harmonics [60,62].

The dynamics of the outer bar is quite remarkable, as well (Figure 2). It shows a variability concerning its form, its (a) symmetry, and its offshore and depth location, which is not only inter-annual but annual to seasonal (2008–2010), and even could be observed within a single storm event, as in October 2016. According to the measurements, over the

years the outer bar is always present, and its evolution and displacement commonly occur within distances of 110–220 m at depths 3–5 m. The closest to the shore the sand bar was in 2007 and 2016 at distances 110–190 m and 120–210 m, respectively, and it was localized at depths 3–4 m, while in 2018 it was farthermost off the shore at 150–220 m (depths 4–5 m). On the grounds of measurements in 2016, the beach profile deformations were studied during different stages of the reported storm (Hs ≈ 4.5 m), considered as seasonal due to estimated return period of 1 year [61]. The results stated that the outer bar built during the stages of the maximum storm intensity, which led to the formation of a typical storm profile. Storm's attenuation stage, i.e., wave parameters' gradual decrease contributed to the natural process of the beach foreshore recovery, while a slight shoreward shift of the outer bar and increase of the depth above its crest were registered. Based on the same data, [50,62] reported that the growth and displacement of the outer bar depend on the breaking type and location of the breaking zone within the nearshore area. Large storm waves breaking by plunging and wave asymmetry variations with respect to the vertical axis [63,64] trigger initial and further outer bar development, while shifts of the breaking area by plunging entail respective bar crest relocations.

The availability of regular monthly seabed measurements in 2008–2010 gave opportunity for more detailed analysis of the outer bar dynamics on annual and seasonal basis (Figure 3).

**Figure 3.** Intra-annual morphodynamics of the outer sand bar: (**a**) chronological seabed relief changes in November 2008–April 2010; colors and symbols of bar crest positions correspond to profile groups as described below in the caption text; vertical magenta lines—beginning of each year, horizontal white line—reference winter bar crest location defined from 2009 data; (**b**) outer bar profile envelopes of temporal variations (sweep zones) for November 2008–October 2009 (left panel) and November 2009–August 2010 (right panel), Hs and H/L—seasonally averaged monthly maximum values of the parameters at depth 4.4 m; black lines with triangles—winter pro-files, red lines with circles—summer profiles, and blue line with diamond—transitional profiles; heavy black line with triangles—last measured profile prior to the abrupt seaward bar shift, heavy red line with circles—first profile after the bar's displacement to its summer location.

As previously mentioned, during this period the sand bar migrated within depths ≈ 3.5–4.5 m and offshore distances 130–220 m. Changes of bed relief and crest positions show that the outer bar's intra-annual variations followed a certain repetitive seasonal pattern (Figure 3a). It was characterized by the bar's presence closer to the shore in winter at distances 180–200 m followed by an abrupt seaward shift to larger depths at distances ≈ 210–220 m for spring-summer season, and again a gradual return to its winter location.

It was evident that such bar crest dynamics was accompanied by changes in the bar's form (Figure 3b). After inspection of the measured transects in November 2008–August 2010 and taking into account the seasonal pattern, three groups of profiles were determined according to their similarity in shape:


The proposed "winter-summer" distinction is rather based on the seasonality of the outer bar profile evolution discussed herein, than on the classical concept of seasonal beach profiles—winter and summer ones [16]. Moreover, the possibility to distinguish the group of transitional profiles is connected with the wave regime seasonality in general, which is not an abrupt transition from summer low-energy regimes to winter high-energy ones, but rather a process of gradual wave energy increase in a form of a series of subsequent storms [65].

In the first seasonal cycle of bar migration in November 2008–October 2009 (Figure 3b, left panel) each of the introduced profile groups is very easily distinguished, keeping relatively uniform slightly asymmetric shapes in comparison to the next season in November 2009–August 2010. Regardless of its slight cross-shore migration the bar remains relatively high, as its height varies between 0.99–0.66 m without significant change of its location depth from ≈3.6 m to ≈3.9 m (Table 1).

**Table 1.** Outer bar average characteristics for different profile groups for November 2008–August 2010.


<sup>1</sup> Real profile characteristics for August 2010.

During the second seasonal cycle in November 2009–August 2010 (Figure 3b, right panel; Table 1), regardless of partial availability of data, it is evident that the pattern of intra-annual bar evolution is being repeated, but from shoreward shifted location of origin. Even though, only the winter profiles are relatively uniform in shape, generally all profiles are more symmetric. At the beginning of the second cycle, the initial average bar height was 0.60 m and as the repetition of migration pattern proceeded the offshore bar relocation was accompanied by significant bar crest erosion, thus, increasing the average depth above the crest to 4.4 m. A common feature for both seasonal cycles is that in winter the sand bar has larger heights than in spring-summer season.
