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Crenulate-bay beaches, common geomorphologic features associated with rocky headlands, were studied on Kodiak Island, Alaska. Grain size, sediment-transport direction, and slope variability were analyzed at four mixed sand, granule, and pebble crenulate-bay beaches ranging from 600 to 2,000 m in length. Nine stations were set up along each beach with a profile measured at every other station. At each station, two replicate samples were similarly collected at three sites normal to the beach. Sampling involved collection of 960 cc of beach material, measurement of the volume of each pebble over 2 cc as well as intermediate axis, shape, and analysis of composition. Individual grains <2 cc were labeled as "fines," cumulatively measured volumetrically, and a small sample wa brought back to the laboratory for standard size analysis. A computer-generated wave-refraction diagram was used to determine wave-refraction, diffraction, and reflection patterns which helped to determine transport directions.
Results indicate: (1) Grain size coarsens toward the center of each crenulate-bay beach. Within the four beaches, an average of 80 to 90% coarse material (> 2 cc) is present in the center, and 30 to 65% coarse material is found at the tangential and "shadow zone" ends. (2) Grain size and sorting decrease seaward. An average of 23% more coarse material is present at the landward than at the most seaward sample site. (3) There is a constant increase in beach slope from the "shadow zone" end to the tangential end of each crenulate bay. (4) Grain-size data do not support the tangential end sediment transport for crenulate-bay beach morphology that has frequently been reported in the literature.
Because equilibrium-beach development due to construction of an artificial headland by coastal engineers takes on a crenulate-bay morphology, understanding of natural crenulated-bay beaches is especially important.
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