Disasters causedby torrential downpours have recently been increasing throughout ournation. Inparticular, there have been many cases of flood damage on small and medium-sized rivers with relatively small design flood discharge in valley flats. Specific instances of this type damage include that which occurred in Tochigi Prefecture's Yosasa River in 1998, the Appetsu River in Hokkaido in 2003. These disasters are characterized by the flow of flood waters that were not confined to the river channel and caused damage to many houses, roads and bridges. In this study, we examined a method for adopting double Fourier analysis, which is used for shape analysis of the results of experiments on sediment hydraulics, to assess the flow channel geometry of an actual valley flat. The originof the terrain of the valley flat was estimated based on the results of sediment hydraulic studies, including experiments on compound meandering channel anddouble-row bar. The potential for utilizing the estimate results to predict the flow of flood waters and measures to reduce the impact of disasters at times of large-scale flooding in a valley flat was then indicated.