Transport of gravel and cobble on a mixed-sediment inner bank shoreline of a large inlet, Grays Harbor, Washington

Gravel and cobble transport measurements were obtained by particle tracing experiments from a mixed sand, gravel and cobble beach at the head of a crenulate-shaped shoreline, Half Moon Bay in Grays Harbor, Washington. The direct measurements and results provide insight regarding the differential transport of gravel and cobble sized material on mixed-sediment beaches. The results are of relevance to the development of both predictive formulae for coarse-grained sediment transport and guidance for practical gravel and cobble beach design. Net alongshore transport of gravel and cobble is generally several times greater than the net cross-shore transport at Half Moon Bay. Larger and smaller particles both move preferentially alongshore; however, smaller particles tend to move across-shore more than larger particles. Particle transport distance during a tidal cycle increases with particle size (or mass) up to a point, beyond which the particle transport rate begins to decrease with increasing size. The direct relationship between transport rate and particle size may reflect the selective entrainment and the rejection (or overpassing) of larger particles, which are more exposed to fluid forces on the beach surface than smaller particles, which are sheltered within the matrix of larger particles, as well as the higher susceptibility to burial of smaller particles. The decrease in transport rate for the largest sizes may reflect the limited competency of the fluid forces to transport larger and heavier particles under the observed conditions. The direct transport measurements are consistent with the overall particle size and shape distributions observed on this crenulate-shaped inner bank beach whereby the larger and flatter particles have tended to outrun the smaller and more spherical particles in the downdrift direction.