Distribution of gravel-sand on high water bed by Typhoon Hagibis (T1919) flood in the middle reaches of the Tama River, central Japan

Abstract

Distribution of clastic grains transported and deposited on high water bed during Typhoon Hagibis (T1919) flood in the middle reaches of the Tama River was investigated. Gravel deposition tends to be predominant from upstream part to river channel side of central part of banks. On the other hand, middle-fine grained sand deposition tends to be predominant from far side from river channel of central part to downstream part of banks. The sands often make low-relief 2D dune bedform. The gravels were often deposited on a trough which has been fixed on a bank during several decades. Average rate of the gravel movement during T1919 flood on a trough was estimated as being less than several 10 m/hour. Even though the T1919 flood, one of the biggest flood during several decades in Japan, transported and arranged various sized grains (boulder-sand) on sand-gravel bank conspicuously, hardly changed the form of the banks. Considering devastating flood caused by the rainfall beyond an expected level might increase in future, it is important to record and publish the situation of transport and sedimentation of the clastic grains by T1919 flood at various place in the Japanese islands.