The topography of a sand bar is continuously affected by river runoff, ocean currents and sea wave action. Disruptions in any of these factors can alter the geometry of the sand bar, causing sedimentation problems and increased risk of flooding. In order to mitigate the risks associated with such changes, it is important to understand the characteristics of the topographic changes of river-mouth bars, and to develop a method for controlling the bar geometry. This study examined the effect of variations in the geometric properties of a river-mouth bar using hydrological data (river water discharge and sea wave height). The application of spur dikes to promote erosion of the sand bar by altering the direction of river flow was evaluated by means of flume experiments. A two-dimensional numerical model (iRIC Project) was also applied to examine the effectiveness of spur dikes on river-mouth bar control. The results of this study clarified the efficiency of measures for managing sand bar formation at the mouth of the Yuragawa River in Kyoto Prefecture, Japan.