# Impact of the Revetment on the Seashore in the Region of Babie Doly (KM 93.6-93.9), Poland
## 2 Materials and Methods  
According to [5], the dominant type of coast in this area is the cliffed coast, characterised--in contrast to the dune coast--by a very narrow beach (usually at the foot of inactive cliffs) or its complete absence (mostly at the foot of active cliffs). A clearly distinguishable coastal headland is located at the administrative boundary between the city of Gdynia and the commune of Kosakowo, i.e., in the vicinity of KM 93.4-93.5 (Figure 3). A strip of beach stretches south of the headland, with a width decreasing from about 50 m in the immediate vicinity of the headland to disappear at a distance of about 350-400 m (near KM 93.0). To the northwest of the headland, the beach covers only a short (ca. 100 m) stretch of coast (Figure 4), largely near the remains of hard coastal protection structures. At a distance of  
Figure 1: Location of the study site.  
Figure 2: Revetment on the seashore in the region of Babie Doly (KM 93.6–93.9).  
Most of the 30 analysed satellite images taken in 2005-2019 show sandy shallows of at least 50 m in width, measured from the coastline, located in the nearshore zone within several hundred meter long stretches of shore, adjacent to the headland from the south and from the north. In the \"southern\" section, these shallows usually appear as two distinct sandbars (at a distance of 40-70 m and 150-190 m from the coastline), whereas in the \"northern\" section, they assume irregular forms, sometimes with two less pronounced sandbars (20-40 m and 100-130 m from the coastline; Figure 3). Except for the aforementioned location, with a relatively wide beach along the short stretch of shore immediately adjacent to the headland on the southern side, the analysed area is characterised by a very narrow beach or the absence thereof.  
Coastline development in a historical context can be demonstrated using free images with GIS analysis (see [6; 7]). In the presented specific case study, however, the analysis of the historical maps is carried out, supported by results of the sediment transport modelling. In particular, it is important to identify both the qualitative and quantitative features of transport of sandy sediments along the shore, namely its direction and annual rates. These characteristics are useful in understanding the shoreline evolution occurring at the site under consideration, and the role of the existing revetment.  
Figure 5: Revetment at KM 93.6–93.9; (**a**) seashore directly before the revetment at KM 93.55; (**b**) southern end of the revetment at KM 93.6; (**c**) middle section of the revetment at KM 93.8; (**d**) northern end of the revetment at KM 93.9; sea state: 501 cm; 13.00 UTC, 10 May 2021 (photo by [PERSON]).  
The processes leading to the shoreline development are the result of sediment transport, which occurs in the immediate vicinity of the seashore, especially in the surf (breaker) zone, i.e., in water less than a few metres deep. Outside this zone, in deeper waters (several metres or more) and with typical wave conditions, sediment transport generated by waves is negligible [8; 9].  
In order to quantitatively assess the rates of longshore sediment transport along the stretch of shore between KM 83.6 and KM 102.5, the authors of [10] calculated the volume of sediment transport in four representative bathymetric profiles located at KM 101.0 (western side of the Rewa Cape), KM 98.5 (eastern side of the Rewa Cape), KM 97.0 (near the village of Mechelinki) and KM 89.0 (near the district of Oksywie), in an average statistical year. Calculations of sediment transport rates were performed for all onshore wind directions, i.e., N, NE, E and SE directions. Calculated rates of longshore sediment transport are, on average, 5000 m\({}^{3}\)/year, with transport moving towards the north (Figure 6).  
Figure 6: Directions and approximate volumes of mean annual longshore sediment transport. The blue arrow indicates the approximate location of KM 93.6–93.9 [10].