Abstract
Morphological changes during a flood event in July 2010 were observed with X-band marine radar at the mouth of Tenryu River, Shizuoka, Japan. Radar images were collected hourly for more than 72 h from the beginning of the flood and processed into time-averaged images. Changes in the morphology of the area were interpreted from the time-averaged images, revealing that the isolated river dune was washed away by the flood, the width of the river mouth increased gradually, and the river mouth terrace expanded radially. Furthermore, image analysis of the radar images was applied to estimate the migration speed of the brightness pattern, which is assumed to be a proxy of bottom undulation of the river bed. The migration was observed to be faster when the water level gradient between the river channel and sea increased.
Highlights
Morphological data are essential to evaluating and understanding the long- and short-term behavior of a sandy river
An X-band radar can be used as an alternative to remotely observe the behavior of river morphology
The intensity of a pixel in the radar image corresponds to the relative amount of backscatter signal of the emitted radar beam reflected from the flood plain, such as vegetation, water surface, etc., and it is usable during the night and under slightly rainy and high-wave conditions
Summary
Morphological data are essential to evaluating and understanding the long- and short-term behavior of a sandy river. Data collection is limited to fair weather conditions and daytime periods, which makes it difficult to wholly track a sudden event like a flood In this context, an X-band radar can be used as an alternative to remotely observe the behavior of river morphology. Observation by a land-based X-band imaging radar helps us understand the morphology and hydrodynamics by capturing spatial distributions and temporal variation of water lines of the river channel and coast lines, and wave propagation in the shallow area [5,6]. Pianica et al [7] recently reported a video-based observation of bedform deformation of ebb delta over 23 days in the United States They tracked the migration of bedform features, which was mainly induced by the tidal action under fair conditions and estimated their speed. This study discusses migration of bedform features, but induced by a river flood, and shows another aspect of morphological dynamics at a river mouth
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