| About 30% Changj iang suspended sediments are dispersed southward to the Hangzhou bay and the coast of Zhejiang and Fujian to form the mud wedge extending from the Yangtze River mouth southward off the Zhejiang and Fujian coasts into the Taiwan Strait. The resuspension in the winter plays a critical role on this kind of transportation. Several salinity fronts occur off the Changj iang estuary, which significantly influences the suspended sediment dispersal. The present study carried out a sea investigation off the Changj iang estuary during the spring season. The objectives of this study includes: 1) to determine the location of the salinity fronts; 2) to reveal the occurrence and formation mechanism of the high-speed current at the salinity front; 3) to compute the sediment flux at the frontal area; and 4) to discuss the implication of the high-speed current on the suspended sediment transportation.On 8-14 May 2004, a hydrologic and sediment logical investigation was conducted off the Changing river mouth (121°57.5'E ~122°30'E, 30°50'N~31°42'N). Nine sites were set for continuous observation of 13-25 hours. During the investigation, 3 boats observed in phase according to the tidal fluctuation. At each site, temperature, salinity, and turbidity were measured by OBS; current speed and direction were examined by ADP and direct-reading current meter. Besides, water samples were taken at the surficial, middle and bottom layers. The water samples were then tested for suspended sediment concentration (SSC) and grain size.According to the distribution of the temperature, salinity and the sediment concentration, the studied area was divided into 3 regions: I) well-mixed estuarine water dominates in an area shallower than 10m (sites S3, M3, S2 and M2). This area is characterized by high temperature and SSC, low salinity; II) diluted water at the surface water column versus salt water at the lower water column within the area of water depth from 10 m to 20 m (sites S1 and M1). The water of this area is featured by low temperature and SSC, high salinity. A halocline and thermocline occur at water depth of about 4 m with the highest salinity gradient reaching 2.9/m; III) Subei coastal current water (sites N2 and N3). The water of this area is characterized by low temperature and SSC, and high salinity. The salinity gradient reached 0.4~0.6/km on the sea surface between the region I (S2 and M2) and the region II (SI and Ml). Therefore, sites S1 and M1 are located in the frontal area.During the investigation, anomalous high-speed currents (160cm/s and 200cm/s) were found at the initial stage of the flooding of the higher high water. This high speed occurred generally at the upper water column shallower than 6 m and lasted for about 7 hours. Diluted water of lowest salinity and highest temperature dominated in the upper water column of site S1 at the initial stage of the flood tidal, whilst salt water of 29‰ occupied the lower water column. Therefore the anomalous high-speed current is suggested to be caused by the salinity front.High speed current also occurred at site M2, reaching 220 cm/s at the max. ebb tide of the lower low water, and directing southeasterly. Both max. ebb and max. flood were found of high speed at site S2. The speed of the max. ebb tide (168cm/s and... |
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