Formation, Evolution And Engineering Significance Of Submarine Sand Waves And Sand Ridges, Southeast Of Hainan Island

Lots of sand waves and sand ridges have been identified offshore southwesternHainan Island, which make threats to submarine facilities. The sand waves and ridgesneed to be further studied because related researches are still in the initial stage.Surface sediment, hydrodynamics, latest repeated multi-beam bathymetry, andsubbottom profiles have been obtained to study morphology, distribution, formation,and migration of sand waves and sand ridges. Through the study, followingconclusions are obtained:(1) Sand waves and sand ridges are not always produced bymodern sediment dynamics. Sand ridge Ⅰ and ridge Ⅲ are reformed from residualbedforms by modern hydrodynamics while sand ridge Ⅱ and ridge IV are formodern origin. Sand wave asymmetries, consistent with grain size trends andhydrodynamics, are in opposite directions in the two sides of sand ridges. Sediment ischaracterized with “finer in groove and coarser on ridge” and is finer far offshore.(2)The morphology of barchans is affected by the finite water depth, reversing tidalcurrents and sediment supply. The intensities of flow and sediment supply influencedbarchans’ activity and morphological stability. The separation of small dunes from thehost barchans provides a new sediment transport mechanism in the barchan area,which may play an important role in the evolution of barchans.(3) Symmetrical sandwaves with residual properties are identified. These large sand waves are thesynthetical results of hydrodynamics in different periods during sea level rise. Thebeddings in symmetrical sand waves are found reversed and are inferred to appearafter the Qiongzhou Strait formed. Small modern symmetrical sand waves betweensand ridges take sediments from two sides.(4) In situ observation indicated cyclicalcm level fluctuations of bed elevation. They were confirmed to be related to theevolution and migration of sand ripples and be caused by asymmetrical tidal currents.(5) Simulated sand wave migration velocities on basis of measured hydrodynamicsand sediment parameters are mostly consistent with observation. The calculatedvalues increase8.6%-81.2%in view of surface waves. When a typhoon movedwestward to the mouth of the Beibu Gulf, nearshore hydrodynamics and sedimenttransport were strengthened and showed the phasic changes. A typhoon leads highsediment transport and seriously affects sand wave migration in short term, but cannot change the migration direction in the long time scale. It can only influence thedistribution of sand waves.(6) Study on the sand wave and sand ridge is helpful to pipeline risk detection, safe maintenance of pipeline and also cost saving.