Morphological variations and sediment transport in crenulate-bay beaches, Kodiak Island, Alaska

Abstract

Grain size, sorting, beachface slope variability, and sediment transport directions were examined at five mixed sand, granule, and pebble crenulate-bay beaches located at Kodiak Island, Alaska. Fitted log-spiral curves were used to differentiate zones of erosion and deposition and to determine the equilibrium morphology of each beach. Convergence and divergence of wave orthogonals from a computer-generated wave refraction diagram provided wave energy and sediment transport direction patterns. Delineation of this information combined with ground truth data permits the development of a crenulate-bay beach model. Crenulate bays are divided into three distinct shore segments based upon beach position. The shadow zone at the hooked end of the bay exhibits low wave energy, small grain size, relatively well-sorted sediment, gentle beachface slope, and an eroding shoreface relative to the remainder of the shoreline. The converse is true for the center of the beach which is subject to high wave energies and is characterized by large grain size, poor sorting, a moderate beachface slope, and a shoreline which is transitional between erosion and deposition. The characteristics of the tangential end are similar to those of the shadow zone except that the beachface slope is steep and the shoreline is depositional. Sediment transport is predominantly toward the tangential end. A headland, natural or artificial may result in the formation of a downdrift crenulate bay which eventually develops a stable equilibrium morphology, resulting in wave crests breaking simultaneously along the entire bay shoreline and a zero net longshore transport rate. Wave regime, grain size, sorting, beachface slope, erosion, deposition, and sediment transport all interact to determine the morphology of a crenulate-bay beach. The model of this study is partially based upon crenulate-bay theory which assesses the future morphology of a beach. Additionally, much of the previous work on crenulate bays is synthesized, with beach processes intergrated into the description of such bays.