In previous studies, washing out of trees can be evaluated using Breaking or Overturning
Index(BOI) and Washing-Out Index(WOI). However, the applicability of those indices were only
validated some middle-stream rivers where the bed slope is not so steep. For validating the indices at
different river condition from past studies, this survey carried out at 7 rivers with different bed slopes.
Accordingly, a relationship between WOI and particle size distribution became clear. Bed slope is
found to have no influence on the critical condition of BOI, but particle size distribution can affect the
critical wash-out condition for WOI
