Abstract
On June 28, 1961, a disastrous flood took place at various parts of the Tenryu River course caused by heavy rainfall accompanied by the Baiu front.
In the district of Kawaji, that is situated at the southern end of ma Basin occupying the upper part of the Tenryu River drainage, a violent inundation occurred and the deposits brought by the flood water covered the inundation area thickly.
Except those of branches, the flood deposits are divided into three categories, that is, gravelly, sandy and silty materials. In the cases of the sandy and silty materials, it is recognized that the deposits are well sorted. Namely, grain size distribution of most samples from these two is limited between the two sizes as is shown in . Fig. 4. Then, the writer determined graphically the finer limit of the sizes of a sample and presumed that the particles of sediments smaller than this size moved in the form of suspended load and flowed out with flowing water.
If we represent the value of this critical size by “d0”, and the value of settling velocity of do in standing water by “w0”, w0 may be proportional to values of the vertical component of the turbulent flow which are functions of the velocity of the main flow.
The distribution of w0 is shown in Fig. 5. Considering the foregoing aspects and Fig. 5, it may be interpreted that the flood-flows reduced abruptly its velocity near the entrance of the studied reach, and its high velocity zone forms a meander in the middle part of the flooded area while the flood deposits were being settled.
