Graduate Thesis Or Dissertation
 

Beach profile changes and onshore-offshore sand transport on the Oregon coast

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  • Two beaches with significant differences in grain size and thus in beach profile morphology and response to wave conditions were studied on the Oregon coast. Gleneden Beach, just south of Siletz Spit and Lincoln City, has a median grain size of 0.36 mm (medium sand) and a steep beach face slope, while Devil's Punchbowl Beach has a median grain size of 0.23 mm (fine sand) and low concave-up beach face slope. Eleven beach profile surveys were obtained at Gleneden Beach and twelve at Devil's Punchbowl Beach between August 1976 and April 1977; on average once every two weeks during spring tides. Gleneden Beach showed the typical change from a swell profile with a wide berm that prevails during summer months to a storm profile with little or no berm that exists during the stormy winter months. This transition occurred in August and September, being completed by early November. The finer-grained Devil's Punchbowl Beach also showed general erosion during the fall. However, a transition from a swell profile to a storm profile is not as clear there as the beach has little berm, even in mid-summer, and always has a concave-up appearance typical of the winter storm profile. Gleneden Beach and Devil's Punchbowl Beach did not always agree in their responses to the changing wave conditions. One may be eroding at the same time the other is accreting. These differences in response to changing wave conditions appear to result from their differences in grain size. Volume changes of the erosion or deposition at the two beaches were computed from successive beach profiles. The coarser-grained Gleneden Beach showed larger changes in erosion and deposition, the maximum erosion being 0.71 m³ per meter of profile length, while the finer-grained Devil's Punchbowl Beach showed a maximum erosion of 0.25 m³ per meter of profile length. Attempts were made at relating the erosion or deposition and the volumes of erosion/deposition to the wave breaker heights and deep-water wave steepness that occurred between the beach profile sequences. There is only a vague relationship between the volumes of beach erosion/deposition and the wave heights, the probability of erosion increasing and the volume of sand eroded increasing with increasing wave breaker heights. The maximum wave heights that occur during the time interval appear to be most important to the volume of erosion, erosion volumes being large if storm breaker heights reach 5 to 6 meters or greater. Deposition prevails when the average breaker heights fall below 4 meters and storms are limited to breaker heights less than 5 meters. The deep-water wave steepness shows little relationship to the erosion or deposition volumes, indicating that the wave period is not as important a parameter as the wave height to beach erosion.
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