The purpose of this study was to investigate the changes of the sea bottom topology during construction of Saemangeum seadikes. Sea water depth and bottom topology in the Saemangeum area were measured every year and the surveyed data under construction of Saemangeum seadikes were analyzed and compared to the initial conditions before construction, There was erosion in the overall surveyed areas and the depth of erosion was approximately 53cm compared to the data in 1988. The center sandbanks of Seadike 2 did not show the great changes due to two main channels those were developed in northeast and southeast. The inland sandbanks of Seadike 4 showed the development of great erosion and deposition partially because of the changes in tide direction which developed as a result of the completion of Seadike 3 and the completion of a seadike of the Kunsan-Changhang industrial park at the north of Saemangeumm area.