南海北部海底沙波地貌

南海北部水深８０—２５０ｍ的外大陆架和上大陆坡海底，有大片沙波地貌分布，它们主要是晚更新世末次冰期低海面时遗留下来的滨海沙波、沿岸沙丘、沙堤和沙垄，属残留沙体地貌。它们明显沿着本次冰期时形成的最低海岸线两侧分布，但在现代海底底流作用下，其两翼及沙波谷部发育有次一级的小型沙波或沙纹，它们的形态细小、排列紧密，与残留沙体地貌组成复式海底沙波。这就是流动沙波，是不稳定的地质因素，对海底工程有一定危害，但根据外大陆架实测底流数据，运用计算机数学模型方法计算，表明在细沙和中细沙大面积覆盖区，海底沙波每年均以０．１６６—０．５３４ｍ的速度向深海方向迁移，移动速度相当缓慢，其对海底工程设施不会造成直接危害。

SEAFLOOR SANDWAVES IN THE NORTHERN SOUTH CHINA SEA

A large area of sandwave geomorphology has been found on the seafloor of the outer continental shelf and upper slope within a water depth of 80-250 meters in the northern South China Sea.Most of them are nearshore sandwave,coastal sand dune,sand barrier and sand ribbon that belong to relict sand body geomorphology generated during the low sea level period of the last glacial epoch in Late Pleistocene. The distribution pattern of these sandwaves were located along the two sides of the lowest coastal line produced during the last glacial epoch. Because of the action of modern seafloor currents, second-class sandwaves or sand ripples with a small geometry have been found closely arranged on the two wings and valley of the sandwaves. Both relict sand body and second-class sandwaves produced duplicate seafloor sandwaves. Such sandwave geomorphology has been commended as moving sandwaves, unsteady geological factor or constraints to the seafloor structures. According to the seafloor current data investigated on the outer continental shelf and computation with mathematical model method, the results show that such seafloor sandwaves on the area which are covered with fine and fine to medium sands, move very slowly toward the deep sea area at a speed of 0. 166-0. 534m/yr.Such condition would not directly endanger the seafloor engineering structures.