Study On Flow Structure And Friction Characteristics Of Near Bottom Layer In Estuarine And Coastal Waters

On the basis of fluid dynamic principle, the log-linear model of the near-bed tidal velocity and its extending model in estuarine and coastal waters were established by introducing Prandtl's mixing length theory and von Karman self-similarity theory. Furthermore, the friction characteristics of tidal current were studied.The bottom shear stress could be determined by τ = pu_*~2, where ρ is density of water, u_* isfriction velocity, both are important parameters for studying bottom boundary layer. Three types of method solving u_* are described: 1) Turbulent kinetic dissipation method; 2) Velocity profile method; 3) Experiential drag coefficient method. For the usual velocity profile method, four kinds of method are given: 1) Estimating roughness length from velocity time series of two near-bottom layers; 2) The least square method; 3)Hough transform method; 4) Enumerating combined with the least square method. The Hough transform method is adopted to determine the parameters of velocity profile, and the validation of the method and the application to ADCP field date were carried and compared with the least square method. Meanwhile, the enumerating zO combined with the least square method was used to determine the parameters of velocity profile, the detailed process was given and the fitting and analysis to field data were carried out.Many observed data show that the near-bed tidal velocity profile deviates from the usual logarithmic law. The amount of deviation may not be large, but it results in large errors when the logarithmic velocity profile is used to calculate the bed roughness height and friction velocity and bottom shear stress. Based on their investigation, Kuo et al,(1996) indicated that the deviation amplitude may exceed 100%. By the analysises of the velocity profile data observed at the west Solent south mainland of England, and Eckerforde Bay southern Baltic Sea, using the least square fit combing with enumerating roughness height Zo, the following conclusions can be obtained from the comparison of the log-linear velocity profile with the usual logarithmic law: 1) The log-linear velocity profile is closer to measured value than the logarithmic velocity profile and has higher precision. 2) The roughness height and the friction velocity determined by the log-linear velocity profile are more correlative than that of logarithmic velocity profile, and the former is more consistency in phase.