Abstract
It is known among practicing river engineers that a block rolls down easily when it is dropped into flood water. Originally, block to be used in the river has not been made in the assumption that it will be put into running water, it is designed with the assumption that it does not move in the installation. The reason of why when a block is put into running water, the block rolls easily is not clear. For this reason, the hydraulic experiments supposing the situations of dropping and stillness was conducted to the blocks of some kinds of leg shape. And it was verified whether it would be easier to rotate a block than the time of setting on bed when the block was dropped into water. Furthermore, it was considered from the dynamic model why it would be easier to rotate a block than the time of setting on bed at the time of dropping into water.
