Współczesna dynamika plaż i wydm w środkowej części Mierzei Wiślanej oraz prognoza ich zmian po wykonaniu falochronów kanału = Contemporary dynamics of the beaches and dunes in the central part of the Vistula Sandbar, as well as forecast changes following the construction of canal breakwaters

Abstract

The aim of the work described here has been to analyse contemporary changes along the shore along the central part of the Vistula Sandbar located on the Baltic’s south coast. There, breakwaters are being installed to protect the canal cut that has been under construction since 2020 (fig.1). Shoreline changes involving both dunes and beaches have been predicted to arise following construction of breakwaters, which will obviously influence the dynamics of the previously natural shore of the Sandbar. Factors exerting done in the 2003‑2020 period. The section of the Sandbar under discussion (the middle part, at km 18‑25, fig. 1, 2) has so far shown only limited accumulation trends. Plate 1 presents different relief of the foredunes caused by storm surges and aeolian processes. The wind regime for 2001‑2017 features the more marked presence of wind from the W and SW sector. Aeolian accumulation caused by such westerly winds is rebuilding the beach and dune in the investigated area (fig. 6‑8). The strongest winds are those from the NW that arise during the autumn-winter period. The orientation of the sandbar coast ensures that erosion in the course of storm surges is different. The middle part of the Vistula Sandbar is only eroded during the highest storm surges (fig. 3). Over the research period, it was possible to observe erosion of various types caused by storm surges recorded in Gdańsk. Since 2003, there have been several storm surges featuring a water level higher than 1.2 m AMSL. Each such surge ensures severe erosion of dunes (as in 2004, 20006, 2007, 2012, 2017 and 2019). Storm erosion is a major factor in dune development: the higher the surge, the higher the levels of water run up and coastal erosion (fig. 4). During the highest surges (featuring water of H>1.2 m AMSL, the run-up is of almost 4 m AMSL. The mean rate of retreat at the base of the foredune is 3‑4 m, while the maximum reaches 7‑8 m. In the periods between storms, the areas at the bases of dunes increases by 0.8 to 1.5 m/y. Reconstruction of the dunes took place up to 2‑3 years after a major storm surge. The sand building foredunes is fine and medium (average 0.20‑0.21 mm). There are fines sands along the whole Vistula Sandbar, while beach dune sand is coarser. The coarsest sand is the type that builds the beach ridge (0.3‑0.4 mm). This type of material comes from the cliffs of the Sambian Peninsula located in the NE part of the Gulf of Gdańsk. Plants scattered across the beach ensure aeolian accumulation on the upper beach. Beach height is often higher than the highest water run-up during storm surges, with this serving to safeguard dunes against erosion. Beach width is almost widest on the investigated part of Sandbar (other than at the mouth of the Vistula delta), exceeding 41 m on average (figs. 1C and 5). The foredune dynamics are not great (figs. 6 and 7). However, there are more major increases in years lacking storm erosion, with the height of foredunes exceeding 4‑5 m. Neighbouring sections of coast lack such foredunes, however – meaning that erosion is more marked there than along the Sandbar’s central stretch. The area under study is one of two accumulating sections of the Vistula (the second being in the Vistula Delta). Analyses show that breakwaters constructed will block sediment transport on both the shore and the beach. All aeolian sediment will accumulate on the western side of the Canal that is to be completed (at km 26‑25). There will be new dune growth and beach widening. Sand will not be transported eastwards (at km 23‑18 and further east to Krynica Morska). Dunes will not develop in this section, and that will ensure the development of erosive tendencies. The erosion of dunes will take place on the eastern side of breakwaters and may exceed 0.5 to 1.2 m/y (fig. 8). Annual rates of erosion will depend on the number of strong storm surges with sea level H > 1 m. Such surges occur more frequently than they did in the 20th century.