La Mixed Sediment Beach Processes : Kachemak Bay , Alaska

Mixed sediment beaches are morphologically distinct from and more complex than either sand or gravel only beaches. Three digital imaging techniques are employed to quantify surficial grain size and bedload sediment transport rates along the mixed sediment beaches of Kachemak Bay, Alaska. Applying digital imaging procedures originally developed for quickly and efficiently quantifying grain sizes of sand to coarse sediment classes gives promising results. Hundreds of grain size estimates lead to a quantitative characterization of the region’s sediment at a significant reduction in cost and time as compared to traditional techniques. Both the sand and coarse fractions on this megatidal beach mobilize into self-organized bedforms that migrate alongshore with a seasonality reflecting the temporal pattern of the alongshore component of wave power. In contrast, the gravel bedforms also migrate in the cross-shore without significant seasonality suggesting that swash asymmetry is sufficient to mobilize the gravel even during low energy summer conditions.