ACTIVITY MAGNITUDE OF THE SMALL-MEDIUM SUBAQUEOUS DUNES IN THE YANGTZE SHOAL

The Yangtze shoal, located in the east of the Yangtze River Estuary and about 40 m deep below sea level,has widely spread small-medium subaqueous dunes,whose wave height is 0.6 m and which are generally in activity. Statistical result of 32 subaqueous dune areas shows that the movement direction is within 90°~180°.150°~180° direction accounts for 30% and 120°~150° for 69%. The movement velocity of the small-medium subaqueous dunes, calculated using the formulas from Rubin and Hardisty, is usually 19.70 m/a and controlled by tidal currents, and is 12.7 m/a in storm season.And the total migration rate is 32.4 m/a. Compared with observation data from other small-medium subaqueous dunes in the world, the activity of small-medium subaqueous dunes in the Yangtze shoal belongs to a slow-medium magnitude.