Temporal assessment of sediment transport from beach nourishments by using foraminifera as natural tracers

Abstract

This paper presents the application of a technique that can be easily applied in monitoring programs following beach fills, where sand is dredged from subtidal zones, seabed or bays, usually rich in foraminifera shells. Its utility lies in a simple and rapid estimation of the prevailing longshore transport paths and determination of the main eroding/accreting zones. A beach monitoring program was undertaken in a embayment on the South Atlantic Spanish coast, where several nourishments were carried out in order to recover an eroding beach. Once the nourishment was complete, a temporal analysis of foraminifera shell distribution was made in a nearby beach inside the embayment by monthly sediment sampling. Foraminifera shells were used as natural tracers for estimating sediment pathways. The results showed a complex pattern of sedimentary transport between both beaches, where wind action and reflected/diffracted waves interfered with the dominant longshore current, depending on the prevailing hydrodynamic regime. These results were later confirmed by a 3-year morphodynamic study. Although foraminifera dispersion only accounts for the behaviour of fine fractions, the monitoring of shell dispersion after dumping demonstrated to be a useful tool for studying the stability of nourished beaches. In the case of nourished beaches foraminifera shells could be considered as mixed tracers, with many advantages over both natural and artificial traditional tracers.