Yagisawa, JTanaka, N2013-11-112013-11-112013-11-12http://dl.lib.mrt.ac.lk/handle/123/9000In 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.entree trunk breakagetree wash outbreakage or overturning index(BOI)wash-out index (WOI)particle size distribution of river bed materialsConference-Full-text2012International Symposium on Advances in Civil and Environment Engineering practices for sustainable developmentFaculty of Engineering - University Of Ruhuna. Galle119-126yagisawa@mail.saitama-u.ac.jp