Longshore Transport Variability of Beach Face Grain Size: Implications for Dune Evolution

Abstract

This study investigates grain-size sorting through longshore transport processes and how it influences dune evolution.The analysis is based on a data set of 58 sediment samples distributed alongshore over a 6.5-km-long sandy beach in Angelholm, Sweden. Grain size differs significantly from north to south, where median grain size varies from about 0.4–0.15 mm. The long-term coastal evolution is derived from shoreline change analysis of a series of aerial photos from the1940s until today and from longshore sediment transport rates calculated on the basis of wave data simulated by the SWAN wave model employed in the Coastal Engineering Research Center (CERC) formula. The results show an almost unidirectional longshore transport from north to south; the beach is eroding in the northern part and accreting in thesouthern part. The McLaren model, a grain size–based model to predict transport direction, was tested against the grainsize data. The test indicated transport in the opposite direction. This result supports previous studies suggesting that the McLaren model has limited applicability for sandy beaches with a dominant longshore transport. The sediment sampleswere collected at the mid–beach face position in an area where sediment is supplied to the beach during accreting conditions. Sediment in the appropriate grain size to build dunes—at this beach, 0.2–0.3 mm—was found in the parts ofthe beach where the dunes are growing. In the eroding parts, the sediment was coarser, suggesting that the longshore transport influences the supply of sediment for aeolian transport. The gradients in longshore transport rate were also found to affect dune morphology; the dunes were higher in the eroding and stable parts of the beach and lower in the accreting parts.