Two-dimensional tide-induced residual sand transport: Applications to the Jiangsu coast, China

The residual sand (D > 62.5 μm) transport (RST) refers to the tidally averaged rate of horizontal sand transport. Albeit much smaller than the peak rates, it determines the morphological evolution. The analytical expression for the tide-induced RST has been reported in the condition of one-dimensional tidal currents by van de Kreeke and Robaczewska (1993). Here we extended the analytical solution to two-dimensional. The constituents of the residual, M₂, and M₄ tidal current velocities were used to characterize the collinear tidal current in the orthogonal X and Y directions, i.e., cross-shore and alongshore, respectively, while the M₂ tidal constituent is dominant in both directions. Based on the solution, RST is controlled by a series of triple interactions of the M₂ tidal velocity in the X direction, M₂ tidal velocity in the Y direction, and the residual or M₄ velocity in the X or Y direction. The solution was applied to the tide-dominated continental shelf of the Jiangsu coast, China, where the alongshore current is much stronger than the cross-shore component. The results show that the triple interactions of the cross-shore M₂ tidal velocity (U_M2), the alongshore M₂ tidal velocity (V_M2), and the alongshore residual velocity account for the major parts of both the alongshore and cross-shore RST in the nearshore area. The RST patterns promote sand accumulation in the shallow nearshore area and generate a narrow and active conduit of coastal alongshore sand transport. These findings can be applied in similar marine environments to understand the effects of non-linear tidal interactions in RST.