Apart from the dredging furrows and pits, the sonar mosaic (Figure 8b) also shows that the areas around the pits and furrows became covered with very fine to fine sand fractions, which find more flowed over the dredger’s side and settled on the seabed near the dredging sites. The sonar mosaic shows them up as a bright buffer zone of 50–100 m around the dredge marks. This fine sand cover was up to 0.1–0.2 m thick. Comparison of the bathymetric records made directly before and directly after the sand extraction operations (Figures 8a, 9a) allows one to assess the volume of the fine sand cover formed as a result of the dredging operations at about 15 000 m3. The total volume of the dredging furrows

and pits was estimated at ca 111000 m3, which, after subtracting the fine sand volume left in the

area of dredging operations, makes about 96 000 m3 of sand used for nourishing the Hel Peninsula beaches. This appeared to be 45% of the amount assigned by the Gdynia Maritime Office for beach nourishment there in spring 2009. Measurements selleck inhibitor carried out in April 2010, eleven months after the cessation of sand extraction, showed that, depending on the method of extraction, the dredging traces had partly or completely evened out. The depths of the dredging pits were between 2.5 and 3.0 m, i.e. they had become 2–2.5 m shallower, and the bottoms of the pits were flattened. The diameters of the pits were between 120 and 170 m, i.e. they had increased by 40–50 m (Figures 9a,b). The gradients of the dredging pit slopes were also reduced. The maximum gradient was no steeper nearly than 10° (Figure 9c). After 11 months, the total volume of

the 4 pits from stationary dredging was about 56 500 m3, i.e. about 2 000 m3 smaller than directly after the dredging. The bottom of the stationary dredging pits is covered with fine to medium sand (Figure 10). The sonar mosaic obtained 11 months after the completion of extraction operations (Figure 9b) shows no more bright patches around the post-dredging pits. This is also confirmed by the grain size distribution of sands from box-cores taken between the post-dredging pits (Figure 11). The composition of the surface layer of sediments is the same as before the dredging operations. The proportion of fine sand transported over the seabed surface and accumulated in the pits is also indicated by the variable 137Cs content. While the normal 137Cs content in bottom surface deposits in this region does not exceed 1.5 Bq kg−1 (Figures 7, 12), the concentration in the pits was as high as 4.26 Bq kg−1 (Figure 13). The traces left by the smaller dredging pits derived from chaotic stationary exploitation (Figure 14 – Profiles 03 and 04) were transformed and filled to a greater extent than the pits from planned stationary operations. In the area with several adjacent pits having diameters of 20 to 70 m, depths of 2.