| Abstract: | A 2,3-dimensions hydrodynamic numerical model on extended x-coordinate was used to calculate the tidal current before and after trenching in Hangzhou Bay. A piecewise reversible transformation is used in the x direction to map the variable grid into a uniform grid used in the computation. The transformation has the form x=phi(x')=bx'+abx'3, where x'and x are uniform and variable grid respectively, a and b are 1/90 and 38.3 for the computational regions, so that there are 4 grid points across the channel section (width 160 km). In order to reduce the computational time and have good stability. Two dimension calculation alternating with 3 dimension calculation method is employed, the former being the alternating direction implicit (ADI) finite-difference method, the latter being 3 dimension is the explicit method.
The velocity and direction of tidal current before and after trenching during the spring and neap tide are obtained by numerical modeling. The maximum and vertical average vertical velocity are 169.5 and l06.0 cm/s (before trenching) 143.5 cm/s and 92 cm/s (after deepening) during the spring tide, the vertical average velocity after trenching decreases about 13%, the above corresponding velocities are 136.5 cm/s and 78.5 cm/s (before trenching), 113.5 cm/s and 66.5 cm/s (after trenching) respectively during the neap tide, the vertical average velocity in the deepened channel after trenching decreases about 15%. On the vertical direction, and the surface velocity increase about 2% and the bottom velocity decreases about 33%-39% due to trenching. During neap tide, the reducing of bottom velocity is largest. |
