Scouring problems around the head of the west breakwater of Mailiao Industrial Harbor in Taiwan were investigated. The initial water depth at the head of the breakwater was 26 m. A scour hole of which deepest point 20 m deeper than the original sea bottom was formed 11 months after the completion of the breakwater. To cope with this problem, systematic investigations including field surveys, numerical modelling and physical modelling were conducted. The field data could almost identify that the current is the major mechanism causing this scour. To further confirm this point, a hybrid two-dimensional numerical model, the TW2DSD1, was used to simulate the flow fields and bathymetric changes. Then physical models aiming to determine the potential scour depth and to survey an effective protection measure were conducted. It shows that the scale of the scour hole would have reached an equilibrium state.