An experiment designed to compare the relative rates of movement of different sizes of beach pebble

Abstract

This paper describes two simple experiments made by the Hydraulics Research Station which were designed to compare the relative rates of movement of several sizes of beach pebble. The first test was made on the shingle beach north of Deal; the second on the shingle beach at Rye, just west of the River Rother. Natural beach pebbles taken from these localities were marked with marine paint and colour pigments. They were traced almost daily; for four and a half days at Deal, and for nearly three weeks at Rye. In addition to marked pebbles, artificial shingle, consisting of concrete stones incorporating fluorescent particles, were also employed, their progress being followed at night using a portable generator and ultra-violet lighting equipment. The centroids of movement were calculated for each particle size used. Unfortunately, no detailed wave observations could be made during the tests. The most striking feature to emerge from these tests was the differential movement according to particle size. In general, increase in wave height on breaking (and therefore a corresponding increase in the amount of energy available at the shoreline) caused more marked differences in the relative rates of movement. It appears that for a particular size of wave there is a particular size of material which moves at the greatest rate. As the waves become higher, the size of material that travels most rapidly also increases. In general, the larger waves move the largest material most rapidly; smaller waves move the smaller particles most rapidly; while very small waves may move only the smallest beach particles, the larger particles remaining more or less stationary. The findings were largely in accordance with earlier work; and help to explain the complete, or only partial, grading on many natural beaches.