Detection of fronts as paths of river inputs into estuary system, Pearl River Estuary, China

The Pearl River estuary is the first basin of sedimentation when inputs of all branches of the Pearl River reach the sea. The estuary is situated along a low-tidal coast with tide ranges of about 1m at the mouth. Annual runoff for the River is 3.27 x 10¹¹m³, and annually sediment loads discharge is 8.34 x 10⁷t. The bottom topography inside the estuary is relatively flat. Within the Lingdingyang, the largest estuary channel accepting about one-half of water discharge and one third of sediment loads from the whole Pearl River output, there are two main channel, whereas along both sides of the channels there are extensive tidal flats and shoals. The huge metropolitan areas located along the shore of the estuary such as Guangzhou, Hong Kong and Shenzhen seem to be the initial sources of the nutrients and other matter, which might be linked, with the events of pollution and phantom bloom around the mouth of the estuary. However, how the material is transported into the mouth of the estuary and in which system we can collect the information of the events mentioned above as soon as possible, are still far from being understood. Large-scale hydrographic surveys commenced during the 1970s and 1980s. The data suggest that its obvious salt wedge and the difference between east and west sections give the estuary its characteristics. Li Chunchu (1997) has done analysis on this set of data and classified the Lingdingyang into three sub-systems: the river discharged sub-system in the northwest section, the tidal inlet sub-system in the east section and the continental water intrusion sub-system in the southeast section around Hong Kong. The fronts within or especially between those systems play important roles in carrying suspended solids into the sea (Ying Zhipu, 1994, 1995). The aims of those surveys, even the main project conducted recently are not, however, to focus on the fronts and their key structures connected with source, path and sink of the chemical matter. To search the tools to monitor the environment of the Pearl River estuary, we conducted observations, using both satellite and ship-aboard sensors, in the Pearl River estuary during the winter of 1996 and summer of 1997. In this paper we will discuss the characteristics of the fronts and their roles in river input into the sea