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In this study, the hydraulic function of the Hyakken-gawa Floodway, which was constructed in the 17th century to reduce the flood risk to Okayama Castle City, was evaluated by numerical flow simulation. The calculation conditions were determined by referring to the records about the floodway in old documents as well as from the numerical data based on the present conditions. The alluvial plain topography used for the inundation calculation was obtained from recent GIS data. The computation results showed that the flood control function of the floodway was composed of two stages: (1) Just before inundation occurred in the castle city, river water flowed into the floodway by collapsing the earthen dike located at the floodway head; this limited the increase in the river flow rate in the city area. (2) Just before the floodway capacity was exhausted, the backwater generated by the transverse masonry dikes in the floodway induced inundation to the paddy field region, which was on the side opposite to the castle city; this reduced the flood damage in the city area. The results also suggested that the civil engineers in the 17th century, who had neither the knowledge of modern hydraulics (including the concept of river flow rates) nor the advantage of using machinery for construction, presumably developed flood risk management strategies by combining the measures possible in those days. Finally, the results indicate that the floodway design used to distinguish the paddy fields from the urban areas for flood acceptance can be utilized for risk management against extraordinary floods even in present times.

Estimation of flood risk management in 17th Century on Okayama Alluvial plain, Japan, by numerical flow simulation