Mobility and transport directions of residual sediments on abrasion platforms in front of active cliffs (Southwestern Baltic Sea)

Pebbles, cobbles, and boulders are commonly found on abrasion platforms of retreating soft rock cliff-coasts, which are widespread along the southwestern Baltic Sea coast. So far, little has been known about the behavior of these residual sediments under the influence of wave impact. Thus, detailed field measurements were carried out at three different coastal stretches of the southwestern Baltic Sea between 1997 and 1999. Underwater photo stations were established and marked pebbles and cobbles (tracer) were placed on the seafloor. During an 18-month period, these stations were revisited up to 10 times, taking reproducible photos of the seafloor and measuring any tracer movement. Local wave data were collected by wave buoys. In water depths of up to 5.6 m and distances from the shoreline of up to 300 in pebbles, cobbles and even boulders were moved by wave action. Displacements were in the range of up to tens of meters, but highly dependent on water depth and wave energy input. Nearly all marked pebbles moved onshore, coinciding with the direction of the main wave propagation. It can be assumed that residual sediments are an important sediment source for the development of beach ridges in the adjacent coastal lowlands. There is evidence that the whole residual sediment layer, even of several centimeters to decimeters in thickness, has been reworked repeatedly. Former assumptions, that residual sediments covering the solid seafloor might protect it from abrasion, seem no longer valid.