عنوان مقاله [English]
Sand hills are formed by erosion in very large mega-cusps (200 meter along the coast). This erosion is caused by returning currents. In very large mega-cusps, coasts reach their narrowest limit. So progressive waves of huge storms and high tides can reach the claws of the coastal hills undercutting them, which finally results in coastal hills being eroded. Measurements and field observations of dunes, sandy beach and returning current morphology were performed in an 18 kilometer coast line of Monterey Bay in California. This part of sandy coast line is uplifted more than 40 meters due to the spread of sand dunes. Under the cape and toward the gulf center, waves converge due to their breakdown on the Monterey underground canyons and their height increase significantly. Large gradient of wave height in coast length creates a continuous gradient in morphodynamic scale. Thus, strong returning waves and narrow mouth of the bay have resulted in the development of returning currents throughout the coast.
With 95% confidence level, longitudinal coastal changes which happen due to the volume of eroding hills have a significant correlation with longitudinal changes occurring due to coast line cusps. Moreover, longitudinal changes in coasts caused by the cusps in the coast line has a very significant relation with longitudinal changes of the coast in the range of rip currents. Therefore, it is possible to say that very large cusps are related to rip currents and the position of eroding hills is also related to the range of mega-cusps.
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