The turbulent flow in shallow-water coastal regions controls substance mixing and exchanges within these regions. These mixing mechanisms get highly involved in processes such as air-sea exchanges, diapycnal transport, and sediment resuspension in the coastal ocean, which are key focuses of the marine environment and climate change. Oceanic boundary layers in coastal regions are driven by diurnal external forces such as wind, surface gravity, tide, and current, resulting in a highly complex dynamical system. Previous studies have mainly focused on either wind-wave driven surface boundary layer (SBL) or tide-current driven bottom boundary layer (BBL) in the ocean. Recent studies have shown that the SBL and BBL in shallow water can have strong interactions and merge into one layer. However, the physical mechanisms of such a process have not been revealed. In this study, we use the large eddy simulation (LES) to investigate the evolution of SBL and BBL in the coastal area and the merging processes. LES can reproduce high-fidelity turbulent flow with extremely high spatial and temporal resolutions, directly resolving interactions between two boundary layers in the coastal ocean as well as the turbulent mixing processes. The relation between the merging and the diurnal external forcings is characterized. And the different patterns of vertical mixing before and after merging are shown.