CHARACTERISTICS OF UNSTEADY FLOW IN AN MEANDERING COMPOUND CHANNEL WITH EMBANKMENT

Abstract

 The heavy rainfall in July of Reiwa 2 caused significant damage in the vicinity of the Chaya settlement in Watari, Kumamoto. The floodwaters exceeded the levees and rapidly flowed along the railway line, resulting in house collapses in the levee area. In Japan, rivers often have dual-section configurations with low channels and high floodplains to accommodate different flow rates during normal and flood conditions, and the Kuma River is no exception. This study aimed to investigate the flow structure and turbulence characteristics in unsteady flow by installing levees in a dual-section meandering channel. Laboratory experiments using stereo PIV measured water depth and flow velocity in steady and unsteady conditions with and without levees. The results showed that without levees, there was little difference in water level, but with levees, water levels dropped before and after surpassing the levees, and the water level increased in the inner part of the levee. Observations of the main flow velocity distribution revealed faster flow in both the low channel and the high floodplain, correlating with the distribution of house collapses in the Chaya settlement caused by the heavy rainfall in July of Reiwa 2. Additionally, turbulence characteristics showed larger values during the reduced water flow period.