乐清湾近期海岸演变研究

乐清湾是一个与东海相通的半封闭型强潮海湾。通过对1934,1968和1992年的海图对比研究表明:近期乐清湾各等深线所围水域面积持续减小。后一阶段,围垦使岸线推进速度加快,滩涂面积由前一阶段的不断增加转变为不断减小。1934到1968年,西南部最大潮滩华岐潮滩岸线和零米线平均外推速率分别为4.88m/a和13.24m/a;1968到1992年平均淤进速率分别为6.17m/a和8.08m/a。实地采样分析表明:表层水样平均悬沙浓度为0.1043kg/m3,从湾顶向湾口呈不断增加的趋势。悬沙浓度虽较低,但湾内处于低能动力沉积环境,有利于细颗粒沉积物落淤,这是湾内地貌演变的要因。长江入海南移和陆架区再悬浮的细颗粒物质是乐清湾主要的泥沙来源。泥沙自然淤积和人类活动影响是近期乐清湾大部分岸线处于淤进状态的原因。漩门二期工程前后乐清湾的纳潮量减小了5.73%,落潮流相对携沙能力降低为工程前的79%。而根据实测落潮平均流速计算得围垦后落潮流的相对携沙能力减弱为原来的54%。而从1934年到漩门二期围垦工程后,纳潮量减小了22.57%。开辟盐田和围垦造陆工程等人类活动,对海岸演变的影响显著。

Study on Marine Environment and Recent Coastal Evolution of Yueqing Bay, Zhejiang Province, China

Yueqing Bay is located in Zhejiang Province, which is a half-closed bay, with the strong tidal dynamics, and connected to the East China Sea through the tidal channels in the southern part. Comparing the water depth maps surveyed during 1934, 1968 and 1992, it is obvious that water areas enclosed by different water depth contours decreased continuously in the whole area of the bay. Furthermore, during the period from 1968 to 1992, the progressing velocity of the coastline seawards accelerated because of the tidal flat reclamation, while the progressing velocity of the zero-meter depth contour decelerated. As a result, the area of the tidal flat reduced dramatically, even through it increased continuously during 1934 to 1968. Taking the Huaqi tidal flat area for example, which is the biggest tidal flat in the bay located in the southwestern area, the average progressing velocities of coastline and zero-meter depth contour in this tidal flat area were 4.88 m/a and 13.24 m/a during 1934 to 1968, but 6.17 m/a and 8.08 m/a during 1968 to 1992.Based on the surface water samples got in the eastern part of the bay from north to south, the average concentration of suspended material is 0.1043 kg/m3, increasing from the head of the bay to the bay mouth. Since it is a low-energy dynamic environment, even though the suspended sediment concentration is not that much high, it is easy for the fine material to deposit inside of the bay, which becomes the main feature of the geomorphologic evolution processes in the Yueqing Bay. There are two main sources of the fine materials in the Yueqing bay, one is the materials discharged from the Changjiang River mouth and moving southwards along the coastline, and the other is the re-suspended materials in the continental shelf. The natural siltation processes of the fine materials and human activities, especially reclamation of tidal flat, turn to be the main reasons for the accelerative progressing of the coastline recently. Before and after the second stage of Xuanmen reclamation project, the tidal influx of Yueqing Bay reduced 5.73 %, and the capacity for the ebb tidal current to carry sediment load decreased 79 %, while, using the surveyed speed of the ebb tidal current, the capacity decreased 54 %. In fact, after this reclamation project, the tidal influx has reduced 22.57 % since 1934. Thus, human activities such as opening up salt fields and reclaiming tidal flats have impacted the coastal evolution processes remarkably.