Alongshore transport and variations of cut-and-fill cycle of beaches in marginal seas

Prof. Dr. Tarmo Soomere

Some properties of wave-driven sediment transport are specific to marginal seas. I shall consider two aspects of this transport in the context of the Baltic Sea. Waves in such seas are commonly generated at a short distance from the shore and they approach the shores often under relatively large angles. As a consequence, the magnitude of wave-driven alongshore transport is often much larger on these shores compared to the open ocean shores and frequently has a variable direction. This feature may modify the classic cut and fill cycle of beaches. For example, on many open shore segments of the eastern Baltic Sea sediments are transported to a completely different location, deposited there and never enter back the coastal system. 
A specific mechanism may stabilise pocket beaches under bi-directional wind fields when different strong wind and wave directions are systematically accompanied by different water levels. In such conditions sand that is removed from the higher part of a particular beach and deposited in the shallow nearshore during elevated water levels and severe waves may be transported back during periods of low water level and moderate winds from the other direction. This is a variation of so-called "rotating" beaches where sand is transported in different water depths under different wind directions.

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