Density gradation in cross-shore sediment transport

Abstract: One of the fundamental properties of a coastline is its sediment composition. Coastal sediments are rarely composed of one type of sediment. Due to these differences, the sediments are sorted on the beach and foreshore. The effect of density variations of the sediment on coastal sediment transport has been studied in a wave flume experiment. Two sands with an equal grain size distribution but with different densities (heavy minerals and quartz) have been used. Detailed measurements of profile evolution and of sediment composition have been used to assess the sediment transport rates for each sediment fraction. The experiments show that the presence of heavy minerals in the sediment results in reduced erosion on the beach face; the breaker bar is smaller and its crest is more pronounced. The transport rates of different sediment fractions are not only determined by the availability at the bed. In the inner surf zone close to the beach, armouring occurs; in the inner surf zone close to the breaker bar, sediments are entrained more easily than expected. For effective armouring, the heavy mineral fraction in the bed should exceed a certain level. This threshold concentration is reached first close to the beach and extends later in seaward direction. [Copyright &y& Elsevier]