Hiding effect among sediment particles exposed on surface of riverbed
with an extremely wide range of sediment grain sizes
Yuki HIRAMATSU, Ryota NAKAMA, Koki HONGO, Masato SEKINE (Waseda University)
Introduction

Summary of experiments

In this study, the experimental investigation was conducted on the sediment transport onriverbed composed of materials with extremely wide range of sediment grain sizes. Suchphenomena are seen in river reach just downstream of a dam in Japan. In order to analyze theriverbed deformation process, it is necessary to fully evaluate the hiding effect of cobbles ongravels and sand. The hiding effect means that the tractive force acting on sand decreases as
the exposure degree of cobbles increases. Ashida and Fujita (1986) investigated the hiding
effect of gravels and proposed a formula to evaluate the sediment discharge of fine sandsuspended from the void among gravels.
The riverbed materials are classified into three grain size groups of sediment: L-, M-, andS-particles. L-particles are cobbles that cannot move at all. M-particles are gravels that canmove as bedload. S-particles are fine sand or silt that can move as suspended load. We haveconducted fundamental experiments on movable bed under the condition that each grain sizegroup is represented by only one grain size (Figure 1), and the relationship of the hiding effect
was formulated as follows:2𝜏
1 ∆M= 2+1(1)𝜏𝑀 𝛼 𝐷𝐿

Alumina balls with diameter of DL = 50 (mm) were used for L-particles (specific gravity is 3.98). Glass beads with grain
sizes of 5, 4, 3, 2, and 1 mm were used for M-particles (specific gravity is 2.5). These are called Ma-, Mb-, Mc-, Md-, and Meparticles in order of decreasing grain size. These were painted black, yellow, green, red, and blue, respectively. Silica sandwith grain size of 0.21 mm was used for S-particles (specific gravity is 2.65). The flume used for the experiments was anopen channel with a rectangular cross-section with the total length of 16 m, the width of 0.2 m and the slope of 1/250. The
bottom of the reach between 8.5 and 14 m from the upstream end of the channel is 0.05 m lower than that of the otherreach, and this lower reach is called ‘movable bed reach’. In the initial riverbed M- and S-particles were filled to the top of Lparticles in this reach (Table 1).
The experiments were conducted under the condition that flow rate was Q = 15 (L/s), and the discharge was reduced tozero when the riverbed attained the static equilibrium state. Any sediment was not supplied from the upstream end. Under
the equilibrium state, no sediment was transport at all, and therefore bed can not change any more. After the bed attained
this state, measurement about the vertical structure of the riverbed was conducted. For example, the height of individual
particles in the void space among L-particles was measured in the middle reach of the movable bed and in the center of thetransverse direction.

in which α is proportional constant (= 0.3) (Sekine and Hiramatsu, 2017) (Figure 2). However,since the grain size distribution is continuous in actual rivers, the experiments in this study
were conducted on riverbed with M-particles consisting of several grain sizes. In the static
equilibrium, the vertical structure of the riverbed was examined. Based on these results, weinvestigated whether the results obtained in authors’ previous experiments are general.

Figure 1. Vertical structure of riverbed in staticequilibrium: (a)L-M bed, (b)L-S bed.

Results

Figure 3. Schematic view of vertical structure of riverbed in static equilibrium: (a) Case A, (b) Case B, (c) Case C.

Figure 4. Cross-section of riverbed in static equilibrium: (a) Case A, (b) Case B, (c) Case C
Figure 2. Hiding effect of L-particles

Table 1. Experimental conditions and results: The friction velocity was u* = 0.058 (m/s).

✓ A layer consisting of only M-particles was formed in the void space among L-particles in all cases. This layer is called ‘Mparticle layer’ (Figure 3). The maximum grain size of M-particles group is defined as DMmax. In Cases B and C, LM / DMmax
were 0.9 and 1.0 (Figure 4). This means that the thickness of M-particle layer was approximately equaled to DMmax,
which was consistent with the previous results (Sekine and Hiramatsu, 2017).
✓ Top surface elevation of M-S particles DM becomes almost same (Figure 4). The difference in elevation among eachgrain size was almost the same as the difference in grain size. In other words, the bottom elevation of each size of Mparticles is consistent with the same plain which corresponds to the bottom surface of the M-particle layer.

Conclusion✓ The experiments were conducted in which the riverbed framework is composed of cobbles that cannot move at all.
✓ The vertical structure of riverbed under the static equilibrium state was made clear by conducting a series of experiments.
✓ In the the void space among L-particles, it was found that M-particle layer was formed. Characteristics of the structure of this layer was also understood quantitatively.
✓ The thickness of this M-particle layer is approximately equaled to DMmax, and the bottom elevation of each size of M-particles in this layer was the same level on averaged.

References
Ashida and Fujita, 1986, Proceedings of JSCE.
Sekine and Hiramatsu, 2017, Gravel-Bed Rivers.

SekineLaboratory, Department of Civil and Environmental Engineering, WASEDA UNIVERSITY