Author(s): Wenjin Zhu; Na Wang; Xiaotian Dong
Keywords: Cross flow; Residual velocity; One-dimensional convection-diffusion equation; Gauss function; Migration rate
Abstract: The mechanism of coastal sediment transport is that wave stirs up sediment and current transports the sediment. The waterway of the coastal region is subject to current and waves. Usually, currents are perpendicular to the longitudinal axis of the waterway. The currents are called as cross flow. The phenomenon of channel axis deviation and cross-section change will occur under the action of the cross flow. The velocity of cross flow comprises the residual velocity and main tidal constituents velocity. It is assumed that waves only act as stirring when the channel axis deviate and cross-section change. Moreover, it is also assumed that the morphological time is much larger compared to the tidal period. The seabed deformation equations caused by the motion of bed load and suspended load are averaged over the tidal period separately. For the bed load, Bagnold transport formula is used for bottom boundary condition. For the suspended load, the bottom net sediment flux equation is used and the reference concentration is proportion to the square velocity. The higher order terms are retained for the results, while lowed order terms are neglected. Then, a theoretical model is established for describing the law of channel axis deviation and cross-section change in the coastal region. The new theoretical model has the form of one-dimensional convection-diffusion equation which has constant convection-diffusion coefficients. The Gauss function is used to describe the waterway cross-section. According to the theoretical solution of the one-dimensional convection-diffusion equation, the quantitative description of channel axis deviation and cross-section change under the action of cross flow is obtained. The channel axis deviation and cross-section change is a function of residual velocity. The new method is applied to the channel of Lianyungang Port in the Southern Yellow Sea. Under present hydrodynam ic conditions, the migration rate of channel axis in Lianyungang Port is about 5m yr-1, and the change rate of cross-section is 292.69m2 yr -1. In addition, it is also applied to the waterway of the Port of Haian at Qiongzhou Strait. The migration rate of channel axis in Haian Port is about 10m yr-1, and the change rate of cross-section is 294.22m2 yr-1. This research has important engineering value for designing channel and dredging maintenance.