Abstract: During the design process of sea-crossing bridges and tunnels,it is necessary to estimate the design flow velocities at different recurrence periods for the project location. However,the lack of on-site long-term flow observation data makes such estimations very difficult. Based on typical storm surge processes with different return periods,a numerical method was proposed for estimating design flow velocities for estuaries and coastal areas,for which the coupling of flood runoff and storm tidal currents was considered for the estuarine areas. According to the measured tide level data from the Macao tide station from 1925 to 2003,the characteristics of historical storm surge processes in the Pearl River estuary were analyzed,and typical storm surge processes of different return periods were obtained based on the results of the analysis of annual extreme frequencies of tidal levels and tidal ranges. Then,a horizontal two-dimensional hydrodynamic numerical model was established to simulate the flow field in the Pearl River estuary under the combined conditions of storm surges with different return periods and general upstream flooding. With such a model,design flow velocities at control points along the Hong Kong-Zhuhai-Macao Bridge were determined.