三门湾猫头深潭及附近海域底床冲淤演变及其动力机制

Study on evolution and mechanism of seabed erosion and siltation in Maotou Trough and the adjacent area, the Sanmenwan Bay

对1995年采自三门湾猫头深潭及附近海域的原状土样的沉积结构、沉积速率的计算及在历史海图地形对比、剖面重复水深测量等结果进行了分析,研究了该区底床的冲淤演变及其动力机制,结果表明:强劲的水动力条件维持了三门湾猫头深潭及附近海域底床冲淤的动态平衡,近百年来该区冲淤平衡且略有淤积,平均沉积速率为1～3cm/a。由于水体中的悬沙浓度和水动力条件存在季节性差异,使得底床存在冬、春季微淤,夏、秋季微冲的季节性循环。近30年来,围涂等频繁的人为活动使该区内湾纳潮面积和纳潮量分别减少了15%和22%,削弱了落潮优势流,造成深潭中心出现了较快的淤积,平均沉积速率超过10cm/a,且以风暴残留沉积为主。

A comprehensive analysis is done based on the data of sedimentary structure,sediment radioactive isotope age determination,comparison of historical sea chart together with duplicate measurement of profile depth in Maotou trough and the adjacent area. Results show that the study area has kept stable with a little accumulation for a long time, there also exists distinct erosion and siltion by turns in different seasons.The Sanmenwan Bay is surrounded by hills except the southeast. There are a large number of tidal flat alternated with channel within it. Maotou Channel is the dominant tidal inlet linking up the inner and outer the Sanmenwan Bay. It's very difficult for the suspended sediment to deposit due to forceful tidal current (especially the strong bottom current) within spring and middle tide, and the seabed erodes slightly. There is possibility for some suspended sediment to silt during the neap tide and seabed deposit slimly. The strong tidal current has kept the Maotou Channel in the equilibrium of erosion and siltion with slight siltation in the past 100 years, the average sedimentation rate is 1~3 cm/a by using\+\{210\}Pb,\+\{137\}Cs methods. With the alteration of hydrology and suspended sediments, the seabed is easy to be silted in winter and spring, eroded in summer and autumn. When the storm hits the bay, accumulation occurs obviously in the trough and erosion in tidal flat. It's easy for the rapid accumulation to reside if several storms continue to act and the storm consists with astronomical spring tide. Because of a great deal of reclamation and mariculture after 1960s in the Sanmenwan Bay, the tidal current (especially the dominant ebb current) becomes weaker, which leads to the sedimentation rate become faster in some area of Maotou channel such as the center of Maotou trough.