METHOD FOR EVALUATING THE DYNAMICS OF FOREST IN RIVERS CONSIDERING TWO INDICES REPRESENTING THE BREAKING AND WASH-OUT CONDITIONS OF TREES ON GRAVEL-BED BARS

Abstract

  For evaluating the possibility of forestation in gravel bed bars, Breakage or Overturning Index (BOI) and Wash-Out Index (WOI) were derived considering the difference of tree breaking phenomena due to floods, i.e. trunk breakage or overturning, and wash-out of trees, respectively in this study. Coupling the two indices, the method for classifying the river habitat into four, not breaking and not washout (Region A), breaking and not washout (Region B), not breaking and washout (Region C), and breaking and washout (Region D), is proposed. The applicability of the two indices, BOI and WOI, were validated in the middle stream reach of the Arakawa River and the down stream from the upper stream reach of the Tamagawa River. In addition, in seven locations where relative height from ordinary water level is different, the breaking modes of trees were classified in the BOI-WOI plot considering floods with different return periods. There are two patterns for the dynamics of plots with flood return period in the BOI-WOI figure. One is changed from Region A to Region D (Pattern I), and the second is changed from Region A to Region B (Pattern II). In Pattern II, trees are not washed out even though they are damaged. Then, the location has high possibility to be a stable forest. Therefore, the countermeasure for preventing the forestation should be considered to change the habitat from Pattern II to Pattern I.