Sediment dynamics on Wemyss beach, South Fife

Abstract

The beaches of South Fife consist of an unusual mix of heterogeneous pebbles, sand and coal waste, and consequently provide a complex environment in which to examine sediment transport. The use of tracers revealed that low density coal pebbles were 'rejected' from the bedload layer of ironstone and sandstone pebbles, and were transported to extreme distances alongshore and potentially offshore. Whereas, the high specific gravity of the ironstone pebbles restricted longshore transport, but enhanced movement in the backwash to low water. Coarse pebbles protruded from the mixed bed of closely packed small pebbles and generated turbulence, thus enhancing the lift forces for entrainment. Once in transport, the coarse pebbles 'overpassed' the small pebbles. Therefore, 'Selective transport' according to pebble composition and size, was an important phenomenon on the mixed beach. On the mixed beach, the sand was readily moved offshore in the backwash and transported by longshore currents. Whereas, pebble transport was restricted to the swash and backwash, in the direction of maximum wave energy. Pebbles were either buried in the sand or lay flat on the surface. The limited protrusion of the pebbles lowered the shear stress available for pebble entrainment. Furthermore, once in motion the sand absorbed the impacts of the saltating pebbles. Vertical cores were used to investigate the structure of the beach. This technique revealed that a homogenous upper layer of coarse pebbles was transported along and upshore, over a less mobile beach layer of compacted poorly sorted sediment. Dense ironstone pebbles armoured the beach face and trapped finer sediments below. Consequently, a mix of sediment and dense coarse pebbles could be used to increase beach stability. Important factors influencing the dynamics of pebbles on mixed beaches were identified, however an adequate formula for mixed sediment transport remains to be calibrated.