X-band radar observation of morphological changes due to flood events at the mouth of Tenryu River, Japan

摘要

Morphological changes due to flood events at the mouth of River Tenryu are observed using an X-band radar. A lateral river sand bar exists mostly and is eroded intermittently in different manners due to floods and high waves. Time-averaged images are processed hourly to track the variations of the river channel, river-mouth width, bar locations, and other features. 59 flood events are analyzed in an 8-year observation period: 1) patterns and extents of bar erosion with respect to flood intensities, and 2) detailed processes of bar erosion or flushing during flood events. Bars are eroded mainly owing to river floods, which are sometimes accompanied by large waves. The extent of erosion of the bar is dependent on the width of the river mouth before the flood and the intensity of the flood. The river mouth can be expected to widen by more than 150 m owing to a flood exceedance probability of 3 years. An image analysis of the radar images captured during floods was used to estimate the migration speeds of the light-dark brightness pattern, which is assumed to be a proxy of the bottom undulation of the river bed. The migrations were faster when the flood intensities increased.