Power Spectrum Measurement with a Strain Gage in the Fluctuation of a Movable Bed

Abstract

In river-water intake in the agricultural water utilization, it is important to estimate the fluctuation in river bed. As a means of solving the problem of bed load, the power spectrum analysis is introduced; and in this connection, the experiments have been made.
For the measurements, a strain gage, which is handy and inexpensive, was employed. With the strain gage attached at the end of a cantilever, the fluctuation in a channel bed free from the water flow was only recorded. The strain gage was placed within the sand bed in an experimental waterway. And an electrical signal correspondent to the fluctuation was produced through the gage; these signals were taken in a tape recorder. In this way, during the course of an experiment, the fluctuation in the channel bed with time could be measured automatically, with water flowing.
The data in the tape recorder were mechanically sampled and digitalized in an A-D converter in corporated in the spectrum computer; and the auto-covariance function and the power-spectrum density function were computed.
As the result, it was revealed that the phenomenon being observed was a typically random one and in the unstable state of sand in a channel bed such as the front of a sand wave the auto-covariance function was with a considerable attenuation slope.
Furthermore, the power-spectrum density function itself was varied periodically. Then though no definite conclusion was possible because of the insufficient data obtained, the fluctuation tended to be large when the dimensionless quantity of the tractive force in flowing water was about 0.2-0.25.
In the future, basical experiments will be made systematically by the procedure described, thereby accumulating the necessary data. And thus the actual situation of bed load and river-bed fluctuation will be made clear from the dynamic characteristics; this information obtained should contribute to the design in river-water utilization.