PROCESS OF CLAY BED DEFORMATION DUE TO THE ACTION OF MOVING PARTICLES OF BEDLOADSEKINE MASATO, SHUN ISHIHARA, NOKO FUJIURA, and FUMITAKA YOSHIKAWA (Waseda University, Tokyo, Japan)
Introduction
Many studies had been conducted in the field of sediment transport and deformation of riverbed with non-cohesivesediment. But research on clay riverbeds was hardly conducted, and the knowledge about it was restricted. The authors
have been conducting a systematic basic experiment on the erosion of "clay bed" generated by the effect of only waterflow as the first stage of research (Nishimori and Sekine, 2009). Furthermore, an experimental study on also affected bybedload supplied from upstream, have been attempted by Sekine, Oka and Nomura et al. (2013), and Sekine andHiramatsu (2017). Based on these studies, this study focused on the deformation process of clay bed to sand or gravel bed.

Summary of experiment
In this study, a series of experiments wereconducted in two types of channels (seeFigure 1). Movable bed was composed ofclay only, and the sand or gravel wassupplied uniformly at the boundary crosssection of upstream end of movable bed.
In a straight channel, the flow rate of water
is 0.270 L/min, and the dimensionlesstractive force for silica sand No.3 is t* =0.079. The relative sand supply rate qb / qe
was set to be 0.25, 0.5, and 0.75. In thecurved channel, on the other hand, the flowrate was set to be 200 L/min, and relative
sand supply rate qb / qe was set to be 0.2.

Effect of sand transported as bedload on the deformation process of clay bed was investigated quantitatively.
➢ From Figure 2, it can be seen that the erosion rate decreases as the sand coverage rate on the bed surface increases. And therelation is independent of relative sand supply rate qb / qe. This means that sand covered with the bed surface play a role tosuppress the erosion of clay bed.
➢ In the initial stage of this phenomena, on the other hand, the erosion rate increases as time passes, because the transported sandparticles contact the clay bed surface and promote clay erosion significantly.
As was explained above, this process is controlled by both "erosion promoting effect" and "erosion suppressing effect" on clay bedby sand or gravel transported as bedload.

Process of mixed layer formation➢ As is seen from left photograph in Figure 3, sand remaining on the riverbed surface is in the state where half of the particle isburied in clay bed and forms a layer together with clay. This layer of sand and clay is called the "mixed layer" in this paper.
➢ From Figure 4, eroded inner-channel like a ditch was formed, and the channel widening occurred in transverse direction.
The process of this bed deformation can be summarized as follows; (1) a streak-like erosion occurs in the central portion due to theconcentration of bedload, (2) erosion in the vertical direction proceeds but is suppressed as the coverage of sand proceeds, (3) lateralerosion occurs gradually in transverse direction, and (4) finally, the sand coverage proceeds over the wide range of bed.

Phenomena of clay bed erosion in uniformly-curved channel
Figure 1. Experimental setup:(a) Straight channel (b) Uniformly curved channel

Clay bed deformation in straight channel

Figure 2. Relation between sand coverage
ratio and erosion rate

Figure 4. Cross-sectional profile of the riverbed

Erosion promotion and suppression effects due to moving particles as bedload

Figure 3. Riverbed situation and contour diagram

Phenomena of clay bed deformation was investigated byusing a uniformly curved channel where a secondary current
can develop considerably.
➢ Photograph 1 (a)-(d) shows that sand is initially depositedalong the inner side-wall due to the influence of thesecondary current, and erosion of clay bed occurs initiatesfrom the area. Dark portion along the inner wall in
Photograph-1 denotes such area. White portion in
Photograph-1, on the other hand, is the clay bed hardly
eroded yet. The eroded area extends laterally toward outerwall gradually as time goes on.
➢ Figure 5 shows the cross-sectional profile of riverbed ateach time. The left half with minus transverse distancecorresponds to the outer half of bed, and the right is innerhalf. It can be seen that the remarkable erosion initiates in
the inner half and the point of maximum erosion migrates
toward the outer side wall as time goes on. The location
where this erosion is most active is along the boundary
between the area mixed layer developed and the area clay
is exposed on the bed surface.

Photograph 1. Riverbed situation in a curved channel

Figure 5. Cross-sectional profile of the riverbed at different times

Conclusion

References

In this study, we conducted an experimental study to elucidate the deformation mechanism of clay river bed under the influence of transporting sand and gravel as
bedload as well as running water. It was found from a series of experiments that sand or gravel moving as bedload promotes the erosion of clay bed significantly.
Such effect was made clear in both the straight channel and the curved channel. In addition, the formation of the mixed layer composed of clay and sand (orgravel) on the bed surface also proceeds, which reduces clay bed erosion by being covered with deposited sand or gravel. This process initiates in the central
portion of the riverbed gradually, and then the lateral widening proceeds in a straight channel. In case of uniformly curved channel, such process is from the innerside to the outer side due to the influence of secondary current, but the essential mechanism is same as the one in straight channel.

Nishimori, K. and Sekine, M. (2009). Journal of JSCE, Ser. B1(Hydraulic Engineering).
Sekine, M. Oka, K. and Nomura, M. (2013). Journal of JSCE,
Ser. B1 (Hydraulic Engineering).
Sekine, M. and Hiramatsu, Y. (2017). Gravel-Bed Rivers:
Processes and Disasters.

Sekine Laboratory, Department of Civil and Environmental Engineering, WASEDA UNIVERSITY