Lithodynamic processes along the Lake Jamno Spit

Abstract

The contemporary trend of Lake Jamno Spit development is evaluated based on data from the periods 1889-1975, 1960-1983 and 1988-1991. During the last hundred years, erosion proceeded at a slow rate: 0.07 m/yr. The average rate of retreat in the period 1960-1983 was 0.57 m/yr. Shallow-lying Pleistocene boulder till with uneven upper surface, on which there arc peat and gyttja layers, is the direct substratum of a narrow zone of sandy bottom. The possibilities of reproducing the eroded coastal forms arc limited, since the gyttja on the bottom surface is no source or sandy material, while the local outcrops of till in the activite zone of the nearshore bottom supply only small amounts of sand. During sea level rise from about 12 m at Littorina II, the whole upper surface of Holocene deposits was reworked. On the one hand, preserved erosional sills influence the dispersion of wave energy, on the other, they also exert a negative influence on the lithodynamic processes, strenghening the division in the sediment transport in the coastal zone. In transverse motion, after passing over the sills, the material is discharged directly into deep water, and has little chance of returning shorewards. The average rate of coastline retreat during the last 6000 years was ca. 0.2 m/yr, and the average rate of sea level rise was ca. 2 mm/yr.