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The left branch of the Bagua reach of the Yangtze River in China is vital for the regional economy and shipping. This paper describes a numerical modelling study, based on an extensively verified shallow-water solver, to simulate the effects of a diversion dyke on the flow around Bagua Island. A boundary-fitted curvilinear mesh of high resolution was generated, covering the model domain which stretched from Sihao Wharf to Gangchi. The simulation was calibrated against the field observations and physical model tests for the dry season at some velocity measurement cross-sections and water level gauging stations. The results showed that a diversion dyke can modify the water levels, flow fields and the flow rates to the left branch of the river. The water levels on the left side of the upstream channel and in the left branch were mainly affected. The left branch's diversion ratio also varied greatly when the length and the angle of the diversion dyke changed. However, when the angle of the diversion dyke was about 70°, the diversion ratio barely changed with the length of the dyke. The results are useful for channel management and regulation. [ABSTRACT FROM AUTHOR]