BEACH CHANGES AND SEDIMENT MOVEMENT IN THE SURF ZONE

Zbigniew Pruszak, Ryszard B. Zeidler

Abstract


The South Baltic Sea coastal zone investigated in the years 1983-1989 belongs to multi-bar dissipative ones, with an average slope of 1.5% and sand grain size .050=0.22 mm. EOF have exhibited the most common locations of bars (e2x) and the most intensive bed changes (e3x). Particularly conspicuous are the bed changes about the inner bar (depths about 3 m). Radioisotopic irydium glass tracers have been employed in three different subzones of the shore profile; 50-60 m from shoreline (depths h of 0.8-1.2 m), 200-250 m (ft=2-3 m), and some 350-450 m (ft=4-5 m), respectively. During storms the most intensive sediment motion occurs in the second subzone, where the longshore sediment transport rate q reaches 40 to 100 kg/(mh). Closer to shoreline (first subzone), the transport rate ranges from 15 to 40 kg/ (m-h), while the smallest rate (q « 3.5-20 kg/(m-h)) is encountered in the third subzone. Under conditions of weak or moderate oblique waves (e.g. mean wave period and height T < 3-4 s and H < 0.2 m in the second subzone), the longshore mode also prevailed but the ratio of longshore to cross-shore rate was lower, about 1.7:1 (13.5 versus 8 kg/(mh)). Core samples of tracer sand have provided estimates of the vertical extent and distribution of transport rate in the bed layer. It has been found that the thickness of the bed sublayer in which all grains move in bulk reaches 6 « 2-5 cm during storms, versus a few grain diameters to 1.5 cm under weak and moderate waves.

Keywords


surf zone; sediment movement; beach changes

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