Book Description

The observation and measurement of fluorescent tracer sand movement on the sea floor under full-scale sea conditions have outlined major elements of the sediment transport system operative above wave base. Much of this picture had not heretofore been defined in the field. Oscillatory surge accompanying surface waves drives the system imparting movement to sediment predominantly in the direction of wave advance. Sediment is transported from coastal sources in predictable patterns which are dependent mainly on surge velocity and direction, other currents, sediment grain size, and bottom slope. Sediment movement patterns caused by surge are distorted by the currents of other origins. A null line exists for a given sediment size, wave character and bottom slope. Sediment shoreward of the null line migrates onshore; sediment seaward of the null line migrates toward wave base. Sediment moves onshore with increasing velocity in shoaling water, ultimately entering the surf-swash longshore drift which in turn is locally deflected offshore by rip currents or rocky headlands. Eventually sediment shifted offshore is again reworked in the onshore sediment movement induced by wave surge. Sediment moving in traction and saltation is lost from the system either when trapped in submarine canyons or during periods of large waves when sediment is transported beyond the reach of normal waves. Sediment kept in suspension by wave-induced surge forms a dense turbid layer at the bottom which, as wave energy decreases, flows slowly down the sea bed and beyond wave base into deeper water. (Author).