SEDIMENT TRANSPORT IN THE RIVULETS NEAR KAIFUURA, NIIGATA PREFECTURE

Abstract

In a former paper the writer (T_AKAYAMA, S. 1956) mainly treated the problem of bed load transport. The present article describes the results of measurement on the transportation of suspended load of sediment by the three rivulets.
In Fig. 1 are presented writer's results of measurement of the suspended load on these rivers (Hayakawa, Fukôgawa and Ôkawa). Both the appreciable scatter of the points and the limited coverage of discharge range are evident. However, the trends of the regression lines indicate that suspended load discharge increases continuously with increasing discharge. The equations for the lines for average conditions of Hayakawa, Fukôgawa and Ôkawa are respectively:
Ha akawa; Q_S=114.1 Q^2•48
Fukôgawa; Q_S=89.9 Q^1•72
Ôkawa; Q_S=51.3 Q^2•06
in which Qs is the suspended load in tons per second, Q is the water discharge in cubic meter per second.
The coefficient and the exponent in the equations are different from each other due to the condition of the surface of the drainage area at the time, the rate of availability of sediments and other factors. Usually the suspended load seems to vary approximately as the square roots of the water discharge and this means that the values of increment of suspended load with water discharge is generally larger than that of bed load.
Two fairly distinct trends could be discerned (one for the flood occured August through September when high flows take place; the other for the flood of October when flood stage is not so high) from Fig. 1. This difference is thought to be due to the changing volume of fine-grained river bed material which can be easily transported. That is, the river bed is filled with fine-grained particles subsequent to several weeks of low stages; then the flood peak in August and September completely removes these particles.
Total sediment discharge is computed as the sum of suspended discharge and bed load discharge. It is the total quantity of sediment as measured by dry weight, that is discharged during a given time. For example, in Fig. 2, the area under the curves (Q_S-T and Q_B-T) represent the sum of suspended load [Q_S] and that of bed load [Q_B] respectively during a flood (Tab. 1) . Hydrograh for this flood is shown in Fig. 3. The percentage ratio of [Q_B] to [Q_S] (hereafter designated as Q_B/Q_S) varies with the scale of flooding as shown in Fig. 3. There is a tendency that the bed load movement dwindles to small portions as the maximum water discharge increases.
The gross amounts of sediment transported as bed load, suspended load and total sediment load during the observed period are calculated by continuous summation of [Q_B], [Q_S] and [Q_T] of each flood and Σ Q_B, Σ Q_S and Σ Q_T obtained as tabulated in Tab. 2. In order to provide data on comparative basis, these values are reduced to the values per square kilometer of drainage area. There are great spatial variations among them and these differences may be reflected not only in the character of sediment but also in the landforms themselves in each drainage area.
The geomorphic features which appear to be significant to sediment delivery are mean altitude of a drainage basin, maximum relief in a drainage area, mean watershed slope, mean channel slope and mean relief ratio (Fig. 4 & 5, Tab. 3).