Change of biological community and bed topography after the gravel-bar restoration work on the Nagata district in the Tama River

The gravel-bar restoration project, which is conducted in the Nagata district of the Tama River, was reported in this paper. The purpose of this project is to regenerate gravel-bar area and wildlife that is characteristics of gravel-bed river. In making a project execution plan, scholarship persons, citizens and administration discussed the goals of this project and its method together, taking account of both flood control and river environment. As a result, 1) Cutting down and uprooting Robinia pseudoacacia, one of exotic species, 2) Widening of low-water channel by excavating flood channel, 3) Making gravel-bar area by arranging gravel at comparatively high elevation above ordinary water-level, 4) Importation and supply of gravel and so on were carried out. After these works, researchers of the Tama River Research Group, the River Ecology Research Group have been monitoring the number of biological communities such as Aster kantoensis, Eusphingonotus japonicus, Charadrius placidus and riverbed topography as their habitat. The followings were confirmed as that result. 1) The number of the biological communities mentioned above increased in the widened area. 2) Sand accumulated at the time of the flood of about 700 m3/s in a part of the C-section of the widened area, which was planned and excavated not to make sand accumulate. 3) It has the possibility to control the degrading tendency of the riverbed, which goes on so far, by supplying gravel to the upstream end of the Nagata district. 4) Conysa sumatrensis and Erigeron Canadensis are washed out by flood, when the non-dimensional tractive force becomes greater than about 0.06.