2009 Volume 15 Pages 153-158
For evaluating the difference of tree breaking phenomena due to floods, i.e. trunk breakage or overturning, and wash-out of trees, Breakage or Overturning Index(BOI)and Wash-Out Index (WOI) were derived respectively in this study. Based on these indices, the method for classifying the dynamics of forestation in rivers was proposed. This method was applied to the middle stream reach of the Arakawa River The situation of ’breaking’ or ’washing out’ on a gravel-bedbar after 2007 flood can be evaluated by using BOI and WOL In addition, in three locations where relative height from ordinary water level isdifferent, the relationships between flood return period and breaking mode of trees due to floods were demonstrated. At the location where relative height is the smallest, possibility of forestation should be low because wash-out of trees occurs by 5 to 10 years return period of floods. In contrast, possibility of forestation should be high at the location where relative heightis the biggest, because wash-out of trees doesn’t occur by 40 years return period of floods. This method is available to judge the area where artificial management of trees is required.
