The Study Of The Tidal Process On Upper Part Of The Tidal Salt-marsh

Yangtze River Estuary is one of the most typical muddy estuaries, and it occupies an important position in the the field of international estuarine and coastal research. Upper part of the tidal salt-marsh is an important part of the estuary tidal flat, where various microbial flora and fauna gathered. Upper part of the tidal salt-marsh not only protects the coastal zone, but also responds most sensitively to the changes of the environment. With the development of the Yangtze River estuary delta, the available potential resources of the salt-marsh are getting increasingly important. At the same time, the government and experts are putting more concerns on the threat from human activities to the tidal flat. Therefore, in recent years many scholars and experts have been focusing on tidal flat research. With various means and methods, they have made many achievements. However, it remains to be further studied on the research of tidal currents on salt-marsh through the long-period field measurements.Supported by the National Natural Science Foundation (40571012) , the team went to the upper part of the tidal salt-marsh on Chongming Dongtan and did long-period observation on tidal currents in spring and summer in 2007.It was the first attempt to use ADCP to collect data on the tidal flat. The team completed the field measurements, data acquisition and sample collection..The team also obtained the vertical distribution of biomass (Scirpus mariqueter) via laboratory analysis. According to these data, the paper discussed the tidal current on the Chongming Dongtan of Yangtze River Estuary with some main conclusions as follows:(a) The features of the tidal currents on the salt-marsh of Chongming Dongtan are as follows: tidal showed asymmetry, the flood phase lasted longer than the ebb phase for about 0.1-0.5 hours. The flood current velocity was 1.07-1.95 times faster than the ebb current velocity. In calm weather conditions (wind velocity < 5m/s), there was a current velocity peak during both flood and ebb phases. The peak value during ebb phase was much lower than that during the flood phase under the effect of salt-marsh. In windy weather conditions (wind velocity≥5m/s), there was a current velocity peak during the tide process and the maximum current velocity occurred at the late stage of the flood tide. Long-shore tidal currents'forces were usually stronger than those of cross-shore tidal currents', especially in the deep water. Current forces (maximum water depth was equivalent to the tide range) had a significant impact on the vertical velocity. The greater the water depth of the tidal cycle was, the greater the vertical velocity would be.(b) According to the classical velocity profiles model von Karman-Prandtl equation using u - lnz linear analysis, it was calculated that there were more logarithmic velocity profiles during a complete tidal cycle with deeper water and there were more logarithmic velocity profiles during the flood phase than there were during the ebb phase. According to internal consistency demands, there were more water layers during the flood phase. Distributions of mean velocity in vertical were influenced by wind, wave, changes of mean velocity in vertical, bed morphology, etc. Distributions of mean velocity in vertical tended to be non-logarithmic under strong wind waves. In calm weather conditions, distributions of mean velocity in vertical were also non-logarithmic when the maximum water depth was deep and the maximum velocity occurred at the subsurface water layer. Near-bottom velocity profile was non-logarithmic due to the effects of bed morphology. The vertical distribution of biomass (Scirpus mariqueter) was one of the factors that influenced vertical velocity.. The more the biomass was, the slower the perpendicular velocity would be.(c) The vertical velocity gradient was higher during the flood phase than that during the ebb phase, which meant the velocity difference in adjacent water layers was greater during the flood phase than that during the ebb phase. The greater the velocity was, the greater the vertical velocity gradient would be. If tidal velocity was slower during the ebb phase than that during the flood phase, the vertical velocity gradient would be less during ebb phase than that during flood phase.(d) Generally, the instantaneous changes in water depth could be described as spindle type. Smaller changes occurred during the early flood phase, and then changes would increase gradually and reached the peak during the medium-term tidal cycle, and finally changes would become tender again during the ebb phase. However, the reason why there was a low point of instantaneous changes in water depth during the medium-term in 0802D tidal cycle was that the water depth was shallow.