Response of water column to strong wind forcing,southern Brazilian inner shelf: Implications for sand ridge formation |
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| Authors: | Alberto G Figueiredo |
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| Institution: | Department of Geological Sciences, Columbia University, New York, N.Y. 10027 U.S.A. |
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| Abstract: | The result of two sequential oceanographic stations of 36 hours each in the area of sand ridges are presented. One station was located in the trough between two sand ridges and the other was at the crest of a sand ridge. At these stations salinity and temperature of the sea water, currents, winds, waves, and barometric pressure were measured each hour.During the observations, a cold front passed; this generated westerly winds that grew in speed from 24 to 52 km h?1. The average height of the wind generated waves grew from 1.0 to 1.5 m and their periods increased from 7 to 10 s, and the speed of the northeast directed surface current increased from 40 to 82 cm s?1. A bottom current (also directed northeast) increased from 26 to 34 cm s?1.After the cold front had passed, the wind backed to the southeast and decreased in speed from 26 to zero km h?1. The surface current in a northwest direction decreased from 29 to 8 cm s?1. A bottom current (also directed northwest) decreased from 22 to 3 cm s?1. Later, swells from the southeast appeared and their periods increased from 5 to 9 s and their heights grew from 1.0 to 1.5 m. After 3 hours, the speeds of the surface and bottom currents increased from 8 to 72 cm s?1 and 3 to 62 cm s?1 respectively.This cold front induced strong winds and storm-wave currents able to erode sediments (assuming a threshold velocity of 20 cm s?1) and transport them in a north-northeast direction.The origin and the maintenance of these sand ridges is thought to be a function of sediments eroded from troughs and piled up at ridge crests during a storm condition. Some eroded sediments are transported north of Verga lighthouse where they are deposited on a smooth bottom. |
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