Cross-shore grain size and sorting patterns for the bed profile variation at a dissipative beach: Hasaki Coast, Japan

Abstract

Three field observations were conducted in three consecutive years on a sandy beach on the east coast of Japan. Sediment cores were collected from the shoreline to the end of the offshore bar region. Each core was subdivided into layers and the sorting and skewness were analyzed using predefined Trask coefficients. The correlations between sediment properties, that is, median diameter, sorting, and skewness, with respect to the change in the bed profile are discussed in this paper. The weekly bed profiles, offshore wave conditions, and wave energy along the bed profile during the observation periods are discussed. Two significant sediment types were observed: finer well-sorted sediment and coarser poorly sorted sediment, both coarser skewed. The latter sediment properties were observed near the shoreline and at deeper elevations of the cross-shore bed profile. Finer well-sorted sand was transported away from and toward the coast depending on the wave intensity, which causes in-depth materials of coarser poorly sorted sediments to reappear with time. The offshore bar and trough region has a limited erosional depth due to the existence of poorly sorted coarser materials, which has been confirmed using weekly bed profile measurements. The sediment properties have a direct influence on the change of the bed profile.