Longshore Movement of Pebbles and Cobbles in the Southwestern Finnish Archipelago

ABSTRACT

Longshore transport of shore pebbles and cobbles by waves and ice was studied in the Airisto area in the SW Finnish Archipelago for five years (1976–81) by means of painted clasts 1–10 cm in size. Seven gravelly, stony shores were selected as observation sites. The net transport distances of the marked material from the starting-lines were measured once every year. The results were treated statistically.

The maximum net movement of pebbles and cobbles along the shore occurred in the swash zone during severe storms when the water level is often high. It was at a minimum on the backshore and in the surf zone. The effects of the open sea were, in general, reflected in the directions and net distances of the longshore movements at the observation sites. The mean net distance travelled during the five research years was short, only 4.2 m. In the last year it was longest. 1.9 m. which was due to favourable wind conditions and the action of wind-driven ice during breakup. The net distances travelled by some individual pebbles and cobbles along the shore were as much as 10–20 m per year. Pebbles of 2–6 cm and cobbles of 7–9 cm in size travelled on average farthest while pebbles of 1–2 cm and cobbles of 6–7 cm and 9–10 cm in size travelled on average the shortest net distances.