2007 Volume 13 Pages 177-182
For elucidating tree breakage condition with different tree species and tree age, field investigation on tree height hv, trunk diameter at breast height dBH, and tree projected area in vertical section, of Salix subfragilis and Robinia pseudoacacia were conducted on three gravel-bed bars in midstream of the Arakawa River. After a flood on October 2006, hv, dBH and trace water depth were investigated for trees which were broken or bent down by the flood. Loading test was conducted for deriving critical breaking moment of the tree trunk. Critical breaking moment of the two tree species were expressed by the function of dBH,3 The critical breaking moment of Robinia pseudoacacia is lower than that of Salix subfragilis when the diameter is the same. Drag and drag moment acting on trees were calculated with different tree age and magnitude of flood disturbance. Return period of floods (T2) was compared with the maximum tree age (T1) at which tree is broken under critical breaking moment. Comparing T1 with T2 has the possibility to become an effective method for evaluating whether the habitat in a river has a tendency to be a forest or not.
