A Study On The Distribution And Activity Mechanism Of Sand Waves On The Northern South China Sea Shelf

The characteristics, distribution and activity of sand waves in the Pearl RiverMouth Basin (between Platform PY30-1and HZ21-1B) have been studied using thelatest multi-beam bathymetric data and measured hydrodynamic data.Internal-waves characteristics of the strong bottom currents and its effect on theformation and activity of sand waves were analyzed. The mechanism of formationand migration of sand waves was also discussed.The geomorphology becomes more complicated and sand waves become largerfrom north to south in the study area along with the deepening water depth. Smallsand waves and pock marks are found in the flat north part of the study area. Largenumber of sand waves including middle, large and giant scales lie in the middle part.Most sand waves are parallel in NE-SW with straight crest lines and few of them aresinuous. Many slides and collapses are also found on the slope in the middle part.Residual palae-coastlines and small sand waves are the dominant bedfoms in thesouthern part. Repeated observations since2010to2011show that sand wavesmigrated from northwest to southeast with weak activity. Most migration distancesare less than10m and only larger in partial area. The migration of sand wavesincreases from north to south. Most of the bottom currents are nearly southeastwardor northwestward, and the average current speeds range from10.20cm/s to17.99cm/s. Abrupt strong bottom currents faster than30cm/s were measuredfrequently in the study area, and the extreme speed can reach to116cm/s. Thesestrong bottom currents stronger in north and in summer are caused by internal waves.This paper points out that the tide currents in the study area are too weak to formand migrate the sand waves. The variations of scale and activity of the sand wavesare in accordance with the internal-wave. Abrupt strong bottom currents faster than30cm/s are formed when the internal waves spread towards the NW direction. It isbelieved that the strong bottom currents caused by internal waves formed andmigrated the sand waves. In addition, abrupt strong currents caused by typhoons alsocould make sand waves move forward.