从河口发育过程探讨河海界线

Research into the river-sea delimitations from estuary evolvement

当河口三角洲逐渐向海洋伸展时,必定会对河口附近的海洋进行改造,从而改变口外海滨某些海洋要素,据此可作为河海界线的判断标准,即在常态状况下,某些要素的改变,是否已经改变了口外海滨本来的属性,如果已经改变,应界定为河流,如仍以海洋属性(包括动力、海水理化性质、沉积、地形等)为主,则仍界定为海洋。河口是河流和海洋相互连接的枢纽地带,在这里主要动力有径流(河流属性)、潮汐、海流和波浪(海洋属性)等。在它们的作用下,通过泥沙运移,塑造各种地貌形态,反过来,地貌形态又改变了水动力的属性。从河口地貌学的角度,河口可以分为3部分,即近口段、河口段和口外海滨段。近口段是在原来的口外海滨或河口段,经历三角洲外伸,沙洲并岸,河槽束狭,河槽成型、加深等阶段,逐渐形成的。从近口段的演化过程看出,该段属于河流。口外海滨段虽然受到河流影响,但是其仍然表现为海洋属性,因此,属于海洋无可置疑。河口段介于近口段和口外海滨段之间,该区段受径流和潮流的共同作用而造成自然要素的不连续,这种不连续表现在河口形态,底床形态,潮汐特征,水团(盐度、絮凝体)、浊度及沉积物特性等方面。河口形态的变化比较直观。潮波从外海传入河口口门后,因受到地形的影响而改变,包括潮汐性质、潮差,甚至变形破碎形成涌潮。所以在河海界线上,潮波变形造成的最大潮差、最大流速、乃至潮波破碎形成涌潮的位置具有一定意义。河口段存在最大浊度带,其内界与最大絮凝体位置相当。根据我国现行定义的海陆界线为平均大潮高潮位线,从平面上看河海界线时,沉积相是一个重要指标,即只有出现三角洲平原相时,才说明河口对海洋改造已基本完成。

River is an important external power of estuary evolvement, which transport silt to the estuary region and sea. With continuous development of the estuary, the sea is always retreating. When the estuarine delta stretches into the sea, the adjacent sea must be transformed and thus changing the elements of the sea shore. These changes can be used as the judgment criteria for river-sea delimitation. That is to say, if the sea elements such as hydrodynamics, physical and chemical properties of the sea water, sediment and landform etc have been changed, it should be defined as river, otherwise, it should be defined as sea. The river mouth is a vital district where the river links the sea and the hydrodynamics there includes river runoff （river attribute） and tide, current and wave （sea attribute）. It can shape the topography through transporting sediment and the changed topography in turn changes the hydrodynamics. By geomorphology the river mouth can be divided into three parts, the near mouth section, mouth section and outside mouth section or coastal section. The near mouth section is the original coastal section or mouth section, resulted from delta outward stretching, sand bank-shore merging, river channel narrowing, shaping and deepening. From the evolving process of the near mouth Serction it belongs to river. Although the coastal section outside mouth is influenced by river, it still keeps ocean attribute and so belongs to ocean. The mouth section lies between near mouth section and coastal section. Because of the combined action of runoff and tide, the natural elements including estuarine morphology, bed shape, tidal characteristics, water mass （salinity and flocculating constituent）, turbidity and characteristics of sediments, are intermittent. The estuarine metamorphosis is relatively visual. The bed metamorphosis is mainly incarnated by the channel mouth bar resulted from the accumulation of large amount of silt near the outlet. The bar crest is not only a distinct relief feature, but also has shield effect on various factors. It is an important topographic symbol and has some sense in sea-river delimitation. Tidal wave will be changed by the estuarine topography as it comes from open sea into the river mouth. It even breaks to form tidal bore. The maximum tide range and flow speed resulted from the metamorphosis of the tidal wave and even the position where the tidal wave creaks to form tidal bore is important to the sea-river delimitations. The flocculating constituent is resulted from flocculation of the suspended matter in the river mouth. According to the study of the flocculating constituent in Changjiang River Estuary, the salinity of water with the biggest diameter of flocculating constituent is 1 - 2 or so. This index can be referred to in sea-river delimitation. There exists a belt with greatest turbidity, of which the inside boundary is corresponds to the position of the biggest flocculating constituent. Sediment texture reflects the reformation process of the sea when the river mouth stretches seaward. According to our current definition, the coastline is the mean tidal level of spring tide. Sedimentary facies is an important index in sea-river delimitation. Only when the deltaic plain facies merges, it can be educed that the modification of the sea by river mouth is complete.